Glasses and Crystals
History of Glass and Glassmaking
Long before humans learned how to manufacture glass from raw materials, people fashioned tools
and weapons from naturally occurring glasses. Several types of natural glasses were used including
tektites, obsidian, and pumice. Tektites are small oval-shaped pieces of glass believed to be from meteors.
Obsidian, or cooled lava, is a glassy volcanic rock. And pumice is a foamed glass formed when gases are
released from lava as it cools. Among the first glass artifacts were knives and arrowheads of flaked
obsidian dating from Paleolithic times. Among the oldest pieces of synthetically manufactured glass
known is a bead found in Egypt dating back to about 3500 BC. Most historians indicate that the birthplace
of man-made glass was in Egypt or Mesopotamia prior to 3000 BC. It has been speculated that
glassmaking was accidentally discovered while cooking in a sandy, alkaline environment. More likely
glassmaking came about as an offshoot of metalworking. Silicate impurities in metal ores would have
separated during refining and formed an intriguing glassy slag. Another likely scenario may have been
from experimentation with ceramic glazes that were developing during approximately the same time
period.
The Egyptians became excellent workers of glass and produced colored glass sticks as raw
materials for glass objects such as glass vessels, glass eyes, beads, mirrors, and ornaments. The art of
glassmaking died out in about 1200 BC and did not reemerge until about the 7th century BC. During the
4th to 2nd century BC new technical and artistic developments allowed for the making of gold-sandwich
bowls in which a layer of gold leaf was trapped between two layers of glass. Other developments of this
Hellenistic period included the manufacture of mosaic glass rods and cameo glass, a two-layer glass that
could be carved.
It was the Romans who took the Egyptian’s knowledge of glass working and raised it to a new
level of excellence. When the technique of glass blowing was invented in the first century AD, glass
quickly became an object of common use throughout the Roman Empire because it was inexpensive and
easy to produce. Techniques perfected by the Roman glass workers were soon spread throughout Europe
and everything from blown glass vessels to cast glass windowpanes were produced. The Romans also
became expert in coloring glass and modifying the reflective nature of the glass surface. Glass making
centers arose in Italy and glass was exported as far away as Britain. By the fourth century, different glassmaking centers throughout the East and the West were developing their own particular regional styles. As
a result of barbarian invasions after the 5th century, glassmaking experienced somewhat of a decline and it
became a less centralized activity. This gave rise to more widely distributed yet smaller glass centers
throughout Europe.
During the Medieval Period, glassmaking was dominated by the demand for colored window glass
for churches and cathedrals. The earliest surviving painted glass dates from the 10th century in Germany
but it is suspected that painted windows date back to even earlier times. In the late 13th century a strong
glass industry emerged in Venice. Techniques for making stemmed glasses and vessels, mosaic glass used
to produce millefiori glass, and ornately decorated glass objects were fiercely protected by laws which
prohibited glass makers from practicing their skills outside of Venice. Eventually, during the 16th century,
the Italian knowledge of glass working did spread throughout Europe influencing glassmaking in Vienna,
Sweden, England, and Bohemia. By the late 16th century Bohemian glass began to develop a new style
which involved the cutting and engraving of glass. This was the beginning of the famous German “crystal”
industry. The Germans also introduced less expensive methods for producing colored glass by coating the
outside of a clear glass vessel with a layer of colored glass.
Glassmaking was introduced to North America in 1608 at Jamestown. Most of the early glass
produced there was sent back to England. A number of other glass factories were founded including one
begun by Baron Stiegel in 1763 in Manheim, Pennsylvania. Over the years since the American Revolution
a number of glass manufacturers grew up and died out. Many came to influence modern day glassmaking
in the United States and they include Owens-Illinois Glass Company, Wheaton Glass Company, Kimble
Glass Company, Tiffany glass, and the Steuben Glassworks in Corning, New York.
Today glass is one of the most common substances in home and industry, and, at the same time,
one of the most intriguing. Its hardness is evident in windowpanes and jars; its brittleness is unfortunately
seen with table glass and light bulbs. Yet, glass is not a solid! It has little internal crystalline structure and
therefore we call it amorphous. Usually it is described as a super cooled liquid. Like other liquids, the
Source: Dr. Patricia Hill, Millersville University
1
The History of Dyestuffs
Dyes have been known and used by peoples since ancient times. However, before 1856 all dyes
came exclusively from natural sources, either plant or animal. In 1856 William Perkin, an eighteen yearold English college chemistry student, accidentally discovered the first organic dye produced synthetically.
He was trying to produce the drug quinine from coal tar when he noticed a bright reddish-purple residue in
his reaction flask. He named his newly synthesized reddish-purple dye mauveine and soon a synthetic dye
industry was born and began to provide a wealth of colored organic compounds that eventually supplanted
most of the natural dyes. An understanding of the chemical nature of colored organic substances such as
dyes did not begin until 1868 when two scientists (Graebe and Liebermann) observed that all dye molecules
contain conjugated double bonds in their structure. In addition to these conjugated systems, O.N. Witt
noticed the occurrence of certain functional groups in dye molecules. He called these structural features
chromophores. The presence of other functional groups, which he named auxochromes, effected the
molecules’ light absorption, often making it more intense and shifting it toward longer wavelengths thereby
changing the hue of the dye. The combination of these three structural components — conjugated double
bonds, chromophore groups, and auxochrome groups — has been termed the chromogen and is the part of
the molecule responsible for its color. Today all dyes are classified as organic aromatic compounds
containing a conjugated system of double bonds to which chromophores and auxochromes are attached.
This chemical structure determines the following characteristics of the dye
1. Wavelength of maximum absorption — hue (color) of the dye
2. Intensity of the visible band — molar absorptivity above 20,000 L/mol cm
3. Width of the visible band — the narrower the band the brighter the dye
4. Polarization direction of absorption band
As you read about the history of natural dyes, examine the compounds’ structures and see if you can
identify the parts which cause these molecules to be colored.
Vegetable Dyes
Madder
Madder, also known as “dyers root”, is a red dye obtained from the roots of the Rubia tinctorum
plant, a plant indigenous to many countries in the tropical and temperate zones and found abundantly in
Europe and Asia. Most of the pigment occurs in a reddish-colored area of root between the outer skin and
woody heart of the root. Chemically the dye is found in the form of glucosides such as ruberythric acid.
This can be decomposed to yield alizarin, chlorogenin, pupurin, and sugar.
Madder has been known and used as a dye since antiquity from India, to ancient Persia and Egypt.
Madder was also used by the Greeks and Romans. Madder-dyed cloth has been found on Egyptian
mummies from a predynastic era. In 330 BC Alexander the Great purchased madder for use by his invading
army. He dyed their clothing to simulate blood and fooled the Persian army into thinking they were facing a
weak and helpless foe.
During the Dark Ages, madder was not cultivated in Europe, but was grown in the Orient. By the
7th century AD madder began to be imported to Europe from the East. Cultivation of madder for use as a
dye began again in Europe during the 8th century and continued throughout the Middle Ages and up to the
19th century. The “red coats” of the British army were dyed with madder, as well as the caps of the French
army, and the coats of all those engaged in fox hunting in England.
When synthetic dye production reached the commercial market in the mid-19th century, the
cultivation of madder declined worldwide. This was because synthetic alizarin, the red compound in
madder, was about one-fourth the cost of madder and more effective as a dye.
O
OH
Alizarin, the red compound found in madder root,
OH
has the chemical name 1,2-.dihydroxy-anthraquinone and
has the chemical structure shown to the right. It is
manufactured from coal tar, a by-product of the iron and
steel industry, or from petrochemicals derived from crude
O
oil.
Indigo
Source: Dr. Patricia Hill, Millersville University
115
2
Indigo, one of the most important blue vegetable dyes, has been known and used for over 4000
years. It is obtained from the leaves of Indigofera tinctoria a leguminous shrub which grows in Asia,
Africa, the East Indies, the Philippines and America. The dye is extracted by harvesting the leaves, soaking
them in water for about 14 hours, and beating the yellow-orange/olive-green liquid with sticks to infuse
oxygen and convert the soluble leuco indigo to an insoluble, dark blue precipitate. The solid dye is allowed
to settle out and the water is removed. The indigo sludge is heated to prevent further fermentation, cooled,
and filtered to form a thick paste which is formed into bars, dried and cut into indigo cakes for distribution.
Indigo was used by the Egyptians, Romans, Greeks, Incas as well as the Indians. The Venetians
were the first medieval peoples to use indigo imported from India in the year 1194 AD. However, the rest
of Europe did not use indigo until the 15th century. This was because a strong woad industry existed in
Britain, Germany and France.
Woad, Isatis tinctoria, is a cruciferous perennial plant from which a blue dye was obtained in a
similar manner to the extraction of indigo from indigo plants. In fact the chemical composition of these two
dyes is identical. Woad was known and used by the Celts for body paint and tattoos during the time of the
Roman Empire. The word Britain is derived from a Celtic word meaning “painted men”. The woad dyeing
industry in England, France and Germany was protected from competition by laws prohibiting the importing
and use of indigo. Indigo was often referred to as the “devil’s dye”. Only in the early 1600s did indigo
start to be used in Western Europe as the Dutch began to import and sell indigo. Once the Americas were
settled, indigo became an agricultural crop in Georgia and the Carolinas. In 1747 farmers were even
subsidized for growing indigo. Much of this indigo went to color British navy uniforms, which were
designated as “Navy blue” by King George II around 1745. Today indigo is the dye used to color blue
jeans. It is also known as D&C Blue #6 and is used in cosmetics and as a laboratory indicator dye.
The chemical structure of the dye molecule is given below. Because indigo is insoluble in aqueous
solutions, it must be applied by the vat process. In this method the dye is chemically reduced to a soluble
leuco form, usually in an alkaline solution. The material to be dyed is soaked in this solution, and then
removed and exposed to the air where the soluble dye is reoxidized to its insoluble form and is trapped
within the fibers of the material.
OH
NH
ox id ation
HN
OH
reduction
leuco indigo
NH
O
HN
O
indigo
Other Vegetable Dyes
Many other colored dyes can be obtained from a variety of plants. Most of these dyes are
extracted by boiling the leaves, roots, or fruit in hot water. Natural vegetable dyes work best on natural
fibers such as linen, cotton, silk, and wool. However, most of these dyes are not very lightfast or colorfast,
fading in light as well as with repeated washing. Some vegetable dyes are listed below.
Saffron is a yellow dye obtained from pistils of a crocus. Saffron has been used in cooking as well
as in dyeing. Weld is another plant that yields a yellow dye. Weld is the oldest known yellow dye and was
used by the ancient Romans. It is also known as “dyers’broom” or “dyers rocket”. Brazilwood is a red to
red-violet dye obtained from certain leguminous trees called soluble redwoods. Logwood or Campeachy
wood, comes from a tropical tree which grows in Mexico. Logwood is also red to red-purple in color.
Animal Dyes
Tyrian Purple
Tyrian purple dye was first used in Crete as early as 1600 BC. The Phoenicians continued to
prepare and trade this dye well into the 7th century AD. Tyrian
O
purple is a brilliant, colorfast red-purple dye obtained from
NH
Br
Mediterranean snails of the genus Murex. Because it takes nine
thousand snails to obtain one gram of dye, Tyrian purple was
HN
very costly and reserved for regal and ecclesiastical uses, hence Br
O
the term “royal purple”.
Tyrian purple
The chemical structure of this dye is remarkably
similar to the structure of indigo. In fact the chemical name for Tyrian purple is 6,6’- dibromo indigo.
Source: Dr. Patricia Hill, Millersville University
116
3
Kermes
Kermes is a bright red dyestuff obtained from an insect, an oriental shield louse which lives on the
leaves and stems of shrubby oak trees Quercus ilex and Quercus coccifera (scarlet oak). Sometimes it is
referred to as a “scale insect”. This insect is related to the shellac-producing insect, stic lac. This scarlet
dye is one of the oldest colorants known with evidence of its use as a textile dye in Neolithic times. It was
widely used from ancient times until the Middle Ages. In addition to being used as a soluble dye, it has also
been widely used as a lake pigment by prepitating it onto Al2O3 (aluminum oxide). However, it was not
until 1640 that a Dutch chemist discovered the chemical similarity between kermes and cochineal another
red dye. Kermes was most likely discovered by the Phoenicians but it
CH 3 O
OH
was also widely used in the Orient where dyers were able to obtain
COOH
brilliant red colors. Kermes was expensive but not as costly as Tyrian
purple. From the word kermes developed the words carmine and
crimson used to describe various shades of red. In 1467 Pope Paul II HO
OH
ordered that “scarlet is the cardinals color” and kermes was used to
OH
COOH O
dye these robes.
kermesic acid
The chemical structure of kermes, called kermesic acid, is quite similar to the structure of alizarin,
the coloring extracted from madder root.
As a dye and pigment, carmine is a fugitive colorant. It has poor light and weather resistance and
is quite translucent when used in oil or varnish. Therefore it finds modern use primarily in cosmetics and
student grade artists paints.
Cochineal
In the New World another red dye called “nochezli” was being used by the Aztecs. This dye was
imported to Europe and eventually replaced kermes. This dye called cochineal or carmine was collected
from cactus beetles (Dactylopius coccus) in Mexico. The cochineal beetles are small gray-red or dark redbrown insects and it requires about 70,000 dried beetles to produce one pound of cochineal dye. Cochineal
made its way to Europe during the 1500 and 1600s. Until about 1725 many Europeans believed that
cochineal was the seed of some American, tropical plant. Use of cochineal dye declined starting in 1858
with the introduction of aniline red and with the introduction of azo dyes about 1889 its use practically
stopped.
Carmine, also known as carminic acid, has a structure similar to both alizarin and kermes.
C H2OH
O
OH
HO
dried
cochineal
beetles
O
OH
HO
C H3
C OOH
OH
O
carminic acid
Modern Organic Dyes
As mentioned earlier, the birth of the synthetic dye industry began in the late 1800s. In fact
William Perkin quit school, much to the dismay of his teachers, and with his father’s financial backing, went
on to achieve fame and fortune in the synthetic dye industry. Although Perkin was English, it was the
Germans who established companies like Badische Anilin- und Soda-Fabrik Co. (BASF) and Baeyer
(Bayer) which carried out the synthesis of indigo and alizarin starting from organic chemicals found in coal
tar. Between 1880 and 1900 some 950 patents for new synthetic dyes were registered and the growth and
development of the dye industry was in fact the growth and development of synthetic organic chemistry.
Today there are over 100,000 synthetic dyes and over 2,000 of these are produced in commercial quantities.
The Society of Dyers and Colourists produces a five-volume Color Index which classifies about 8,000 dyes
and pigments under some 40,000 commercial names. To begin to make sense of so many dyes we must be
able to classify them into groups with similar properties and behavior. One of the most logical
classifications is according to color. Dyes belong to one of the following color classes:
yellow
violet
black
Source: Dr. Patricia Hill, Millersville University
117
4
orange
blue
red
brown
Because there are so many dyes available and because they are known by a variety of different
names, scientists and dyers have attempted to classify them into three major groups according to: (1)
solubility, (2) method of application, and (3) chemical composition and structure.
Before describing the three categories of dyes it is important to consider what characteristics are
desirable in a useful dye. Some of these characteristics are inherent in the dye molecule itself while others
may be dependent upon the method used in the application of the dye to a substrate or the character of the
fiber to which it is applied. The table below lists six important criteria that make for useful dyes.
Characteristics of a Useful Dye
1.
2.
3.
4.
5.
6.
Intense color with > 10,000 — This allows for the use of small amounts of dye so there is only a 0.13% weight gain by the fibers or fabrics.
Solubility in water. Dyes should be soluble or dispersible in water or have the ability of being
temporarily converted into a soluble form.
Substantivity to the fiber to be dyed. This means the dye molecule and the fiber must possess some
affinity for each other in order to drive the dye from the dye-bath onto the fiber.
Durability to wet treatments. This is for color fastness when washed.
Durability to further treatments in production and normal use.
Safe, easy to handle, and reasonably priced.
Classification of Dyes According to Solubility
As opposed to a pigment, which is an insoluble inorganic solid, a dye is a relatively soluble
organic solid. To be a useful dye the molecules must be able to diffuse into unordered, amorphous areas of
a fiber and become anchored to the fiber by chemical attraction or by physical entrapment. This
necessitates getting the dye into a form that can do this. There are three groups of dyes based upon
solubility criteria.
a. Soluble dyes — These are molecules with areas of anionic or cationic charge which are readily water
soluble.
b. Disperse dyes — These molecules are non-ionic (no charge) and are usually only very slightly watersoluble. They need high temperatures and various additives in order to disperse the dye in solution.
These dyes have affinity for many synthetic fibers such as polyester, acrylic, nylon, and acetate.
c. Pigments — These dyes are so insoluble that they are classified as pigments. However, some of these
insoluble materials may be rendered temporarily soluble prior to application. These dyes, indigo being
the most noted example, are chemically reduced to a “colorless” leuco form in the dye vat. The cloth is
introduced, soaked, removed, and exposed to air to allow the dye to reoxidize to the colored, insoluble
form that becomes physically trapped in the fiber network.
Classification of Dyes According to Method of Application
Classifying dyes by their method of application is of particular interest and use to textile chemists
and dyers whose job it is to choose the best dye to color a particular type of fiber. Dyes have affinity for
fibers and are attracted to them by a number of forces that include: (1) ionic bonding or electrostatic
attraction, (2) hydrogen bonding, (3) van der Waals forces, and (4) covalent bonding. Therefore it is
important to understand the factors which can enhance the attractive forces in both the dye molecule and in
the fibers.
Fibers to be colored can be grouped into those that are hydrophilic and those that are
hydrophobic. Hydrophilic or “water-loving” fibers have chemical structures that are polar in nature.
Hydrophilic fibers include proteins such as wool, animal hair and silk which have many different ionic and
polar groups, and cellulosic fibers which include cotton, linen, jute, and rayon. Cellulosic fibers contain
many polar hydroxyl groups (—OH) which makes these fibers have great affinity toward water.
Hydrophobic or “water-fearing”, fibers on the other hand are quite non-polar in character.
Hydrophobic fibers include modified regenerated cellulosics (acetate and triacetate), polyesters, polyamides
(nylon, nomex®, kevlar®), acrylics (polyacrylonitrile and modacrylics), olefins (polypropylene), and
polyurethanes (spandex®). Historically these fibers have been difficult to dye with brilliant, permanent
color because of their non-polar character.
Source: Dr. Patricia Hill, Millersville University
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Some dyes can be applied directly to fibers
while other dye and fiber combinations require a DYE
MORDANT
FIBER
third chemical called a mordant. Mordants are used
to pre-treat material so as to enhance the binding of the dye to the fiber. Common mordants include various
metallic salts such as alum or potassium aluminum sulfate (KAlSO4) potassium dichromate
(K2CrO7), tin(II) chloride (SnCl2), and iron (II) sulfide (FeS); acids such as acetic (vinegar) and tannic
acids; and bases such as sodium hydroxide (NaOH). Some mordants act as a “linker molecule” between
fiber and dye while others ensure that charged groups are present on the fibers prior to dyeing.
The following table lists various classes of dyes and the type of fiber they are most suited to color.
Type of Dye by Application Method
Acid Dyes
Basic Dyes
Direct Dyes
Azoic Dyes
Vat Dyes or Sulfur Dyes
Reactive Dyes
Disperse Dyes
Solvent Dyes
Fiber Type
Protein fibers and polyamides
Acrylics, wool, and basic dyeable polyester
Cellulosics (cotton, linen, rayon)
Cellulosics, acrylics, nylons
Cellulosics
Cellulosics
Synthetics - polyester, acetates, polyamides
Synthetics
Acid dyes are applied to protein and polyamide fibers from an acidic dyebath. The acid ensures
that the amine groups on the protein acquire positively charged cationic sites that are attracted to the
negatively charged sulfonic acid groups (anionic sites) on the dye molecule. Thus the dye and fiber are
held together through an ionic bond as shown below.
Dye—SO3-Na+ + Wool—NH3+ClDye
Dye—SO3- +H3N—Wool
Fiber
+ NaCl
Colored Fiber
Basic dyes are applied to fibers from a basic or alkaline solution. They act in much the same
manner as acidic dyes, although the fiber takes on a negative charge and the dye a positive charge. Wool is
amphoteric (can take on either a positive or negative charge) and can be dyed using either acidic or basic
dyes.
Direct dyes are those which can be applied with no mordant. These dyes are very polar making
them water soluble and having great affinity for polar cellulose fibers. These dyes are used to color cotton,
linen, rayon, and other cellulose-based materials.
Azoic dyes, sometimes called “developed dyes”are formed by mixing two colorless chemicals in
the presence of the fiber to be dyed. These dyes are intensely colored and tend to be quite wash-fast
because they are formed within the structure of the fiber.
Vat dyes are essentially insoluble in water so they must first be chemically converted to a soluble
form. Most often these dyes are chemically reduced within the dye vat (hence the term vat dye) in the
presence of a strong base (NaOH) to a leuco (colorless or white) form. Then the fiber (usually a cellulosic)
is placed into the vat and the dye is chemically oxidized back to the insoluble colored form. The dye is
physically trapped within the fiber network. These dyes tend to be quite colorfast.
Fiber-reactive dyes are colored organic compounds that have as part of their structure a group
capable of forming a covalent chemical bond between a carbon atom in the dye molecule and an oxygen,
nitrogen or sulfur atom present in the fiber. The types of fibers that will contain oxygen, nitrogen and sulfur
atoms are either cellulose or cellulose-like fibers such as cotton, linen, rayon, paper and wood, and proteinTriazine (3 nitrogens)
based fibers like wool and silk. Cellulosic fibers
Fiber-reactive Dye Structure
contain lots of —OH groups while protein fibers
contain both —NH— and —SH groups.
Cl
Procion MX is one of the commercially
N
available brands of fiber-reactive dyes. These
Dichloro- (2 chlorines)
D
N
dyes belong to a class of compounds called
dichlorotriazine dyes. The structure of the “fiberN
reactive” portion of these dyes is shown in the
Cl
figure to the right.
color-forming
chromophore
Source: Dr. Patricia Hill, Millersville University
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6
The “D” portion of the dye molecule represents the chromophore or color-producing part that makes the
dye a particular color and this part varies from dye to dye. Take a look at the structures for two different
Procion dyes — Procion Brilliant Yellow M-6G and Procion Brilliant Blue M-R — shown below to see
how different the color-forming parts can be while the “fiber-reactive” part remains identical for both dyes.
Cl
OH
Cl
N
NaSO3
N
N
N
NH
N
N
N
Cl
CH 3
NaO3 S
Cl
Procion Brilliant Yellow M-G6
O
SO3Na
Cl
N
O
HN
NH
N
N
Procion Brilliant Blue M-R
SO3Na
Cl
As previously mentioned, disperse dyes require high temperatures and additives in order to get the
dye into solution. These relatively small molecules with very low water solubility, are non-ionic and
possess great affinity for hydrophobic fibers. Therefore they are used to dye synthetics such as polyester,
acetates, polyamides, acrylics and other hydrophobic fibers.
Solvent dyes are only soluble in nonpolar hydrocarbon solvents. These dyes are only suitable for
dyeing nonpolar synthetic materials.
Classification of Dyes According to Chemical Composition
An organic chemist is interested in understanding how the molecular structure of a dye contributes
to its physical and chemical properties. Therefore, they would classify dyes according to their chemical
makeup and predominant chromophore groups. Some of the important chromogen dye classes include:
1. Azo dyes and pigments — mono azo, diazo, etc.— aminoazobenzene dyes
2. Carbonyl dyes and pigments — anthraquinone and indigo derivatives
3. Di- and triphenylmethane dyes
4. Phthalocyanine dyes and pigments
Azo dyes comprise a broad range of low-cost, bright colored dyes with acceptable but not
outstanding performance properties. Many of these dyes have suffered from problems of severe light
sensitivity. Many of the red, orange, yellow, brown and black synthetic food colors are classified as azo
dyes. Some azo dyes have come under scrutiny because of fears of toxicity and possible carcinogenicity.
All azo dyes contain the azo chromophore consisting of two nitrogen atoms bonded together through a
double bond.
—N=N—
Carbonyl dyes and pigments include compounds which contain carbonyl groups such as indigo
and Tyrian purple and compounds which contain anthraquinone rings such as alizarin, cochineal, and
kermes. (See the previous structures of these dyes.)
Source: Dr. Patricia Hill, Millersville University
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Green, blue and violet synthetic food colors belong to the class of triphenylene dyes. These dyes
are characterized by a carbon atom attached to three phenyl rings somewhere in the molecule.
C2 H5
N
C H2
C
SO 3-NH4 +
N
N
FD&C B lu e 2
N
SO 3-
N
N+ C H2
C2 H5
A Triphenylene Dye
N
+2
Cu
N
N
N
SO 3-NH4 +
Thalo Blue
Thalo blue is a pigment that belongs to the phthalocyanine class of dyes. It is an organometallic
compound because it also contains a metal ion. The triphenylene dyes and phthalocyanine dyes have
excellent brightness and do not fade or bleed easily.
Non-Textile Uses of Dyes
Besides being used to color fibers and fabrics, dyes are used in a variety of other ways in some
important technological fields. Dyes form the basis of color photography in everything from traditional
color film processes (Kodachrome, Ectachrome, FugiColor) to Polaroid and Kodak instant color pictures
and slides. Modern ink jet printers rely on dyes to produce full color prints from computerized images. In
the electronics industry dyes are used to prepare lasers, solar cells, and digital watches and instrument
display panels. They are also used in xerographic processes.
Dyes have long been used in the fields of biology and biochemistry. Dyes are used as histological
and bacterial stains so cells can be visualized microscopically. Dyes have played roles as drugs (eg.
Prontosil a sulfanilamide) and anti-tumor agents (anthraquinone dyes), and reagents in analytical methods of
analysis of biologically active compounds. Food dyes are routinely used to color beverages, confectionery,
sausages, dessert powders, pet foods, cereals, and dairy products.
In more recent years, dyes have been adapted to color other types of modern synthetic materials
such as plastics and foams.
Photochemical Degradation of Dyes
Lightfastness is the stability of a dye towards light. Ideally dyes should exhibit low
photochemical activity. The lightfastness of a dye is tested under accelerated conditions that use an intense
light source rich in near UV radiation (xenon lamp). A number rating is obtained by comparing the fading
rate of the dye against those of standard dyeings. Lightfastness is quantified on a scale of 1 to 8 where
standard 1 is very light sensitive and 8 is very lightfast. Each increment on the scale corresponds to a
doubling of the time required to fade the dye to a predetermined amount. Most useful dyes should have a
rating of between 5-6 although some applications should be higher and others may be lower.
Several variables influence the fading of dyes and these can be categorized into two groups: (1)
physical effects such as light source, aggregation, oxygen permeability of the substrate, humidity and
temperature, and (2) chemical effects such as the structure of the dye and the substrate, and the presence of
additives.
For azo dyes, there are three principal types of photochemical reactions that can affect the color:
(1) cis-trans photoisomerization; (2) photoreduction; and (3) photooxidation. Under normal conditions the
azo group exists in the trans form. Light absorption by the dye can lead to the formation of the cis form
with an accompanying deepening of the color. This photochromic effect is reversible when the light is
removed. However, measures must be taken when designing dyes to make sure the cis-trans conversion
happens rapidly so as to not be noticeable.
Photoreduction of azo dyes results in the destruction of the chromogen by converting the nitrogen
double bond to a single bond. Azo groups containing nitro groups or dyes on protein substrates (wool, silk
and polyamides) undergo photoreduction more easily.
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Photo-oxidative degradation of azo dyes occurs on all substrates other than proteins and results in
fading when the azo group is oxidized or other parts of the molecule are oxidized.
Anthraquinone dyes are more lightfast than azo dyes. However, di- and tri-arylmethane dyes have
notoriously poor lightfastness. In fact these dyes are so photochemically reactive they are used as
photochromic dyes, photosensitizers, and laser dyes.
REFERENCES
1. Ancient and Medieval Dyes, William F. Leggett, Chemical Publishing Co., Inc. NY, 1944
2. Developments in the Chemistry and Technology of Organic Dyes, J. Griffiths, Editor, Critical Reports
on Applied Chemistry Vol. 7,, Blackwell Scientific Publishers (1984)
3.
4.
5.
6.
7.
8.
Indigo Textiles: Technique and History, Gosta Sandberg), A&C Black, London, Lark Books (1989
The Chemistry of Food Dyes, Dianne N. Epp, Terrific Science Press, Middletown, OH (1995)
The Chemistry of Natural Dyes , Dianne N. Epp, Terrific Science Press, Middletown, OH (1995)
The Chemistry of Vat Dyes, Dianne N. Epp, Terrific Science Press, Middletown, OH (1995)
The Dyer’s Companion, Elijah Bemiss, Dover Publications (19730
The Dyeing of Textile Fibers: Theory and Process, Joseph Rivlin, Philadelphia College of Textiles and
Science (1992)
9.
The Manufacture of Organic Colorants and Intermediates, Gerald Booth, Society of Dyers and
Colourists (1988)
Study Questions for Chapter 18
1. Prepare a table listing the name, source, color and historical usage of the dyes discussed in the reading.
2. How can dyes be classified?
3. Why would natural dyes work well for coloring natural polymers but not be very good at coloring
synthetic polymers?
4. Why do you think many dyes are sensitive to light and tend to fade easily?
Important Chapter Terms






William Perkin
chromagen
alizarin
woad
carmine
diazo dye








covalent bond
colorfast

Source: Dr. Patricia Hill, Millersville University






mauveine
vegetable dyes
kermes
leuco indigo
mordant
fiber-reactive
dye
hydrogen bond

122
chromophore
animal dyes
cochineal
vat dye
hydrophobic
electrostatic
attraction
carbonyl dye







auxochrome
madder
indigo
Tyrian purple
hydrophilic
van der Waals
forces
lightfast
9
COLORANTS
Substances used to impart color are called colorants. Colorants can be categorized as pigments or
dyestuffs. The major difference between pigments and dyestuffs is their respective solubility. Dyestuffs
or dyes are soluble in water or organic solvents and are mainly used in solutions. Pigments are generally
insoluble substances that do not dissolve in either water or organic solvents. Therefore, they require a
binder or vehicle to hold them onto a substrate.
Colorants can further be divided into categories of organic and inorganic compounds. Organic
compounds are composed chiefly of carbon, hydrogen, oxygen, nitrogen and sulfur atoms and have rather
complicated structures. Inorganic compounds often contain a wider variety of atom types but they usually
have a much simpler structure or chemical formula. With a few notable exceptions pigments are inorganic
compounds and dyes are organic compounds. One other category of colorants is called a lake. Lakes are
soluble dyes which have been attached to an inert, semi-opaque, usually white support material such as
barium sulfate (BaSO4) or alumina (Al2O3) to render the dye insoluble.
Inorganic Pigments and Fillers
Inorganic pigments may be colored, colorless, black, white, or metallic in appearance. They can
belong to any one of the following chemical classes:
 Elements (C)
 compounds
oxides (TiO2, Fe2O3, ZnO)
sulfides (HgS, As S)
chromates (ZnCrO4)
silicates
phosphates
carbonates (CaCO3, PbCO3)
Historically, most inorganic pigments were derived from naturally occurring minerals, which were
collected and finely ground. Although several inorganic pigments have been used or manufactured for
centuries, most pigments were not commercially manufactured until the 19th century. Today inorganic
pigments are made predominantly by synthetic methods. Pigments should possess the following ideal
physical properties. They should
1. be lightfast — not fade or change color when exposed to light
2. be chemically inert — not undergo chemical reactions with other pigments, binders, grounds,
or with chemicals and pollutants in the atmosphere
3. resist bleeding — not migrate through dried binder into adjacent paint layers
4. possess good opacity or hiding power
In addition to these properties, pigments are often selected based upon other factors such as hue or tint,
brightness, texture, dispersability, oil absorption, weatherability, and flow characteristics. These physical
properties depend on a number of factors including chemical composition, pigment particle size and shape,
particle size distribution, and the surface texture of the particles. In particular, bleeding of pigments is
dependent upon the solubility of a pigment in a nearby paint or binder layer and the pigment’s reactivity
with other nearby pigments. If the pigment is slightly soluble or if it is chemically reactive toward another
compound, then the pigment will migrate toward that layer or pigment and bleed.
The opacity or hiding power of a pigment depends upon its ability to scatter light. The more a
pigment is able to scatter light, the more opaque the pigment will appear. This light scattering ability
depends chiefly upon two factors: (1) the size and shape of pigment particles, and (2) the refractive index of
the pigment relative to the refractive index of the binding medium. The small size of pigment particles is
achieved by hand grinding using a muller or by pulverizing in ball mills or between rollers. As the size of
pigment particles is reduced, their ability to scatter light increases and they become more opaque. The
most effective light scattering occurs if the particle size is between 200-400 nm (approximately 1/2 the
wavelengths of visible light). To obtain maximum coloring power (brightening power for white pigments),
the particle size must be maintained when the particles are suspended in the binder. Agglomerations or
coagulation of pigment particles will reduce both coloring and hiding power. The maximum color intensity
is obtained when each pigment particle is completely surrounded by binder. Along with the increase in
pigment hiding power as particle size decreases, however, comes a shift in hue of the pigment. For
example red pigments tend to become more yellowish as they are ground to finer particle size and dark
Source: Dr. Patricia Hill, Millersville University
85
10
green pigments (ie. malachite) become pale green in color when finely ground. It is therefore important to
control pigment particle size when grinding to avoid “desaturation” of the pigment color.
Another important factor influencing opacity of pigments is the differences in refractive index
between pigment and binder. Refractive index (RI) is a measure of the degree to which a substance bends
light as the light passes into or out of the substance. If the RI of a pigment and the RI of a binder are very
similar, then the paint will appear to be more transparent. However, if the RI of pigment and binder are
very different the paint will be very opaque. For example, mixing chalk (CaCO 3, RI = 1.6) with water
(RI =1.3) will yield a more opaque paint than mixing chalk with linseed oil (RI = 1.5). To increase the
opacity of paints, white pigments with high refractive indices such as TiO 2 and ZnO (RI>2.0) are often
added as opacifying agents and fillers. The following table lists the RIs of some common binders,
pigments and fillers.
Refractive Indices of Binders, Fillers and White Pigments
1. Air
2. Binders
Water
Urea-formaldehyde resin
Linseed Oil
3.White Pigments
Zinc sulfide (ZnS)
Zinc oxide (ZnO)
Titanium dioxide (TiO2) anatase
Titanium dioxide (rutile)
0
4. Fillers
Silica
Kieselguhr (diatomaceous earth)
Aluminum silicate
Kaolin (China clay)
Calcium silicate
Sodium aluminum silicate
Chalk
Alumina (Al2O3)
Barium sulfate (BaSO4)
1.3
1.6
1.5
2.4
2.0
2.55
2.75
1.45
1.45
1.6
1.6
1.5
1.5
1.6
1.6
1.65
Manufacture and Use
Today most inorganic pigments are prepared by precipitation reactions where the pigment is
deposited as a crystalline solid when two solutions of chemicals are mixed together. The conditions of the
reactions (temperature, pH, speed of mixing) must be controlled carefully so that the desired particle size is
obtained. After precipitation the pigments are separated, washed and dried. In some cases the crude
pigment must be further processed to insure optimal particle size and crystal structure before the pigment
can be finely ground.
White pigments are the most important group of pigments both in quantity produced and in
economic value. Titanium dioxide (TiO2) production accounts for about half of the world’s production of
inorganic pigments. After that, the next most important compound is zinc oxide (ZnO) contributing to
about 20% of world production. Iron oxide pigments follow these two white pigments (10% of world
production). Other pigments produced include
Lead pigments
PbO
Pb3O4
2 PbCO3 . Pb(OH)2
Lithopones
Chromate pigments
PbCrO4
ZnCrO4
BaCrO4
Chromium oxide
Molybdate orange
Cadmium pigments
CdS
CdSe
Prussian Blue
Ultramarine Blue
Source: Dr. Patricia Hill, Millersville University
Lead Oxide
Red Lead
White Lead
co-precipitated BaSO4 and ZnS
Lead Chromate
Zinc Chromate
Barium Chromate
CrO3
Cadmium yellow
Cadmium red
K3Fe4[Fe(CN)6]3
(Na, Ca)8(AlSiO4)6(SO4, S, Cl)2
86
11
Pigments and fillers are often manufactured together and the same substance can often serve as either a
filler or a pigment. When comparing pigments and fillers

pigments are more finely divided than fillers
 pigments are usually more expensive than fillers
 white pigments have a higher refractive index than white fillers
Numerous commercial products rely upon the addition of fillers. The most important fillers, with the
exception of carbon black, sawdust and shredded paper or textiles, are white inorganic powders. These
materials are insoluble in binders and solvents. They are used as cheap additives and also are used to
improve the useful properties of the product to which they are added.
Fillers are used in paper to decrease its transparency and increase its whiteness and produce
smooth surfaces that retain ink without bleed-through. They increase the volume of paper and prevent it
from becoming powdery on printing, they even regulate the combustibility of cigarette paper.
In paints, inks, and artists’ colors, fillers thicken the product, improve the covering power and help
prevent sedimentation and coagulation of pigment particles. If added in larger quantities, they will produce
a matte finish in the dried coating. Fillers are also used as thickening agent in such consumer products as
toothpaste, creams, lotions, ointments, adhesives, polishes, and shoe creams. Fillers are added in small
quantities to items like salt, instant coffee and soup powders to prevent caking. Fillers can be classified as
those occurring naturally and those synthetically produced (extenders).
Natural
heavy spar or barytes
Fillers
barium sulfate (BaSO4)
chalk
calcium carbonate
(CaCO3)
CaCO3
diatomaceous earth
kaolin and clay
calcium aluminum
silicates
china clay
powdered asbestos,
quartz, slate rock
pure kaolin
silica containing
Synthetic
carbon black
precipitated calcium
carbonateCaCO3
hydrated alumina
Al2O3.H2O
satin white calcium
sulfate and alumina
(CaSO4 and Al2O3)
Fillers
calcium silicate
CaSiO3
blanc fixe barium
sulfate (BaSO4)
silicic acids
SiO2. H2O
aluminum silicate
Al2O3.SiO2 + water
All fillers are finely divided with large surface areas. The particle sizes are 10-100nm with
surface areas of 10-400 m2 per gram of material. This large surface area generates attractive forces
between the particles of filler and binder that provide for the reinforcing effect of active fillers.
For
example, silicic acids are used to produce “non-drip” paints. These paints are thixotropic— they flow
under pressure of brush or roller but when the pressure is released they become solid again.
Color in Inorganic Pigments
If one examines the colors of chlorides (MCln) of period four elements you find that at the
beginning and end of the period the compounds formed are colorless or white. However, chlorides of the
transition elements are almost all colored.
Color of Period 4 Chlorides
IA
IIA
IIIA
IVA VA
VIA VIIA
Transition Metals
K Ca
Sc Ti
V
Cr
Mn Fe
Co
Ni
Cu
Zn* Ga
Ge
As
Se
Br
1s 2s
1d 2d
3d 4d
5d
6d
7d
8d
9d
10d 1p
2p
3p
4p
5p
Colorless
Various colored compounds
Colorless or white *
Most of these transition elements can produce cations of more than one charged state (oxidation state) and
each different oxidation state produces a different colored compound. For example titanium can form three
different chlorides — TiCl2 which is brown, TiCl3 which is dark violet, and TiCl4 which is light yellow.
Iron can exist in the +3 (ferric) and +2 (ferrous) state. Ferrous iron compounds are a yellow or green
color while ferric iron compounds are brown to yellow orange. The trend of colored transition metal
compounds would repeat itself if we examined transition metal compounds in periods 5 and 6. Also if we
Source: Dr. Patricia Hill, Millersville University
12
examine compounds of selected elements in Groups IV, V, VI, and VII, we find compounds with strikingly
brilliant colors (cadmium, lead, and mercury sulfides and oxides for example). Why do these inorganic
compounds exhibit color and why are others colorless? Three major factors govern the colors observed in
inorganic compounds: (1) the degree of covalency of the bonds in these inorganic compounds, (2) the
electronic configuration of the metal ion (cation) within these compounds, and (3) the geometry of the
negative ion (anion) around the cation in the compound. These three factors lead to the absorption of light
energy by inorganic compounds through several different mechanisms.
Chemical bonds exist on a continuum from ionic (complete transfer of electrons from metal to
non-metal element with the formation of ions) to non-polar covalent (equal sharing of electrons between
two atoms). Most chemical bonds fall somewhere in the middle of this continuum and possess some
C+
A

-
ionic
C


CA
no npo lar covalent
po lar covalent
The Bonding Continuum
degree of both ionic and covalent character although compounds are often simply categorized as ionic or
covalent. However, increasing covalent character of the bonds in metallic salts influences the color one
observes.
To explain the phenomenon of colored transition metal compounds we must examine what all of
these transition elements have in common. When transition metals form ionic compounds they exist as
cations with partially filled d-subshells (orbitals). It is these d-electrons which can undergo light-induced
electronic transitions between energetically different d-orbitals. When light strikes these compounds some
of the light energy goes into making electrons move to empty d-orbitals. These d-d transitions cause
selective absorption of wavelengths of visible light and the compounds exhibit color. Many artists’
pigments as well as gems and minerals are colored as a result of these d-d transitions. Examples include
chromium oxide green (Cr2O3), iron oxides (Fe2O3), ruby, and various copper minerals such as azurite,
malachite, and turquoise.
Another mechanism of color production in inorganic compounds is the charge transfer
phenomenon. Charge transfer pigments exhibit brilliant colors ranging from blues (Prussian blue) to
yellows (cadmium yellow, orpiment) to reds (vermilion, cadmium red). In these compounds light energy
induces the movement of electrons between the metal cation and the anion (ligand) or between two metal
ions with different oxidation states within the compound. Examples of charge transfer pigments are as
follows.
light
Cadmium Yellow
Cd2+S2-
Vermilion
Hg2+S2-
Orpiment (As2S3)
As3+S2-
Prussian Blue
K3Fe4[Fe(CN)6]3
Fe2+Fe3+
light
light
light
Cd1+S1Hg1+S1As2+S1Fe3+Fe2+
Several other mechanisms of color production in inorganic compounds exist. For example the
bright yellow pigment lead chromate (PbCrO4) derives its brilliant color from a combination of d-d and
charge transfer electronic transitions. In some compounds, the shape of the crystals can influence the color
as can the presence of impurities trapped within the crystalline structure of some minerals. For example the
pigment lead oxide (PbO) has both a yellow and a red crystalline form. While the pigment ultramarine,
obtained from the mineral lapis lazuli, exhibits a blue color due to the presence of “color centers” of
trapped S3- in the crystalline matrix.
Organic Colorants
Source: Dr. Patricia Hill, Millersville University
13
In order for an organic compound to exhibit color, the molecules must contain certain structural
features that allow them to selectively absorb visible light. All dyes are aromatic compounds with a
conjugated double bond system. In addition to this basic framework they must also contain chromogens
(color-generating centers), which are composed of two parts — chromophores and auxochromes. The
chromophore (from the Greek word khroma-pherein or color bearer) is the structural portion of the
molecule responsible for color generation. It must be present for an organic molecule to possess color.
Several important chromophore groups are shown below.
Important Chromophoric Groups
—C=C—
alkenes
—C=O
carbonyl groups
—N=O
—N=N—
azo groups
phenyl group
O
—C=S
thial or thione
O
Quinoid group
—NO
nitroso groups
2
nitro groups
In many cases the presence of just one of these groups in insufficient to produce a colored molecule. For
example, a compound containing 9 conjugated carbon-carbon double bonds (shown below) is colorless.
But if the number of conjugated carbon-carbon double bonds is extended to 11, the molecule absorbs blueviolet light and the compound appears orange in color.
colorless
-carotene (orange)
If an azo group is present in the molecule, the number of conjugated carbon-carbon double bonds needed
to produce color is reduced even further to only 6-7.
Cl
C2 H5
O2 N
N=N
N
C2 H4 OH
Disperse Red 13 — an azo dye
Auxochromes or “color augmenting” groups are organic functional groups which contain nonbonding
electron pairs. These groups act to strengthen or deepen the color produced by the chromophore. Some
important auxochrome groups are
Amino Groups
Hydroxyl Groups
Ether Groups
—NH2
—OH
—O—R
Both of these structural features, the chromophores and auxochromes, provide the organic molecule with
bonding and nonbonding electrons that are able to readily absorb energies in the region of visible light.
Source: Dr. Patricia Hill, Millersville University
14
Increased conjugation brings ground state and higher electron energy states closer together so the electrons
can become excited more easily.
This is what leads to selective absorption
of light and the production of
color in these molecules.
O2 N
N=N
X
an azo dye
X = OH
yellow color
X = NH2
yellow-red color
X = NR2
deep red color
Most organic colorants are classified as dyes and are soluble in either water or organic solvents.
Historically dyes were obtained from natural plant and animal sources. It was not until 1856 that synthetic
dyes were discovered by William Perkin, an 18-year old English college student. Perkin went on to
establish a thriving dye industry. As the organic dye industry developed during the later part of the 19th
century, chemists were able to provide a wide variety of new, brilliant colorants. Today many of these
organic compounds have replaced traditional mineral-based pigments in paint formulations. However, in
order to convert these relatively soluble dyes into insoluble pigments the organic dye molecules must be
attached to insoluble, inert supports. The most commonly used support is hydrated alumina (Al 2O3 . H2O)
and the resulting pigment is called a lake. There are a few organic dyes, such as natural and synthetic
indigo, which are practically insoluble solids. These dyes can readily be used as pigments and in order to
use them as dyes they must be chemical reduced to a soluble form. One of the difficulties encountered with
synthetic dyes, however, has been their lack of colorfastness. Because they contain conjugated aromatic
systems they tend to be quite light sensitive and susceptible to degradation upon prolonged exposure to
light as compared with traditional mineral based pigments. This has necessitated the addition of chemicals
to artists’ paints that absorb ultraviolet light in order to prevent fading.
The following table summarizes the major differences between inorganic and organic colorants.
Property
Solubility
Tinctorial Strength (coloring power)
Hiding Power
Specific Gravity
Heat Resistance
Intensity or Brightness
Inorganic Pigments
Insoluble - non-bleeding
Low
High, opaque
High
High
Low
Organic Dyes
Slightly or completely soluble
High
Low, transparent
Low
Low
High
Characteristics of Some Common Inorganic Pigments
Black Pigments
Ivory Black and Bone Black
These black pigments are produced by burning ivory or bones. Ivory produces a fluffy, blue-black
substance while bones often produce a black with a brownish tint. This is because the pure carbon content
of these black pigments is only around 10%. The other ingredients are Ca 3(PO4)2 (84%) and CaCO3 (6%).
These black pigments produce smooth, slow drying paints and they fairly good hiding power. They are
both quite light-fast and non-reactive with other pigments.
Lampblack or Carbon Black
Lampblack is produced by burning petroleum products such as mineral oil, tar, pitch or resin in an
oxygen-deficient atmosphere. This produces an intense black, fluffy, lightweight pigment that is almost
pure carbon. This pigment tends to be quite sooty and almost greasy and therefore does not wet easily in
either oil or water. The pigment must be thoroughly ground in binder before use. Lampblack dries very
slowly to a soft film and produces a very permanent, light-fast color.
Mars Black or Iron Oxide Black
Mars black is not a carbon-based pigment. It is a ferro-ferric oxide (magnetite) with a formula of
Fe3O4 or more accurately FeO . Fe2O3, a combination of the two different iron oxides. This pigment is
Source: Dr. Patricia Hill, Millersville University
15
synthetically produced beginning with metallic iron. It is a light-fast pigment that is compatible with all
other pigments but it also dries very slowly when used in an oil binder.
White Pigments
Calcium Carbonate
Also known as Whiting, Natural Chalk, Lime White, and Shell White, calcium carbonate (CaCO 3)
has been used since earliest times in art both as part of grounds and support structures and as pigment and
extenders in paints. It is the major component of “whitewash”. It is the basic structure of fresco wall
paintings as well as the basic material of marble and limestone statues.
The mineral form, calcite, is found in marble, limestone, chalk, lime, mollusk shells, and coral.
Most is mined, washed, finely ground and sold in several grades of chalk. It is also prepared as
“precipitated chalk” as a by-product of many chemical industries.
Because the RI of this pigment is close to that of linseed oil, it has poor hiding power as an oil
paint. It is relatively stable but all forms will decompose upon heating or exposure to strong mineral acids
such as HCl. In both cases CO2 gas is rapidly evolved. All forms of calcium carbonate are also alkaline
with a pH ranging from 7.5-9.8. The pigment is lightfast and it is not darkened by sulfides. It is compatible
with most other pigments except alkali-sensitive colors such as Prussian blue. It reacts slowly with H2SO4
in the atmosphere to form CaSO4. Although CaSO4 is also white, it has a greater volume than CaCO3 and
this can cause extensive damage to surfaces of frescos and limestone and marble buildings and statues.
Lead White
This pigment is one of the oldest synthetically produced pigments and has been used since early
historical times by the Greeks and Romans. It was used in China as early as 300 BC. It is probably the
most important white pigment in the history of Western painting because it was the chief white pigment
from ancient times to about 1834.
Chemically this pigment, also known as Flake White and Cremnitz White, is basic lead carbonate
[2PbCO3.Pb(OH)2] and its naturally occurring mineral form is called cerussite (PbCO 3). Lead white is a
fairly opaque pigment with an RI=1.94-2.09. It is soluble and reactive to acids. Although it is very
permanent and unaffected by light it will turn black in the presence of sulfides in air and possibly in
neighboring pigments according to the following equation
PbCO3 + H2S
PbS + H2O + CO2
black
If the pigment is sealed in a layer of binding medium this reaction is much less likely to occur. As a paint
pigment, Flake White possesses the finest mechanical and physical properties of all pigments. It has low
oil absorption, it brushes well, and it dries rapidly to tough, flexible films in various drying oil binders. One
of the major disadvantages of lead white is its toxicity. It is highly toxic when swallowed or inhaled —
leading to lead poisoning or “painters’ colic”. Due to its toxicity, lead pigments were banned for use in
house paints in the mid 1970’s in the US but they still are available today in artist’s paints.
Zinc White
In 1834, the compound zinc oxide, ZnO, was introduced as a watercolor pigment and by 1845 this
pigment enjoyed large-scale commercial production. This pigment, commonly known as Chinese White,
provided artists with a “cold white” color with hiding power comparable to lead white without the toxicity.
Despite its advantages it also had disadvantages such as high oil absorption and slow drying in linseed oil,
poorer brushability, and formation of dry brittle films which easily crack upon drying. ZnO also will react
with H2S, however, the resulting ZnS compound is also white so the pigment does not darken like lead
white.
Barium White
Another white pigment used in place of lead white was barium white, also known as baryta, blanc
fixe, and permanent white. Barite, BaSO4, can be mined or produced synthetically. It was first used as a
watercolor pigment in 1782 after which it was routinely used in paints until the 19th century.
Unfortunately it is not very opaque when used in oil binders. Because it is quite inert, has a moderate RI
(1.64), has low toxicity, and is easy to wet and disperse, it has found greater use as an extender or filler for
other pigments in paints, as a base for lakes, and as the material used to prepare grounds for painting. After
1874 it was manufactured in combination with ZnS and a mixture of 30% ZnS and 70% BaSO4 became
Source: Dr. Patricia Hill, Millersville University
16
known as Lithopone. It is estimated that 60% of the grounds and 20% of the pigments in 19th century
paintings contain BaSO4.
Titanium White
The newest white pigment to be introduced for artists’ use was titanium dioxide, TiO 2, It was first
manufactured in 1916 but it is not found used in paintings before 1920. In fact its presence in paintings has
been used to detect modern forgeries of old art. Of all the white pigments it has the greatest hiding power
with refractive indices (RI) between 2.5-2.75. It is an extremely stable pigment unaffected by heat, dilute
acids and bases, air, light, and drying oil binders. And it is also nontoxic. It is the whitest of the white
pigments and is used in many applications from cosmetics to paper coatings.
Red, Orange and Yellow Pigments
Iron Oxides
The iron oxides are a group of pigments often referred to as earth colors. Their origin as
pigments traces back to prehistoric cave paintings. They come in a wide variety of hues from yellows to
browns to reds and even greens. Most are hydrated iron (III) oxides with the chemical formula
Fe2O3.xH2O. The colors vary depending upon how many water molecules are associated with the iron
oxide.
Red Lead
This pigment, known also as minium, red lead oxide or lead tetroxide (Pb3O4), is one of the oldest
synthetic pigments. It was accidentally discovered in antiquity when a shipment of white lead turned red
during a fire in Roman times. The mineral, minium, occurs naturally with galena (PbS). It is produced by
the oxidation of metallic lead, lead white or litharge (PbO) by heating in air. Usually more orange than red
in color, red lead is a heavy, dense pigment with excellent hiding power. It speeds the drying of most
binders. It reacts with atmospheric sulfur compounds to form lead sulfide and turns dark. It is poisonous
because of its lead content. It is soluble in dilute acids. It was widely used in illuminated manuscripts in
both Western and Asian art. It is used in anti-rust paint (ie “red rust primer”).
Vermilion or Cinnabar
The brilliant red pigment cinnabar was known in the 6th century BC. in Greece and Asia Minor, It
was also well known to the Romans and Chinese. The chemical composition of this pigment is mercuric
sulfide or mercury (I) sulfide (HgS). Its naturally occurring mineral form is called cinnabar, a dense red
mineral widely distributed throughout the world.
Cinnabar has a very high RI (2.8-3.2) giving it excellent hiding power. And although the pigment
varies in color from strong red to vivid red-orange, it is not considered to be very lightfast because it can
darken upon exposure to light. Chemically it is quite nonreactive to acids, bases, and other pigments,
however, this pigment is extremely toxic, causing neurological problems upon chronic exposure.
Nevertheless, vermilion can be found in numerous paintings dating from the 1st century AD up to the 19th
century.
The Cadmiums
Three colored pigments — cadmium yellow, cadmium orange, and cadmium red — were
discovered in 1817. Two inorganic chemicals, cadmium sulfide (CdS) which is yellow, and cadmium
selenide (CdSe) which is red, can be mixed together alone or with white pigments to produce pigments
which vary in hue from light yellow to light red. Although first discovered in 1817 and used as oil and
watercolor pigments in the 1820-1830s, these pigments remained quite costly and difficult to obtain until
about 1917. These pigment have high RIs and excellent hiding power. The color tint varies depending not
only on the percentage of sulfide versus selenide but also on the pigment particle size. Although the
pigments exhibit good resistance to alkalis, they exhibit incompatibility with lead- and copper-based
pigments due to the darkening of lead and copper when combined with sulfur. The type of binder will
influence this reaction with it being most rapid in watercolors and slowest in oils. CdS is not very water
soluble. However, it is a toxic compound when ingested or inhaled. Cadmium pigments have been used
since the early to mid 1800s and continue to be important artist’s pigments today.
Blue Pigments
Source: Dr. Patricia Hill, Millersville University
17
Azurite
Azurite is the mineral basic copper carbonate, 2CuCO 3 . Cu(OH)2, which occurs with malachite or
green basic copper carbonate. A synthetically produced compound of the same composition is called blue
verditer or blue bice. It has been used as pigment since the ancient Egyptians. During the Middle Ages,
Renaissance, and into the 15th and 16th centuries it was the most important blue pigment of European
painting. It was also the most important blue pigment in the Far East. It was even used by pre-Columbian
Indians of the American Southwest and in Spanish Mission Church paintings. The invention of Prussian
blue in 1704 brought an end to the use of this pigment in paintings.
To prepare the pigment, azurite is ground, washed, and sieved. Coarse particle size produces a
dark blue color while finely ground pigment produces a lighter blue color. The crystals are of moderate RI
(1.7-1.8). Azurite is stable to light and the atmosphere but will decompose giving off water and CO 2 and
forming black CuO if heated above 300oC. It is also decomposed by dilute mineral acids and hot alkalis.
If the pigment is encased in oil or egg tempera binder it is relatively permanent. But it is darkened by
exposure to sulfur fumes. And in some instances it appears to be converted to the green form, malachite.
Ultramarine
Ultramarine is the pigment obtained from the mineral lapis lazuli, a complex sulfur-containing
sodium aluminum silicate with the molecular formula (Na, Ca)8(AlSiO4)6(SO4, S, Cl)2. Lapis lazuli is a
relatively rare mineral found in Afghanistan, Siberia, Chile, Canada, Argentina and the U.S. Although
lapis was used as a semi-precious stone in ancient times, it was not used as a pigment until the 6-7th
century AD. It was used in Persian miniature paintings, Chinese paintings, and early wall paintings in
Afghanistan. It was used most extensively in European paintings during the 14-15th centuries, particularly
in illuminated manuscripts and Italian panel paintings. The use of natural ultramarine in paintings declined
after the synthetic pigment was introduced in 1828. The synthetic form was used by many of the
Impressionist painters.
To prepare ultramarine from the pure lapis lazuli, simple washing and grinding was done. In
1200-1400 AD an extraction method was developed to separate the deep blue mineral from the colorless
matrix in which it was often found. Because of the limited abundance of this mineral and the intensive
extraction procedures needed to produce high quality pigment, ultramarine was as expensive as gold and
often reserved for painting only the robes of Christ or the Virgin Mary. In 1828 artificial ultramarine was
synthesized. It is similar in structure to the natural mineral with a chemical formula Na6-10Al6Si6O24S2-4 It
is often called French ultramarine or French blue.
Chemically, lapis lazuli or lazurite, is the most complex of all mineral pigments. The refractive
index is fairly low (1.50) although it possesses good hiding power. It is very lightfast. But it is readily
decomposed by dilute acids with rapid destruction of the blue color. It is unaffected by ammonia or alkalis.
It can suffer from “ultramarine sickness” — a grayish or yellowish gray discoloration on the paint surface
possibly due to action of acidic atmospheric pollutants (H 2SO4).
Prussian Blue
Prussian blue, iron blue, Milori blue, or Paris blue was first synthesized around 1704 in Berlin and
is considered the first modern pigment. Earliest examples can be found in Dutch easel paintings of the early
1700s. It has been a very popular pigment used in watercolors and oils but since the 1970s it has been
replaced by Thalo blue. It has been used as an early pigment in house paints and wallpapers and was used
as a dye for silk. It was also used for cyanotypes, an early type of photographic process that was the
precursor of blueprinting. It has also been used to make inks for writing and printing.
Chemically Prussian blue is a hydrated iron hexacyanoferrate complex called ferric ferrocyanide.
It has two different formulas— KFe[Fe(CN)6] and Fe4[Fe(CN)6]3 and modern formulations give it the
molecular formula M+1Fe+3Fe+2(CN)6.nH2O (where M+1 is K+, Na+, NH4+). Prussian blue is a low-density
pigment composed of very fine particles (0.01-0.2 um) that act almost like a colloidal suspension. For this
reason it is difficult to purify Prussian blue and separate it from the starting materials in the reaction. It can
be mixed with alumina (Al2O3) to help settle the pigment. Prussian blue is a transparent pigment that is
isotropic and cannot be seen under crossed polars. Prussian blue is very sensitive to alkali, turning brown
upon exposure to bases. It is totally nonreactive toward acids. Prussian blue is difficult to mix with water,
but will mix better if alcohol is added. The pigment itself is considered to be nontoxic however, if burned
it will evolve cyanogen which is highly toxic.
Source: Dr. Patricia Hill, Millersville University
18
Green Pigments
Copper-Containing Compounds
Malachite or green basic copper carbonate, CuCO3.Cu(OH)2, was used as a pigment in ancient
Egypt. As a mineral, malachite is found in conjunction with azurite. It was prepared in the same manner as
azurite, that is washed and ground. It was one of the most popular green pigments used in medieval
paintings and illuminated manuscripts. Its chemical and physical properties are quite similar to those of
azurite.
Another green pigment used in medieval painting was “green earth” or terre verte. This is a
copper silicate mineral, chrysoccolla, with a dull, pale, grayish green color. It was ground and washed
before use.
Verdigris is a bright green colored copper acetate salt (CuCH 3COO-) which has been produced
since ancient Roman times by treating copper metal with acetic acid (vinegar) fumes. Verdigris is
incompatible with lead white causing the white pigment to decompose. Therefore substitutes for verdigris
were sought from an early time.
Scheele’s green is a copper arsenite compound. It was the first synthetic green copper arsenic
pigment produced in 1778. Its popularity was brief due to its instability (darkens over time) and its high
toxicity. However, Scheele’s green was a precursor to a more important 19th century copper-containing
pigment, emerald green.
Emerald green, also known as Paris green, is a copper acetoarsenite with the formula
3Cu(AsO2)2.Cu(CH3COO-)2. It is a highly toxic pigment with a brilliant blue-green to green hue. It was
first produced around 1808 in Germany by Wilhelm Sattler. It soon was produced on a large scale and
became a popular pigment among French Impressionist and post Impressionist painter.
Viridian
Anhydrous chromic oxide green, Cr2O3, was first synthesized in the early 1800s. It is a dull,
opaque green color — the color of “army green”. However, in 1838 a Parisian color maker, Pannetier,
prepared and marketed a hydrated form of chromic oxide which was perfected and commercially produced
in 1850 by the French chemist Guignet. This pigment, Cr 2O3.2H2O, is a very permanent, transparent, bright
green pigment called viridian or vert. This pigment was relatively expensive and moderately toxic.
However, it was used throughout the 19th century especially by French Impressionists.
REFERENCES
1.
2.
3.
4.
5.
6.
The Handbook of Applied Chemistry, V. Hopp and I. Hennig, Hemisphere Publishing Corporation,
Washington, 1983.
Surface Coatings Science and Technology, 2nd Edition, Swaraj Paul, Ed. John Wiley, 1997.
The Artist’s Handbook of Materials and Techniques, 5th Edition, Ralph Mayer, Viking Press,
1991.
The Physics and Chemistry of Color, Kurt Nassau, Wiley, 1983.
Light and Its Interaction With Art and Antiquities, Thomas Brill, 1980.
Artists’ Pigments: A Handbook of Their History and Characteristics, Volume 1, Robert L. Feller,
National Gallery, Cambridge U. Press (1986)
7.
Artists’ Pigments: A Handbook of Their History and Characteristics Volume 2, Ashok Roy, Ed.,
National Gallery, Oxford Press (1993)
8.
Artists’ Pigments: A Handbook of Their History and Characteristics Volume 3, Elizabeth West
Fitzhugh, Ed., National Gallery, Oxford Press (1997)
Study Questions for Chapter 14
1. Give examples of commonly used artists’ pigments (name and chemical formula) from the following
color groups: black, white, red, yellow, blue, green.
2. What is the difference between a pigment and a filler? Give an example of each.
3. What are the distinguishing features of the three types of colorants: pigments, dyes and lakes?
4. What factors can be manipulated to make a paint more opaque (have greater hiding power)?
5. How are inorganic pigments prepared or obtained?
6. Why do most transition metal compounds exhibit color>
Source: Dr. Patricia Hill, Millersville University
19
7.
8.
Which pigments are hazardous because of their chemical toxicity?
What do you look for in the structure of an organic molecule to know if the molecule exhibits color?
Important Chapter Terms
 lightfast
 particle size
 d-d transitions
 auxochrome
 refractive index
 colorfast





Source: Dr. Patricia Hill, Millersville University
bleeding
precipitation reactions
charge transfer
azo group
inorganic pigments





opacity or hiding power
fillers
conjugated aromatic
lake
chromophore
20
PAINTING MEDIA
The art of painting and technology of paint making have been around since humans first coated the
walls of caves with pictures of animals and hunt scenes some twenty thousand years ago. During the time
of the early Egyptians and Babylonians, painting
began to serve more than just expressive, aesthetic
and decorative purposes. These civilizations used
surface
coatings
(paints)
for
protection,
preservation, and waterproofing of a variety of
everyday objects. Techniques and materials of
painting were influenced and developed by
numerous cultures including the Greeks, Romans,
and Chinese. Many of these classical recipes and
techniques of paints and painting were improved
upon by artists during Medieval and Renaissance
times. With the advent of the Industrial Revolution
in the mid 1700’s, advances in science and
technology profoundly influenced the materials and
techniques of painting, providing artists with a whole new palette of brilliant colors and relatively easy
access to these materials from a developing paint industry. Today we hardly give thought to the ready
availability of artists’ paints such as oils, acrylics, watercolors, and poster paints, or to the ease of obtaining
just the right color wall paint for our home. Paint, painting and the availability of painting media are so
common that we take them for granted and rarely if ever give much thought to what these materials are and
how they are made.
What Are Paints?
Paints are used to protect and decorate wood, paper, metal, textiles, and a variety of other
substrates. Paints typically contain four major components: (1) a colorant; (2) a binder or vehicle; (3) a
solvent or diluent; and (4) additives or fillers. Often the colorant is of greatest interest to the artist,
however, the other components are just as important to the working properties and final appearance of the
paint.
The colorants used in painting are called pigments. Ideally these substances should be smooth,
finely divided powders with brilliant clear color. They should be free from impurities, insoluble, and
absolutely permanent or lightfast. Of course not all pigments used can totally meet these stringent
requirements.
Chemically pigments are classified as insoluble inorganic chemical compounds
manufactured from either natural or synthetic sources. In some cases soluble, colored organic compounds
called dyes, are converted to insoluble pigments by attaching them to inert, white supports such as whiting
(calcium carbonate, CaCO3) or alumina (aluminum oxide, Al2O3). The pigments used in the various types
of paints (oils, acrylics, alkyds, watercolors, egg tempera) are the same although some pigments are better
suited for one type of paint than another. The difference in the paints lies mainly with the binder.
The binder or vehicle in a paint serves four major functions: executive, binding, film-forming, and
optical. First it puts the pigment into a liquid form which can be brushed onto a surface. Secondly it acts
as a “glue” which adheres the pigment layer to the surface on which it is applied. Thirdly it usually dries to
a hardened continuous film which encloses and protects the pigment particles so that the paint can
withstand use. Finally, when dry, the vehicle surrounds the pigment particles altering their color and light
reflecting and refracting properties. To make a useful paint the binder must possess the correct balance of
these four characteristics.
The solvent or diluent component of paints is used to make the paint more fluid and easier to mix
and apply. Sometimes the term “thinner” is used to refer to the solvent. Paints generally fall into two
major categories with regard to solvents: water-based paints and oil- or organic solvent-based paints. Once
the paint is applied, the solvent evaporates and is no longer present in the dry paint layer. Certain paint
media such as crayons, pastels, and some inks only require solvents during their preparation. These types
of paints are sometimes referred to as dry media. Nevertheless they also contain the most basic paint
ingredients of pigments, binders and various additives.
A variety of additives and fillers are also used in paint formulations. The choice of additives often
depends upon the final use of the paint. Inexpensive white pigments such as calcium carbonate (whiting,
Source: Dr. Patricia Hill, Millersville University
21
chalk, CaCO3) and barium sulfate (BaSO4) are often used to increase the opacity or hiding power of paints
so that less of the expensive colored pigments need to be used. Simple clays such as kaolin are used in the
preparation of pastels to decrease costs and improve their drawing properties. Chemical preservatives may
be added to paints to prevent mold growth and prolong shelf life. Emulsifiers may be added to paints to
keep the pigment particles from settling out over time. Even toxic substances may be added. For example
marine paints often contain copper sulfate to discourage the growth of marine organisms on the bottom of
boats.
Types of Paints
Over the millennia since humans first began to paint a number of painting methods have
developed. Some have proved more durable or less fragile than others but all have produced beautiful
works of art. The following is a list of the most common methods of applying a colorant to a surface.
These are listed roughly in chronological order of when the method of painting first appeared.
Encaustics - ancient Egypt
Fresco - early Greeks
Watercolor and Gouache - ancient Egypt
Egg Tempera - 10th-12th century Byzantine
Oils - 1400’s
Pastels - 1400’s
Crayons - 1903
Synthetic Resins — Alkyds and Acrylics - 20th century
Encaustic Painting
Painting in wax (encaustic painting) was practiced by Greek artists as far back as the 5th century
B.C. The earliest surviving examples of encaustic paintings are from Faiyum in Egypt. These mummy
portraits were painted by Greek painters during the 1st and 2nd centuries AD in colored wax on thin
wooden panels which had been coated with a layer of gesso-like (gesso is a white layer of calcium
carbonate or calcium sulfate) material. Encaustic, from the Greek word meaning “burnt in”, painting was
carried out by the Romans as well and this technique was used heavily up until the rise of tempera and
fresco painting in the Byzantine era. Many Greek and Roman marble statues are believed to have been
painted with colored waxes and polished to a high sheen. The main ingredients of encaustic paints are
pigments and beeswax as the vehicle. Sometimes oils or resins were used to improve the finish or handling
characteristics of this paint. One major difficulty in using encaustic paint is that it requires heating during
application so that the wax is in a fluid state. Beginning in the 18th century encaustic painting was revived
and today there is a resurgence of this technique among artists.
Fresco Painting
When most people think of fresco painting they think of
Michelangelo and the Sistine Chapel. However, the technique of
fresco painting was known in ancient times and examples of these
wall paintings survive from as early as the Minoan period (1700
B.C.) and from Pompei (100 B.C.) In this painting technique,
pigments are ground in water and then painted onto a layer of fresh
lime plaster. Plaster is made by mixing quicklime and water to
form calcium hydroxide, Ca(OH)2. As the plaster dries, it combines
with carbon dioxide (CO2) in the atmosphere to form hardened
limestone (calcium carbonate, CaCO3). The pigment particles
become trapped in the plaster and are bound into the surface of the
wall becoming part of the wall rather than just a film on top of it.
Under dry, warm conditions, in relatively unpolluted atmospheres,
fresco paintings can last for many years. However, dampness
combined with acidic conditions from polluted air can lead to their
rapid degradation.
Early fresco from the palace of King Minos,
Island of Knossos (1500 B.C.
Source: Dr. Patricia Hill, Millersville University
22
Watercolor and Gouache
In most cases, the name of a painting technique corresponds to the name of the vehicle or binder
used, however in the case of watercolor the name is derived from the solvent or diluent. Watercolor
painting can encompass both transparent watercolors and opaque gouaches. Both techniques were used by
ancient Egyptians to decorate coffins and illustrate their Books of the Dead. During medieval times,
similar materials were used by monks to create illuminated manuscripts. It was not until the 1700’s that
watercolor techniques as we know them today became an established method of painting. For both
transparent watercolors and opaque gouache, the vehicle consists of a plant gum binder. Usually this is
gum Arabic, an extract from the Acacia tree. In addition, sugar, honey, or glycerine is added to keep the
gum film supple and prevent cracking. Wetting agents or surfactants (like Ox gall or synthetic detergent)
are added to lower the surface tension of the water and improve the paint’s flow characteristics. Unlike
most other paints, there is not enough binder in watercolor paint to form a protective film around the
pigment particles. Therefore, watercolors can be easily redissolved if rewet.
Egg and Caseing Tempera
Temperare is Latin for blending or mixing and in the past the term tempera was used to denote all
types of combinations of media. However, today the meaning of tempera is much more specific. Tempera
paint combines aspects of both watercolor and oil based paints and usually contains either water-soluble
gums or proteinaceous materials and oils or resins. These substances form an emulsion of aqueous and
nonaqueous binding materials in which pigment particles are mixed. Emulsions are formed when drops of
an insoluble liquid are suspended in another liquid usually with the aid of an “emulsifying agent” which
helps to keep the droplets dispersed. Tempera paint is classified as a “water-based paint” and the aqueous
portion dries through evaporation while the nonaqueous portion “dries” through chemical means to form a
tough film that encases the pigment particles.
Although true tempera paints have been used since ancient times, it was during the 10th century
when egg tempera began to replace encaustic painting as the dominant medium of painting. Its popularity
continued and reached a peak during the Renaissance period. Gradually egg tempera was displaced by oil
painting, which was introduced in the late 15th century.
Classic egg tempera employs egg yolk or whole egg as the vehicle. Egg yolk is composed of
about 50% water, 22% egg fats and oils some of which are drying oils, and two emulsifying agents, lecithin
(9%) and albumen or vitellin (15%). The white of the egg, if included, contributes an adhesive protein
called albumin to the paint formula. Some egg tempera recipes call for only egg yolk, others use whole
egg, some dilute the egg yolk with an equal amount of water, and yet others call for the addition of drying
oils such as linseed, poppyseed, or walnut.
Egg white alone has also been used as a binder, particularly for paints used during medieval times
for the illustration of manuscripts. The clarified egg white was known as glair and was mixed with
pigments and painted primarily on a parchment surface.
Casein tempera uses the protein obtained from milk (casein) as the pigment binder. This protein
has much stronger adhesive power than egg emulsion paints and has been used for the production of glues
(eg. original Elmer’s Glue®).
Both egg and casein tempera paints dry to tough yet somewhat flexible films. Egg tempera
produces a matte finish while casein tempera produces a more luminous finish. Both types of paints were
traditionally painted on gessoed wooden panels. So called “milk paint” was often used during the 19th and
early 20th century on household furniture.
Oil Paints
Although the use of drying oils in paint media was known from earlier times, the “invention” of
oil painting is credited to artists in the Netherlands during the 15th century. From the 17th century on it has
been the most common technique of easel painting. Oil paints consist of pigment mixed with one or more
drying oils. A number of drying oils have been utilized throughout the history of oil painting including:
linseed, poppyseed, walnut, and sunflower seed. The important feature of all the drying oils is their ability
to “dry” and form a tough, leathery film that encases the pigment particles. This drying process does not
involve simple evaporation. Rather it is a “curing process” in which the oils undergo a chemical reaction
with the oxygen present in the air, forming strong cross-links between oil molecules. The linked oil
molecules form a polymeric film on the surface of the paint. The speed of drying of oil paints can be
controlled by the addition of chemicals that either speed up or retard the oxidative cross-linking process.
Source: Dr. Patricia Hill, Millersville University
23
One disadvantage of drying oils, especially linseed oil, is their tendency to yellow as they dry. They also
have a tendency to become more transparent as they age due to changes in the refractive index of the dried
polymer film. Because of this pentimento or the artist’s changes in the painting often become visible as
paintings age. Oil paints are not soluble in water and therefore solvents such as turpentine and mineral
spirits must be used to thin these paints.
Pastels
The use of natural chalks and soft stones for drawing purposes dates back to prehistoric times.
But Leonardo da Vinci began using pastel sticks in his sketches and paintings in the 15th century. By the
17th century the application of colored chalks to a surface was a popular method of creating art. In order to
create pastel sticks, a colored pigment is mixed with enough binder such as gum Arabic or gum
tragacanth (an extract from a leguminous shrub) to allow the formation of a solid stick. Then the pastel
stick is rubbed over an irregular surface where the dry pigment is deposited. The binder in ineffective once
the pastels have been applied and the pigments stick to the surface merely by static and frictional forces.
This leaves the surface of pastel paintings quite fragile and susceptible to damage.
In the late 1800s Edwin Binney and Harold Smith established a company (Binney & Smith) which
specialized in producing pigments. In 1902 they set up a factory along the Bushkill Creek in Easton,
Pennsylvania and began making slate school pencils and dustless chalk. A year later they introduced a new
product that has become known as the Crayola crayon. In 1996 Binney & Smith produced its 100
billionth Crayola crayon. Crayons are prepared from colored pigments and wax as the binder. The wax is
either beeswax, paraffin wax, or a combination of both. Usually commercial crayons also contain an inert
pigment or filler such as chalk (CaCO3).
Synthetic Resins — Alkyds and Acrylics
With the advent of polymer chemistry during the early 20th century, a large number of new
synthetic organic molecules became available. Many of these were made to approximate the physical
properties of naturally occurring substances. Through the process of polymerization, small molecules that
behave as liquids or gases are converted into giant molecules that behave as plastic solids. Among these
new plastics were polymeric materials known as alkyd and acrylic resins. The development of the
synthetic paint binder industry was literally “driven” by the automotive industry’s need to have more
rapidly drying, high gloss paints that could be applied to cars in an assembly line fashion.
Alkyd resins or polymers were developed around 1926. They are formed from two different
monomers, one the difunctional alcohol (polyhydric alcohol) glycerol, CH 2(OH)CH(OH)CH2OH, and the
other the difunctional carboxylic acid, phthalic acid. When chemically combined these two monomers
polymerize via a condensation reaction to form a polyester. Polyesters contain repeating ester linkages ( —
COO—R). Mixtures of these polyesters with drying oils form the binder for alkyd paints. The advantage
of alkyd paints over traditional oils is their rapid drying time of hours as compared to days. Alkyd resins
have been the most important resins in industrial paints and lacquers for a number of years and only more
recently have they been introduced as artists’ paints.
Acrylic resins, introduced by the Rohm and Hass Company in 1931, are polymers formed by the
polymerization of unsaturated acids or esters such as acrylic acid (CH 2=CHCOOH), methacrylic acid
(CH2=C(CH3)COOH, and methyl methacrylate (CH2=C(CH3)COOCH3. Poly methyl methacrylate, also
known by the trade names Lucite® or Plexiglass® , is a hard, strong, thermoplastic resin with the clarity of
glass. Acrylic resins exhibit strong adhesive properties to most surfaces and they form tough films when
dry. Acrylic polymer emulsions can be formed in aqueous solvents and these emulsions act as the binder
portion of acrylic paints.
Varnishes
A varnish is a non-pigmented coating typically applied to the surface of an oil or tempera painting
once it has thoroughly dried. Varnishes are also applied to fine wooden objects such as furniture. The
purpose of applying a varnish is twofold: (1) to improve the appearance of the painting or wood by
intensifying the colors in the pigments or wood grain present and producing a matte or gloss finish; and (2)
to protect the paint or wood from oxygen, pollutants, dirt, and ultraviolet radiation. Most varnishes are
composed of natural resins such as dammar and mastic obtained from the sap of coniferous trees and
shrubs, shellac, which is obtained from insects, or copal a fossil resin. These resins are usually dissolved in
Source: Dr. Patricia Hill, Millersville University
24
alcohol or turpentine in order to be applied. Unfortunately many resins tend to become brittle and yellow
upon exposure to light, pollutants and atmospheric oxygen. As a result many paintings from the Old
Master’s have darkened over time and require extensive conservation treatment to restore their original
appearance.
Summary of Ideal Pigments and Binders
No single paint formula can meet all the requirements for long lasting decorative, aesthetic, or
protective purposes. But today we have many choices among pigments, vehicles, solvents, and additives to
formulate paints that will serve many diverse purposes. When creating paints, especially for the purposes
of making long-lasting art, it is important to remember the ideal characteristics of both pigments and
binders and strive to use paints that meet most of these requirements.
Ideal pigments should...
* not fade or change color when exposed to light
* not undergo chemical reactions with other pigments, binders, or grounds
* not bleed or migrate through dried binder
* have good opacity or hiding power
* remain the same color when exposed to acidic or alkaline fumes in the atmosphere
* form tough, stable films with the binder which are resistant to wear
* not be poisonous when handled with normal precautions
Ideal binders should...
* not undergo a color change with aging
* retain their binding strength as they age
* remain structurally stable with time and resist cracking and peeling
* not be easily dissolved by mild solvents when dry
* not activate a color change in the pigment
* not be toxic or flammable under normal use
Modern Paint and Coatings Industry
In 1700 the first paint mill in America was put into operation in Boston. By the mid-1880s,
numerous paint factories had been established in most population and industrial centers across the nation.
Today the U.S. paint and coatings industry manufactures a variety of coating products that are used to
preserve, protect and beautify the objects which surround us. Typical products include (1) architectural
coatings (house paints), (2) industrial coatings (applied during the manufacturing process to autos,
furniture, appliances, paper, machinery, etc.), and (3) special purpose coatings such as marine paints, traffic
paints, roof coatings, and aerosol paints. All of these modern coatings have the same basic components
(pigment, binder, solvent, additives) as artists’ paints.
REFERENCES
1. Painting Materials: A Short Encyclopedia, Rutherford J. Gettens and George L. Stout, Dover, 1966.
2. The Artist’s Handbook of Materials and Techniques, 5th Edition, Ralph Mayer, Viking Press,
1991.
3. The Materials and Techniques of Painting, Jonathan Stephenson, Thames and Hudson, 1989.
4. The Artist’s Handbook, Ray Smith, Alfred A. Knopf, NY (1987)
5. Artists’ Materials: Which, Why and How, Emma Pearce, A&C Black, London (1992)
6. Looking at Paintings: A Guide to Technical Terms, Dawson W. Carr, Mark Leonard, J. Paul Getty
Museum, British Museum Press (1992)
7. Looking at Prints, Drawings and Watercolours: A Guide to Technical Terms, Paul Goldman, J.
Paul Getty Museum, British Museum Press (1988)
8. Methods and Materials of Painting of the Great Schools and Masters Vol. 1 & 2, Sir Charles Lock
Eastlake, Dover Publications (1960)
9. Michelangelo and the Creation of the Sistine Chapel, Robin Richmond. Crescent Books (1993)
10. Synthetic Painting Media,Lawrence N. Jensen, Prentice-Hall Inc. (1964)
Source: Dr. Patricia Hill, Millersville University
25
11. The Materials and Techniques of Medieval Painting, Daniel V. Thompson, Dover Publications
(1956)
12. The Materials and Techniques of Painting, Kurt Wehlte, Van Nostrand Reinhold (1982)
13. The Materials of the Artist and Their Use in Painting, Max Doerner, Harcourt Brace Jovanovich
(1962)
14. The Painter’s Companion: A Basic Guide to Studio Methods and Materials, Reed Kay, Webb
Books, Cambridge (1961)
15. The Painter’s Craft, Ralph Mayer, Penguin Books (1991)
16. The Painter’s Methods and Materials, A.P. Laurie, Dover Publications (1967)
INTERNET RESOURCES
 Industrial Paints http://www.paint.org/history.html— National Paint & Coatings
Association
 Crayons
http://www.crayola.com — Crayola, Binney & Smith Company home page
 Egg Tempera Paints http://www.eggtempera.com
 Fresco Painting http://execpc.com/~eksaminc/eksam.html
 Artists’ Paints
http://www.sinopia.com
Study Questions for Chapter 12
1. Paint media contain four types of ingredients: binders, solvents, colorants/fillers, and additives. What
is the purpose of each ingredient?
2. What binder and solvent are used to make the following types of paints?
a. encaustics
d. egg tempera
g. crayons
b. fresco
e. oils
h. acrylics
c. watercolors
f. pastels
i. alkyds
3. How are paint binders related to polymers?
4. By what two methods do paints dry?
Important Chapter Terms
 paint
 additive
 thinner
 egg yolk
 beeswax
 alkyds






Source: Dr. Patricia Hill, Millersville University
colorant
vehicle
watercolor
drying oils
pastels
polyesters






binder
diluent
gum Arabic
oil paint
crayons
acrylic resins






solvent
filler
tempera
encaustics
gum tragacanth
varnish
26
Fresco
Introduction
In Italian, the word fresco means “fresh”. It is a method of painting on the surface of wet (fresh)
lime plaster. Its origins date back to ancient times with examples of beautiful frescos that were created
during the 1st and 2nd centuries BC in Greece. During the Italian Renaissance (15 th and 16th centuries AD)
this painting technique was developed into a fine art with perhaps the best known example being
Michelangelo’s painting of the Sistine Chapel Ceiling. As you might suspect, fresco paintings are very
durable since many have survived in relatively good condition for many centuries.
Of the many types of paints and painting techniques, fresco painting is one of the best illustrations
of chemistry in action. Chemical reactions are involved from the preparation of the materials necessary for
the creation of a fresco to the deterioration processes that can ruin ancient frescos. By studying this art form
you can also learn about chemical reactions and how the reactions can be described by chemical equations.
Anatomy of a Fresco
Frescos are most often painted on walls, ceilings and
other architectural forms. During ancient times, as well as
during the Renaissance, the support for fresco painting was
either a stone or brick wall. The rough surface of the wall
had to be smoothed and prepared for painting by applying
increasingly smoother layers of plaster. The first plaster layer
applied directly to the wall, is called the arricio layer. Once
this layer is dry, a final plaster layer, called the intonaco, is
applied on top and smoothed out. While the intonaco layer is
still damp, an outline of the painting (called a cartoon) is
transferred to the plaster and then the fresco is painted with
colored pigments suspended in water. Once the plaster
begins to dry the painting must be stopped so only a section
about 3-5 square meters can be painted at one time. As the
plaster dries it reacts with the air and forms a new chemical
compound that traps the colored pigments beneath the surface
of the painting leading to great durability and a luminous
quality to the painting.
Fresco Chemistry
The chemistry of fresco painting is the chemistry of limestone and lime plaster. Limestone is
calcium carbonate (CaCO3), an abundant, naturally occurring mineral. It is mined in various localities
throughout the world. Limestone is the starting material for the preparation of the lime plaster used to
make frescos. In order to make lime plaster calcium carbonate must first be converted to quicklime, or
calcium oxide (CaO). This is done by heating the limestone producing carbon dioxide as a by-product of
the reaction as shown in equation 1.
(Equation 1)
CaCO3(s)
reactant
CaO(s)
+
products
CO2(g)
A chemical equation is simply a shorthand way of describing a chemical reaction. Equation 1 could be
written out in a standard English sentence that would read: “Calcium carbonate, a solid, decomposes when
heated to form solid calcium oxide and carbon dioxide, a gas.” The chemical equation says the same thing
in a lot less space! If you examine this equation closely and count the number of atoms on each side of the
arrow you should see that this is also a balanced chemical equation. There is one Ca, one C, and three Os
on each side of the equation. A "balanced chemical equation" indicates three important things:
1. The correct formulas for reactants (starting chemicals — found on the left side) and correct
formulas for products (chemicals formed — found on the right side)
2. The correct proportions in which the atoms, molecules or moles of starting materials react with
each other
3. The correct proportions of atoms, molecules, or moles of products formed
63
Source: Dr. Patricia Hill, Millersville University
27
Chemists refer to these correct proportions as the stoichiometry of a chemical reaction. Another way of
looking at a balanced equation is like a shorthand recipe or model that summarizes the most important
information about a given chemical reaction. To be complete in describing the reaction that forms
quicklime (Equation 1) we should say: “One molecule (or mole) of solid calcium carbonate decomposes
when heated to form one molecule (or mole) of solid calcium oxide (quicklime) and one molecule (or
mole) of carbon dioxide gas.”
Once the quicklime (calcium oxide) is prepared, the quicklime (also called lime) can then be
“slaked” by adding water to it to form lime plaster or slaked lime which is also known as calcium
hydroxide — Ca(OH)2. This reaction produces heat as a by-product and is illustrated in equation 2.
(Equation 2)
CaO(s)
+
quicklime
H2O(l)
Ca(OH)2(aq)
+
heat
slaked lime
Slaked lime or calcium hydroxide to be used for frescos is then “aged” for a period of time before use.
Typically this was done in large pits for several months to as long as several years. Because of this aging
process the slaked lime is sometimes referred to as “pit lime”. Slaked lime is a very alkaline substance that
is caustic to skin. Finally the lime plaster (calcium hydroxide) can be mixed with sand to form the actual
plaster that is applied to the wall for fresco painting. Course grained sand is used for the initial arricio
layer while fine-grained sand is used for the smoother, intonaco layer, the layer to which the paint is
applied. The paint, which is simply colored pigments suspended in water, is applied onto the wet plaster.
The designs for most frescos are first outlined on the wet plaster by a technique called “pouncing”. A
drawing on paper is pierced along the lines with a needle, the drawing is laid over the wet plaster and
charcoal powder is forced through the holes to leave a faint outline of the sketch on the plaster. Then the
actual painting takes place. Only relatively small sections of wall can be done at a time before the plaster
dries.
Fresco technique relies on atmospheric carbon dioxide gas (CO 2) and the capillary movement of
water to harden the painting and bind the pigments into the painting surface. Water is absorbed by the
underlying plaster layers and wall as well as evaporates from the surface of the fresco. At the same time
the slaked lime plaster absorbs carbon dioxide gas from the air and undergoes carbonation, producing
calcium carbonate (CaCO3) which traps the pigments and provides a hard, stable surface. The equation for
this reaction is shown below.
(Equation 3)
Ca(OH)2(aq)
+
CO2(g)
CaCO3(s)
+
H2O(l)
Because the lime plaster is very basic (alkaline), only inorganic pigments that are stable in an alkaline
environment can be used for fresco painting.
Deterioration and Conservation of Fresco Paintings
Although many frescos have survived for hundreds of years, there are a number of factors that can
lead to their deterioration and destruction. Some frescos have been damaged by well-intentioned
conservation efforts others have been physically damaged by earthquakes. However, the most serious
source of deterioration comes from the presence or formation of soluble salts in or on the plaster. All walls
contain salts that could be produced by the weathering of the stone building materials, come from
impurities in the plaster itself, or be carried through the walls from the ground on which the wall rests.
Another increasingly important source of soluble salts is from the atmosphere, particularly a damp
atmosphere high in pollutant gases. Very soluble salts of potassium and sodium, while they can cause an
annoying haze on frescos, do little serious damage to them. However, the slightly soluble salt of calcium
sulfate (gypsum) is most harmful to frescos. Calcium sulfate can be produced in frescos as a result of “acid
rain” or “acid smog”.
Acid precipitation is the result of atmospheric chemical reactions that convert pollutant gases from
combustion processes (natural and manmade) into strong acids like sulfuric (H2SO4) and nitric (HNO3)
acids. For example, sulfur dioxide (SO2), an industrial pollutant gas, is readily converted to sulfurous and
sulfuric acids by the action of oxygen and water in the atmosphere as illustrated in the chemical equations
shown below.
SO2(g) + O2
SO3
SO3 + H2O
H2SO4
64
Source: Dr. Patricia Hill, Millersville University
28
The sulfuric acid (H2SO4) precipitation can react with calcium carbonate of the fresco forming calcium
sulfate (CaSO4(s)). The calcium sulfate often crystallizes just beneath the surface and throughout the paint
layer of frescos causing severe blistering and loss of the fresco surface. The chemical reaction of calcium
carbonate (the fresco surface) with sulfuric acid is shown in equation 4.
(Equation 4)
CaCO3(s)
+
H2SO4(aq)
CaSO4(s)
+
H2O(l)
+
CO2(g)
To repair damaged frescos, conservationists try to chemically reverse these reactions or form more stable
products that will not lead to further damage. To repair and clean frescos damaged during the floods in
Florence, Italy in 1966, scientists, artists and art historians worked to develop the “Barium Method” which
was used successfully to restore the frescos of the Brancacci Chapel. This process requires several steps
and is illustrated in the chemical reactions of Equations 5 and 6 shown below.
Step 1: Calcium sulfate is solubilized (dissolved) by converting it to ammonium sulfate and calcium
carbonate through the application of a “poultice” (ground up paper soaked with ammonium carbonate).
(Equation 5)
CaSO4.2H2O(s) +
(NH4)2CO3(aq)
(NH4)2SO4(aq) + CaCO3(s) + 2H2Ol)
Step 2: Barium hydroxide is applied (again as a poultice) in order to convert the ammonium sulfate into
insoluble barium sulfate, a stable white solid. The excess barium hydroxide also acts to produce barium
carbonate, which binds in the plaster as well as helps reproduce more calcium carbonate in the plaster layer.
(Equation 6)
Ba(OH)2(aq)
+
(NH4)2SO4(aq)
BaSO4(s) + 2NH3(g) + 2H2O
The Sistine Chapel has also undergone recent conservation and restoration. However, unlike the Brancacci
Chapel, the paintings in the Sistine Chapel contained low levels of sulfates. They were however covered
with centuries of soot, grime and coatings of glues applied during earlier conservation attempts.
Conservation measures consisted mainly of washing the walls with a special cleaning agent to remove salts,
glues and dirt, washing with distilled water and then applying a special polymer to attach areas of flaking
paint and plaster. The chapel is now “climate controlled” with its temperature and humidity carefully
monitored for optimum fresco health.
REFERENCES
1. Michelangelo and the Creation of the Sistine Chapel, Robin Richmond, Crescent Books, New York,
1995.
2. Fresco Painting for Home and Garden, Sarah Hocombe, David & Charles, London, 1999.
Study Questions for Chapter 9
1. For each chemical equation given, write a complete English sentence that states what the reaction is
describing.
2. Define a “balanced chemical equation”.
3. Examine equations 5 and 6. Show how you determine if these two chemical reactions are balanced.
Important Chapter Terms




arricio layer
calcium carbonate
calcium hydroxide
gypsum




intonaco layer
quicklime
lime plaster
calcium sulfate




pouncing
calcium oxide
carbonation
fresco




limestone
slaked lime
acid rain
alkaline
65
Source: Dr. Patricia Hill, Millersville University
29
Ceramics
History of Ceramics
Ceramic wares are some of the earliest known artifacts of humankind dating back to between
10,000-8000 BC. Humans first worked with natural stone during the Paleolithic (old stone age) and
Neolithic (new stone age) periods because it was hard, strong, would resist both heat and chemical attack,
and was available everywhere. They used it for weapons, tools, and utensils. Human ingenuity soon
created stone-like materials in the form of baked clay, pottery, bricks, and concrete.
Like glass, there are several working definitions for the term ceramics. We will use the one which
defines ceramics as inorganic materials made by heating clay or other mineral matter to a high temperature
at which the particles partially melt and fuse together. Chemically ceramics contain a combination of one or
more metallic elements with a nonmetallic element, usually oxygen. Ceramics contain primarily ionic
bonds with a lesser number of covalent bonds. They resemble "salts" but the ionic bonds in ceramic
materials are much stronger.
The basic raw material of ceramics is clay. Basically a very fine-grained, crushed and pulverized
rock, clay becomes plastic when wet. The clay particles will adhere together and dry without cracking.
Then the dried clay object can be converted into a permanent rock-like mass if heated to a high enough
temperature. Clay a complex mixture of silicate minerals - composed primarily of metal cations and
polyatomic silicate anions (SiO44-). The silicon and oxygen atoms are held together by covalent bonds in a
tetrahedral-shaped structure. These covalently bonded arrays of silicate tetrahedra are interspersed with
metal cations such as Al3+, K+, and Na+ so that a balance of charges is maintained.
Clays begin life as aluminosilicate minerals such as feldspar (granite)— with a chemical
formula of K2Al2Si6O16 (potassium aluminum silicate) — a ubiquitous material. Through the process of
weathering, that is the addition of water and oxygen, the feldspar is gradually converted into clay — with
the chemical formula Al2Si2O5(OH)4. If the clay forms at the site of the parent rock, it is referred to as a
Weathering of Feldspar
K2O . Al2O3 . 6 SiO2 + 2 H2O + CO2
Al2O3 . 2 SiO2 . 2H2O + 4 SiO2 + K2CO3
feldspar
kaolinite
primary (1o) or residual clay. On the other hand if the clay is transported away from its parent rock by
water, wind, or glacier movement, and deposited at a distant site it is referred to as a secondary (2o) clay.
Primary clays are less common than secondary clays and are more coarse-grained and less plastic. Kaolin
(kaolinite), a very pure form of clay is a white-colored primary clay, usually found in pockets in the parent
rock. Kaolin is a refractory clay (high melting point of 1800oC) with large particle size, limited plasticity,
and a low rate of shrinkage when used in ceramics. Secondary clays contain more impurities such as iron
and carbon and therefore are darker in color. For example, ball clay, a secondary clay, is usually found in
large layered deposits. It is a dark gray color due to high carbon or iron content. It has a much lower
melting point (1300oC) than kaolin and it is composed of finer grained particles which are more plastic
when wet. However, ball clay exhibits excessive shrinkage (>20%) upon drying and firing. The table
below summarizes the major differences between primary and secondary clays.
Primary Clays
Secondary Clays
found at site of parent rock
found away from parent rock
Location
small deposits
large deposits
Occurrence
large, coarse
small, fine
Particle size
lighter
darker
Color
low
high
Shrinkage
low
high
Plasticity
kaolin (kaolinite)
ball clay
Example
The plastic property of clays is a direct result of the microscopic and molecular structure of the
clay. The silicates and aluminates align in layered sheets and these sheets trap the “lattice water” molecules
between them. The trapped water allows the sheets to slide over each other making the clay plastic, that is
able to bend without breaking or cracking. Some clays have three alternating layers while others are two-
Source: Dr. Patricia Hill, Millersville University
30
layer clays. Note the layered structures in the electron micrographs of three types of clay shown below at
approximately 25,000x magnification. (Electron micrographs courtesy of the San Diego State University
Electron Microscope Facility reprinted with permission of The Ceramics Web, http://art.
sdsu.edu/ceramicsweb)
Bentonite Clay
(highly plastic)
Fireclay
(low plasticity)
Kaolin Clay
(low plasticity)
Several other types of clays are used in ceramics. These include stoneware and earthenware clays.
Both are highly plastic secondary clays with lower melting points than either kaolin or ball clays. Some of
these clays are highly colored due to higher levels of mineral impurities.
Ceramics Technology
When clays are used for the production of ceramics they are usually carefully washed and blended
to achieve the desired consistency, color and working properties. This mixture of clays and other nonplastic fillers is referred to as the clay body. The color, texture, plasticity, degree of shrinking, and melting
point of a clay body can be adjusted by changing the clay composition. For example the physical properties
of the clay body can be adjusted by adding more or less plastic clays or by adding clays of larger or smaller
particle size. The reaction of clay to heating and fusing (firing) can be adjusted by adding materials of
greater or less fusibility to the clay body. A specific example of this manipulation can be seen in the
formulation of the white, translucent porcelain given in the table below.
Clay Body Material
Kaolin
Feldspar
Flint (a glass)
Ball clay
Amount
4 parts
3 parts
2 parts
1 part
Purpose
White material, low shrinkage, mp = 1500oC
lowers mp to 1300oC, lowers shrinkage
decreases warping, increases hardness, allows glaze to fit
better
makes wet clay body more plastic
In order to convert a clay body into a ceramic the clay must be heated to a high temperature or
fired. In early times firing was done in open fires, but soon special ovens or kilns were developed for the
purpose of firing pottery and ceramics. As clay is heated, several important changes occur in the clay body
which convert the rather fragile dry clay body, to a hard, heat and chemical resistant solid. Since plastic
clay is almost one-fourth water, the first thing that happens to a clay body is the loss of water by
evaporation upon drying. Clay bodies are typically formulated to shrink between 5-8%. The lattice water,
that is the water held tightly between the clay layers, is not removed until the clay is fired. The first process
occurring upon firing of clay is the evaporation of any residual water when the temperature reaches 100125oC. Then as the temperature rises to between 350 and 525 oC the chemically combined lattice water is
removed. Any organic matter, carbonates, or sulfates present in the clay burn off at about 900 oC. However,
the most important phenomena in the firing of clay occur as the quartz (silicates) and kaolin undergo phase
and structural changes throughout the clay body as the temperature rises. The first of these changes occurs
at 573oC when the quartz is converted from the alpha to the beta crystal form. This is followed by a number
of other changes as the kaolin is converted to metakaolin and finally to mullite and the quartz is converted
to cristobalite. At temperatures ranging between 1000 and 1800 oC the mullite and cristobalite fuse and
vitrify forming the hard, strong ceramic body. It is very important that firing of clay occur slowly to
permit these phase and structural changes to occur without damage to the clay as the clay first expands and
Source: Dr. Patricia Hill, Millersville University
31
then shrinks. Kiln temperatures must be monitored carefully and clay bodies heated up and cooled down
very slowly.
Types of Pottery
The color, strength, porosity, and hardness of the pottery and ceramics depends upon a number of factors
including
 The chemical composition of the raw materials used.
 The surface treatment of the pottery before firing
 The temperature of the firing process
 The redox properties of the atmosphere within the kiln during firing
Primitive cultures used whatever materials they could find which initially were limited to large outcroppings
of secondary clays high in mineral contamination. The firing temperatures that could be achieved were
rather low (only about 900oC). Consequently early pottery was quite dark red or brown in color due to iron
impurities, fragile and porous. This type of pottery is termed terra-cotta. As kilns were developed firing
temperatures were raised to between 900-1100oC and the resulting pottery became stronger and less porous.
This type of pottery is known as red earthenware because of its distinctive red color. At slightly higher
temperatures (1100-1200oC) clays with a substantial amount of calcium present (usually in the form of
limestone) produce a cream-colored earthenware. Both types of earthenware can be covered with glazes
and glazed earthenwares are known as majolica (red) or fiance (cream). As firing methods improved,
temperatures above 1200-1300oC could be maintained in kilns. This allowed for the development of
stoneware, a very strong, nonporous ceramic. The high kiln temperatures are sufficient to vitrify the clay
body and seal the pores. In China, advances in raw materials and firing techniques led to the production of
porcelain, a highly vitrified, tough, nonporous, glassy ceramic which can only be produced by firing above
1300oC. Although porcelain production was discovered before 700 AD in China, it was not until the early
1700s that porcelain was produced in Europe by a German alchemist, Johann Friedrich Bottger.
The redox conditions within the kiln also play a role in the coloration of the ceramic body. Under oxidative
conditions (oxygen from the air present) the color of the ceramic will vary from reds to pinks to oranges if
iron impurities are present in the clay at temperatures below about 1100 oC. Above this temperature, and in
the presence of sufficient calcium, colors range from yellow to cream to white as the firing temperature
rises. However, if the atmosphere within the kiln contains little oxygen and more carbon monoxide, smoke
and soot, the ceramic generally takes on a darker color due to the presence of unburned organic matter and
reduced forms of iron oxides (FeO). We call this a reducing atmosphere and the technique of Raku firing
especially in conjunction with use of glazes on the surface of the ceramic leads to spectacular surface
coloration and effects.
Glazes
Although ceramics are hard and strong, many types are also quite porous making them less useful
as liquid containers. Early civilizations learned to coat either the inside or outside of their pottery with
various materials such as mud and natural latex to prevent leakage of liquids out of clay and ceramic
utensils. The technique of applying a glaze to a ceramic or clay body developed as early as 5000 BC in
ancient Egypt. The Chinese also developed quite sophisticated glazes and the technology of high
temperature firing around 500 BC. A glaze is basically a glassy coating that is melted in place on the
surface of a ceramic body. The purpose of the glaze is to provide color, texture, and a non-porous surface
to the ceramic. This thin glass layer beautifies, imparts strength and chemical resistance, and fills the pores
of pottery so it will hold liquids
Glazes contain the same basic oxide components as glass: silicon dioxide (sand or silica); fluxing
agents; stiffening agents; coloring agents; and opacifiers. However, glazes usually contain a larger
proportion of the refractory oxides such as Al2O3 and aluminosilicates. This generally leads to a higher
melting point for the glaze than for glass and it also creates a greater affinity between the clay body, which
is rich in aluminosilicates, and the glaze. Just like glass, silica is the glass network forming oxide. The
fluxing oxides (RO and R2O) act to lower the silica melting point or adjust the thermal expansion
characteristics and surface properties of the glaze. The stiffening agents keep the molten glaze from sliding
off the clay body during the firing process. Coloring agents and opacifiers are metal oxides used to modify
the light absorbing, transmitting, and reflecting qualities of the finished glaze. The source of these oxides in
Source: Dr. Patricia Hill, Millersville University
32
glaze formulas, however, is sometime not obvious and can be confusing. Many of the fluxes are actually
formed during the firing process from other chemicals included in the glaze formula. The following table
gives the names and formulas for various glaze oxides, their role in the glaze formulation, and the chemical
used as a source for the glaze oxide in the formula.
Name
silicon dioxide
sodium, potassium,
lithium oxide
lead oxide
Formula
SiO2
Na2O, K2O, Li2O
calcium oxide
CaO
magnesium oxide
MgO
barium oxide
zinc oxide
BaO
ZnO
boron oxide
B 2 O3
PbO
Purpose
network former
fluxing agents, lower mp of
silica
flux, lowers mp, smooth
bright surface, low expansion
flux adds hardness and
durability, imparts a matte
finish if used in excess
high temperature flux, matte
finish, opacity
similar to CaO
high temperature flux,
smooths surface
lowers thermal expansion,
smooths surface
Source
Flint (sand)
Feldspar or Nepheline syenite
K2O.Al2O3.6SiO2
Litharge, white lead, red lead
Whiting (CaCO3), limestone,
chalk, marble
Dolomite
Magnesite, MgCO3 Dolomite
BaCO3
zinc oxide
Colemanite
2 CaO.3B2O3.5H3O
Borax (Na2B4O7)
clay, kaolin or China clay,
feldspar, alumina
aluminum oxide
Al2O3
stiffening agent, increases
(alumina)
hardness, durability, opacity
transition metal
e.g. TiO2,
provide color to glaze
oxides, chlorides, or NiCO3, CuCl2
carbonates
Glazes are usually applied to the surface of bisque ware, that is clay that has been fired once. It is
applied in the form of an aqueous suspension of finely divided, insoluble particles called a slip. The
ceramics can be dipped into the glaze or the glaze can be sprayed or painted onto the surface. Then the
painted ceramic is re-fired at 1060-1250oC to fuse the glaze particles together with each other and to the
surface of the ceramic. The preparation of glaze formulations must be exact so that after firing no starting
materials remain unreacted to cause improper color, cracking, and other types of glaze failure.
Some of the first glazed pieces from ancient Egypt
incorporated the glaze material into the clay body. This
Typical Egyptian Paste Formula
39%
material, called Egyptian paste, is a light-weight, self glazing Nepheline Syenite
6%
ceramic material suited to the making of jewelry and small Ball Clay
37%
decorative objects because it is not very plastic. The Flint
6%
materials combined in the paste include both clays and glaze Sodium Bicarbonate
6%
materials. Different metal salt compounds can be added to Kaolin
the basic mixture in order to produce a variety of colors in Sodium Carbonate (Soda Ash) 6%
the finished product. The formulation for a typical Egyptian
paste is given in the adjacent table.
Another simple form of glazing of pottery is salt glazing. In this case greenware or bisque ware is fired to
the melting point of silica at which point moist sodium chloride (salt, NaCl) is introduced into kiln. The
heat vaporizes the NaCl forming sodium oxide (Na2O) which serves as flux for silica to give it a glassy
surface.
Source: Dr. Patricia Hill, Millersville University
33
Salt Glazing Reactions
2 NaCl
+
H2O
2 HCl
+
Na2O
Na2O + x SiO2
Na2O . x SiO2
Glazes come in a wide variety of colors and textures. The colors come from the addition of
transition metal compounds to the glaze formula. The transparency of glazes can also be controlled by the
addition of opacifiers such as tin oxide (SnO) and zirconium oxide (ZrO). These compounds do not melt at
kiln temperatures and remain suspended in the glaze to scatter light. Glazes may have a glossy, smooth
finish or it may have a matte or dull surface. The degree of surface gloss can be controlled in a number of
ways. Gloss is reduced by under firing the glaze, including additives such as clay, whiting (CaCO3), or
barium oxide (BaO), or increasing the percentage of high temperature or refractory oxides (Al 2O3, CaO,
MgO) to cause devitrification of the glaze upon cooling. The following table lists some chemicals used to
produce colored glazes.
Sources of Glaze Colors
Transition Metal Compounds
Formulas
Glaze/Glass Color
Iron oxides
Fe2O3, FeO
Wide range of colors
Copper oxides
CuO, CuCO3
Blues and greens
Cobalt oxide
CoO, CoCO3
Blue
Chromium oxide
Cr2O3
Red, yellow, pink, brown or greens
Manganese oxide
MnCO3, MnO2
Purple or brown
Nickel oxide
NiO, Ni2O3
Green or brown
Vanadium oxide
VO2
Yellow
Rutile
TiO2 + Fe2O3
Tan or brown
Ilmenite
TiO2 + Fe2O3
Spots, specks, texture
Iron chromate
FeCrO4
Gray, brown, black
Uranium oxide
UO2
Red to yellow
Cadmium selenide
CdSe
Red
REFERENCES
1. Clays and Glazes for the Potter, Daniel Rhodes, Chilton Books, 1966.
2. Chemistry of Building Materials, R.M.E. Diamant, Business Books Ltd., London, 1970.
3. Low Fire Ways to Work in Clay, Leon I. Nigrosh, Davis Publications, 1980
4. The Ceramics Web, on-line reference for ceramics information, glaze formulations, ceramics links
address is http://art.sdsu.edu/ceramicsweb/
Study Questions for Chapter 8
1. What is clay and what are the types of clays?
2. What is a “clay body”? What ingredients are usually found in a clay body?
3. What happens to clay when it is “fired”?
4. What is the difference between oxidative and reductive firing?
5. What basic ingredients are used in glazes and what is the purpose of each?
6. What is the process of “salt glazing” and why does it produce a glassy coating on the ceramic?
Important Chapter Terms
 clay

 ball clay

 kiln



earthenware
bisque ware


plastic
primary clay
lattice water



aluminosilicate minerals
secondary clay
vitrification



stoneware
opacifier


porcelain
network oxide


Source: Dr. Patricia Hill, Millersville University
kaolin
firing
terracotta
glaze
flux
34
viscosity (ability to flow) of glass decreases with temperature. Upon heating, it glows, flows, and can
either be molded or blown.
What Is Glass?
Chemically, we can describe glass as an inorganic product of melting that has cooled to a rigid
state without crystallizing. It is rich in silicon and oxygen, coming from its basic component silicon
dioxide (SiO2) or sand, but it also contains other metal ions such as Fe, Na, K, Ca, Mg, B, and Al. Glass
does not have a sharp melting point but rather melts over a range of temperatures in which it first softens
and then flows as the bonds holding the glass structure together gradually break. The temperature at which
the glass first begins to soften is called the glass transition temperature (Tg). Below this temperature the
glass is hard and brittle, above this temperature the glass is soft and flows.
Crystals on the other hand are true solids. There are several categories of crystals some of which
we are already familiar with. They are:
1. Ionic Inorganics — These compounds are held together by strong electrostatic forces giveing them
very high melting points. Many are water soluble due to their ionic character. Examples include
NaCl, CaCO3 and many other naturally occurring minerals.
2. Metallic Crystals — All metals in their solid state exist with the nuclei of their atoms arranged in a
crystalline array surrounded by a “sea of valence electrons”. This structure contributes to the
characteristic bulk properties we associate with metals such as electrical and thermal conduction, and
luster. Examples include Fe, Al, Cu, Ni, Cr, and Ag.
3. Molecular Crystals — Certain covalent molecules such as I2, ICl, and S8 form soft, low melting, water
insoluble crystals.
4. Network Solids — Some elements and compounds are covalently bonded to form one large network of
indefinite size. These substances typically have high melting points and are quite insoluble in water or
other common solvents. Examples include diamond (C), graphite (C), and quartz (SiO 2).
In all these compounds or elements the atoms are arranged in definite and repetitive threedimensional patterns. As a result of this regular atomic arrangement in which the particle are held in rigid,
fixed positions which do not change, crystals show definite external geometrical shapes, bounded by plane
faces. Crystals come in many different shapes and sizes however, crystals may be grouped into seven basic
categories with characteristic angles and shapes.
Cubic (Isometric)
Tetragonal
Hexagonal
Rhombohedral
Orthorhombic
Monoclinic
Triclinic
In contrast to crystals, amorphous solids have an intermediate state between the crystalline and
liquid states. They contain no long-range order because the particles (atoms, molecules) are not in rigidly
fixed positions within the substance. In addition to glass, rubber and most plastics are classified as
amorphous solids. Complicating the situation is the fact that within plastics regions of microcrystalline
structure can exist or be induced.
Source: Dr. Patricia Hill, Millersville University
35
Crystalline
Amorphous
Structure and Composition of Glasses
Glasses are composed of a number of different kinds of oxides. There are four types: (1)
network, glass-forming or acidic oxides, (2) fluxing, network-modifying or basic oxides, (3)
refractory or neutral oxides, and (4) opacifying oxides. The table below lists these oxides and gives their
general chemical formula, examples of each type, and the function they fulfill in the manufacture of the
glass.
Oxide Type
Glass-forming
(Acidic)
Fluxing
(Basic)
General Formula
RO2, R2O3
Examples
SiO2, B2O3
RO or R2O
Refractory
(Neutral)
R2O3
CaO, MgO, BaO, SrO,
ZnO, PbO, Na2O, K2O,
Li2O
Al2O3, B2O3
Opacifying
RO2
TiO2, SnO2, ZrO2
Function
Major glass structure
component
Lower the melting point
of glass-forming oxides
Inhibit flow, control
viscosity, retard
crystallization
To make glass opaque
A variety of glasses can be manufactured by varying the types and amounts of these four oxide groups.
The glass-forming oxides are essentially inorganic, polar covalent compounds while the fluxing, refractory
and opacifying oxides are simple inorganic, binary, ionic compounds.
Pure quartz glass is composed chiefly of
silica, SiO2. It contains an irregular array of silicon
Oxyg en
atoms covalently bonded to four oxygen atoms in a
tetrahedral configuration. It has excellent chemical
Silico n
durability, can withstand sudden temperature
changes, and is transparent to many wavelengths of
light including ultraviolet radiation. For this reason
it is used in the optical systems and sample holders
(cuvettes) of ultraviolet spectrophotometers. Due to
the extremely high melting point (1723oC) and high viscosity of molten silica, this glass is difficult to work
and shape making it an expensive glass which is only used in limited applications.
A relatively new application of pure silica
Coating
glass is in the manufacture of optical fibers. These
glass fibers are becoming increasingly important to
the field of telecommunications and require
extremely pure SiO2 glass. Optical fibers are
composed of a core of pure silica surrounded by a
Optical Fiber
coating of silica doped with chemicals that lower
the refractive index of the glass. Light introduced
C ore
into the central core of the fiber is transmitted through it over long distances by a series of internal
reflections off the boundary between the core and the coating.
The glass most commonly encountered in windowpanes, mirrors, and light bulbs is soda-lime
glass. This glass is composed of silica (SiO2) with the addition of oxides of sodium (Na 2O) and calcium
(CaO) as fluxing agents. The role of fluxing oxides is to break the regular crystalline structure of the SiO 2
network lowering the glass melting point from 1723oC to a more workable 1300oC. Although soda-lime
Source: Dr. Patricia Hill, Millersville University
36
glass is fairly resistant to atmospheric corrosion it can be attacked and weakened or dissolved by strong
acids (H+1) and bases (OH-1). Another disadvantage of soda-lime glass is its high coefficient of thermal
expansion. Because of this, soda-lime glass cracks easily when heated and rapidly cooled. When working
with soda-lime glass it must be carefully annealed to relieve the internal stresses that result from the initial
rapid cooling and hardening of the glass. To anneal glass it is reheated after forming in a special oven to
the annealing temperature (depends on the glass composition). Then the glass is allowed to slowly cool at a
controlled rate to room temperature. Stress is relieved because the glass can still flow somewhat during the
annealing process.
Prince Rupert’s Tears
If soda-lime glass is melted and the molten glass is dropped into cold water, the rapid cooling of
the surface of the glass “freezes” the outside and contracts the molten center so much that a small hollow
bubble containing a vacuum appears in the center of the teardrop-shaped glass. Tremendous internal
stresses exist within the drop with the surface layer of glass being highly compressed and the internal
regions under great tension. The compressed glass is so strong that hitting the glass drop with a hammer
will not break the glass. However, a small scratch near the thin tail section will cause the entire bead to
shatter into powdered glass as the stress is quickly released.
To produce a glass that is more thermally resistant, oxides of boron (B 2O3) are added to the
mixture of Na2O and SiO2. Boron lowers the thermal expansion of the resulting borosilicate glass by as
much as 50% over soda-lime glass. This makes borosilicate glass highly resistant to cracking under
conditions of rapidly changing temperatures. Pyrex® or Kimax® glass is used for ovenproof baking dishes
and scientific glassware such as beakers, flasks, test tubes, and other items. Use of boron as a fluxing oxide
also lowers the melting point and the viscosity as well. Borosilicate glass is a highly durable and
chemically resistant glass.
The term lead crystal describes a type of glass that has been cut and polished to resemble natural
crystal faces. It is actually not a crystalline material, just an amorphous silica-based glass. Lead oxide
(PbO) is added to silica as a fluxing agent making the glass easy to melt and work. In addition the lead
imparts a number of unique properties to the glass. Lead glass has a higher refractive index than other
types of glass making it particularly good at refracting and reflecting light from its cut and polished
surfaces. Because of the presence of heavy lead atoms, this glass also is able to deflect various types of
high-energy radiation and consequently it is used in applications like TV and computer screens and
radiation windows. However, lead flux in glass can pose a health hazard because the toxic lead ions can be
leached from the glass when it is exposed to acidic conditions.
Coloring of Glass
When we look through a windowpane we probably consider the glass to be colorless and
transparent. This is true for thin layers of soda lime glass. However, if we were to look through a very
thick layer of the same glass we would notice that it was not colorless. In fact it would have a yellow-green
color that gets darker the thicker the glass becomes. The color comes from the presence of impurities of
iron oxides in the glass. If during the manufacturing process, other transition metal compounds are added
to the melted glass, various colored glasses can be produced. For example addition of selenium and zinc
oxide to glass will result in transparent red glass suitable for red stoplights. Deep blue glass, sometimes
called cobalt glass, results from the addition of small amounts of cobalt chloride(CoCl 2) to the glass. Color
can also be imparted to glass by suspending submicroscopic particles of metals in glass. Gold and copper
ruby glasses owe their bright red color to the presence of a colloidal suspension of these metals in the glass.
In general, copper compounds are used to color glass blue and blue-green, manganese produces pinks, and
iron compounds impart yellow, green or brown coloration (think beer bottles). Opaque or opalescent
glasses can be made by the addition of opacifiers. In small quantities they make the glass appear to have a
Source: Dr. Patricia Hill, Millersville University
37
pearl-like sheen and in larger amounts they make the glass totally opaque. Antimony, tin, and titanium
oxides are typically used for this purpose.
Other Forms of Glass
Thin layers of colored glass can be applied to the surface of metals in the process known as
enameling. Layers of colored glass melted onto the surface of pottery and ceramics are called glazes.
Both of these processes rely upon knowledge of glass composition and the colors associated with transition
metal compounds. Enamels and glazes are quite similar in that both are mixtures of colored metal oxides
mixed with glass frit. Glass frit is prepared by mixing the raw materials of glass (silica and fluxing
agents), melting them to form the glass, and then cooling the glass and grinding it to a powder (frit). The
melting temperature needed for the glaze or enamel should be lower than that of the substrate onto which
the glaze or enamel is placed (metal or ceramic) and this can be controlled by the flux composition of the
glass frit. True “stained glass” is actually the process of painting on glass with glazes that are then fired to
fuse them onto the surface layer of glass.
Deterioration of Glass
Although glass seems to be quite a non-reactive substance, it is susceptible to various forms of
attack. Depending upon the composition of the glass, it may be affected to a greater or lesser degree by
such things as water, acids, alkalis, complexing agents, sunlight, and “acid rain”. Even constant exposure
to high humidity (water vapor) can be damaging to glass. Microorganisms such as mosses, lichens, fungi,
and bacteria have also produced deterioration of glass surfaces most likely as a result of secretion of acidic
waste products during their growth. Deterioration of glass is exhibited through the presence of chemical
crust formation and pitting of the glass surface. Conservation of glass artifacts requires a thorough
knowledge of the chemical composition and physical and chemical properties of a wide variety of glass
types from ancient to modern.
REFERENCES
1. Conservation of Glass, Roy Newton and Sandra Davison, Butterworth-Heinemann Ltd., Oxford,
1996.
2. Glass and Glassmaking, Gerald J. Fine, J. Chem. Ed., 68, 765 (1991).
3. Crystals and Light, Elizabeth A. Wood, Dover Publications, 1977.
Study Questions for Chapter 7
1. How does glass differ from colorless crystals like quartz or diamond?
Important Chapter Terms




structural formula
diatomic molecule


molecular formula
non-polar covalent molecule



covalent bond
non-bonded electron
pair
valence or valency
covalent bond
crystalline



polyatomic ion
polar covalent molecule
amorphous







annealing
opacifying oxide
leaded crystal
silica




network-forming oxide
quartz glass
cobalt glass
borosilicate glass




electronegativity
refractory oxide
glass transition temperature
(Tg)
fluxing oxide
soda-lime glass
enamel
glaze
Source: Dr. Patricia Hill, Millersville University
38