Pigment Questions
1. Know the meaning of the terms pigment and dyes. How are they similar? How are they
different?
Pigments and dyes are similar in that they impart color to a medium. Pigments are
suspended in a binder (such as linseed oil). While pigment and binders works well on wood,
stone, ceramics, and mounted fabrics, it works poorly on flexible materials that were meant
to be worn. To color such fabrics one needs dyes. Dyes work into the fabrics and retain the
flexibility of the fabric.
2. Know the color and chemical structure for the pigment verdigris.
Verdigris is a green pigment copper (II) acetate.
3. What is the oxidation state of copper in copper acetate?
Copper (II) = Cu2+
4. What ingredients did you use to make verdigris?
Copper metal and vinegar.
5. What is vinegar, chemically? What is its chemical structure?
A dilute (5%) aqueous solution of acetic acid, CH3CO2H
O
H3C C
OH
6. Where does vinegar come from?
Vinegar comes from wine that has gone bad. If wine is left exposed in a non-sterile
environment, bacteria can get in. These bacteria metabolize the ethanol to ethanoic acid,
a.k.a, acetic acid. The conversion of ethanol to acetic acid is an oxidation.
7. What is an acid?
An acid is a substance that gives up (or donates) a proton, H+.
8. Write the chemical equation that shows how acetic acid gives a proton.
O
O
O
+
H
O
acetate
anion
acetic
acid
H
proton
8. What is the pH? How is it defined?
pH is a measure of the concentration of the hydronium ion, [H3O+]. The definition of pH is
pH = –log [H3O+]
9. What is the pH of an acetic acid solution? Less than 7, equal to 7, or greater than 7?
pH < 7
10. What chemical reaction takes place when copper reacts with acetic acid?
Cu +
2 CH3CO2H
+
Cu (CH3CO2)2
H2
11. What has happened to the copper in this reaction?
The copper has been oxidized. It has lost two electrons.
12. What happened to those electrons?
They are used to reduce the H+ to hydrogen atoms:
Cu
+
H
Cu
+ Cu
+
H
Cu
+ 2H
Cu
Cu
+
H
2+
+
H
2+
+
H2 (gas)
Notice that the two hydrogen atoms (H·) combine to form hydrogen gas.
13. What type of reaction is this?
This is a redox (reduction-oxidation) reaction. The copper is oxidized and the acetic acid
reduced. Note that reduction–oxidation reactions always occurs in pairs: one species is
reduced and one is oxidized.
14. The verdigris you made was initially a 1:1 mixture of copper acetate and copper
hydroxide. How did you make this in to pure copper acetate?
Recrystallization from vinegar.
Dye Questions
1. Know the meaning of the terms pigment and dyes. How are they similar? How are they
different?
Pigments and dyes are similar in that they impart color to a medium. Pigments are
suspended in a binder (such as linseed oil). While pigment and binders works well on wood,
stone, ceramics, and mounted fabrics, it works poorly on flexible materials that were meant
to be worn. To color such fabrics one needs dyes. Dyes work into the fabrics and retain the
flexibility of the fabric.
2. Know the meaning of the terms fugitive and colorfast.
A fugitive dye is one that doesn’t hold its color after washing. A colorfast dye is one that
remains even after washing.
3. Why must wool be washed before it can be dyed?
Wool contains a natural grease called lanolin. Washing with soap removes the lanolin that
would otherwise interfere with the ability of the dye to adhere to the fiber.
4. What is soap? Know its chemical structure.
Soap is a particular molecule (aliphatic carboxylic acid) that has both a polar, water soluble
component (the carboxylic acid) and a greasy, non-polar component. Normally, water and
grease don’t dissolve in each other and so water can’t remove grease. Soap dissolves both
grease and water, so that water-soap mixtures can remove grease and oil.
O
O
hydrocarbon end
water repeling
Na
ionic end
water attracting
5. Know the meaning of the terms direct dyeing, mordant dyeing, and vat dyeing. What are
the similarities and what are the differences?
Direct, mordant, and vat dyeing are three different ways for coloring fabrics. In direct
dyeing the dyestuff is applied directly to the fabric. In mordant dyeing a mineral is used to
help the dye bind to the fabric. In vat dyeing the dye is chemically modified to make it water
soluble.
6. Give an example of each of the dyeing methods that you carried out in the laboratory.
Direct: dyeing with walnut shells
Mordant: dyeing with cochineal
Vat: dyeing with indigo
7. What is the main chemical responsible for the color that comes from walnut dye? Know its
name, chemical structure, molecular formula, and its color.
The main colored material in the walnut dye is juglone. It makes a brown dye on exposure
to air.
OH O
OH
C
C
C
C
C
H
C
H
H
=
O
O
C
C
O
C
C
H
H
Note that the “skeletal” structure on the left is the normal way we would written it, but it
contains hidden information: carbons at the vertices and hydrogens added until each carbon
is tetravalent. From is structural drawing we can see that it has 10 Cs, 6 Hs, and 3 O’s =
C10H6O3.
8. What is the main chemical responsible for the cochineal dye? Know its name, chemical
structure, molecular formula, and its color.
The main colored material in the cochineal is carminic acid. It is a deep red dye.
OH
O
HO
HO
OH
O
OH
OH
OH
HO
O
CH3 O
OH
Its molecular formula is C22H20O13.
8. Know the meaning of the term mordant. Give some examples of mordants.
Many dyes can be made colorfast by the use of certain minerals called mordants. Potash
and soda ash can in some cases serve as mordants, but they are not commonly used because
they are basic and thus tend to harm the fabric. Much milder and far more common is the
aluminum mineral alum.
9. Know the chemical names and formulae for potash, soda ash, and alum.
potash
soda ash
potassium carbonate
sodium carbonate
K2CO3
Na2CO3
alum
potassium aluminum sulfate KAl(SO4)2
10. Know the common oxidation state of potassium, sodium, aluminum, carbonate, and
sulfate.
For simple monoatomic atoms like potassium, sodium, and aluminum the common
oxidation states can often be determined from the atom’s position on the periodic table. For
polyatomic species like carbonate, and sulfate the structures and oxidation states needs to be
dedicated to memory.
potassium
sodium
aluminum
carbonate
sulfate
K+
Na+
Al3+
CO32–
SO42–
+1
+1
+3
–2
–2
11. Know how to draw Lewis structures for carbonate and sulfate.
O
O
C
O
O S O
O
O
S 1 x 6 e¯ = 6
O 4 x 6 e¯ = 24
+ dianion 2 e¯ = 2
valence e¯
32
C 1 x 4 e¯ = 4
O 3 x 6 e¯ = 18
+ dianion 2 e¯ = 2
valence e¯
22
12. Know what alum is and how it works.
Alum is water soluble aluminum salt potassium aluminum sulfate that functions as a
mordant to help a dye bind to fabric. The important part of alum is the Al3+. The K+ and
SO42– are “spectator” ions. They are not directly involved in the chemical binding, but they
do help make the aluminum water soluble.
Aluminum binds strongly to oxygen. The mordanting process involves two steps. During
the first step in which the fabric is soaked in an aqueous solution of alum, the aluminum
binds to the fabric. During the second step in which the mordanted fabric is soaked in a dye
solution, the dye binds to the aluminum.
OH
OH
OH
OH
OH
alum
O
Al
OH
O
O
Al
solution
fiber
O
O
dye
O
Al
dye
O
O
O
solution
mordanted
fiber
dyed
fabric
Al
dye
O
13. What is indigo? Know its name, chemical structure, molecular formula, color, and where is
comes from.
Indigo is a blue dye made from the plant Isatis tinctoria or Indigofera tinctoria. It can also
be made synthetically in the laboratory. Based on the structure below its MF is C16H10N2O2.
O
N
H
H
N
O
14. Describe how vat dyeing of indigo works.
Indigo and related compounds such as Tyrian purple are highly insoluble in water and most
other solvents. Lacking the ability to dissolve in water, indigo can not function as a dye.
However, when indigo is placed in a reducing solution such as dithionite (Na2S2O4), it is
reduced to leucoindigo. In the reduction process, the indigo gains an extra electron and is
therefore an anion that makes it water soluble. Now that it is soluble it can move wherever
the water can go, i.e. into the fibers. The leucoindigo lacks color and stays colorless as long
as is remains in the reducing solution. However, leucoindigo is not stable and when the
fabric is removed from the reducing conditions it is exposed to oxygen in the atmosphere and
spontaneously reverts to the blue indigo. The whole process is known as vat dyeing. The
chemistry can be summarized in the following chemical reactions:
O
N
H
H
N
O
S2O42–
N
H
O
H
N
O
leucoindigo,
colorless
indigo,
blue
O
O2
N
H
H
N
O
indigo,
blue
Glass Questions
1. What is the main ingredient in glass?
Silicon dioxide, SiO2.
2. What is the three dimensional structure of silicon dioxide?
Each silicon atom is bound to four oxygens and each oxygen is bound to two silicon atoms,
i.e. the silicon is tetravalent and the oxygen is divalent. Both kinds of atoms are essentially
tetrahedral but in the case of oxygen there are two lone-pairs of electrons.
3. What is the difference between calcite (SiO2) and obsidian (also SiO2)?
In calcite the atoms are aligned in a regular, orderly, repeating pattern and is said to be
crystalline. In obsidian the silicon oxide units are arranged in a random, disordered pattern
and is said to be amorphous. This amorphous solid is also called a glass.
4. Under what conditions are crystalline materials made? Glassy materials?
Crystals form when liquid or molten materials cool slowly. Glasses form when liquid or
molten materials quickly.
5. Why is pure silica not often used in glassmaking?
Its melting point is too high (1710˚C, 3110˚F).
6. What is the chemical composition of soda-lime glass?
Soda-lime glass consists of 10–20% soda (NaO), 5–10% lime (CaO), and 60–70% silica
(SiO2).
7. What is the function of the soda in soda-lime glass?
Soda acts as a flux to lower the melting point of the silica.
8. What is this function of the lime in soda-lime glass?
The sodium in glass actually weakens the glass because sodium cations are highly water
soluble. The calcium “stabilizes” the glass because calcium cations have poor solubility in
water.
9. Soda and lime are generally too unstable to be found pure in nature. What are the actual
sources of soda and lime?
Both soda and lime are made by calcining (roasting) their carbonates during the glassmaking
process:
CaCO3
calcium
carbonate
"limestone"
Δ
CaO
calcium
oxide
"lime"
Na2CO3
sodium
carbonate
"soda ash"
Δ
Na2O
sodium
oxide
"soda"
10. Soda-lime glass is the most common type of glass in ancient and modern times. The glass
you made in the lab however is a more recent invention, a borosilicate glass. What
ingredients go into borosilicate glass?
Borax, B4O5(OH)42–, and silica, SiO2.
11. What is the purpose of the borax in the borosilicate glass?
Borax is the flux that lowers the melting point of the mixture.
Lost Wax Questions
1. What is wax?
There are many types of wax (beeswax, paraffin wax, etc.) and almost are mixtures of waxy
compounds. The common feature of all waxes is a long hydrocarbon chain. They are nonpolar and repel water.
2. How does the lost wax process work?
A model is first carved in wax. The wax model is “invested” in plaster (see below) or clay.
The wax is burned away when the plaster or clay is fired to leave a mold. Molten glass or
bronze can then be cast into the mold.
3. What is the chemical reaction that takes place when the wax is lost? Balance the reaction.
For a paraffin wax with 25 carbons,
C25H52 + 38 O2
Δ
25 CO2 + 26 H2O
Glaze Questions
1. What is glaze?
Glaze is a glass like material that binds to the surface of ceramic vessels.
2. What is the chemical composition of glaze?
There are thousands of different recipes for glazes, but they all have the same basic
ingredients: (a) silica (SiO2), (b) a flux, such as soda (sodium oxide, Na2O), galena (lead
sulfide, PbS), or cassiterite (tin oxide, SnO), and (c) a colorant such as malachite (copper
carbonate, Cu(CO3)).
3. When is a ceramic glazed?
The glaze is usually an external coating either painted or dipped onto a ceramic that has
already been fired. Thus, a second firing is required to bake the glaze onto the ceramic.
4. What is Egyptian paste?
Egyptian paste is a “self-glazing” clay-like material. It is plastic (moldable) when wet, but
dries firm. The glaze is not coated coated on the outside as described above. Rather the
glaze ingredients (silica and flux) are mixed into the wet clay. As the water evaporates from
the clay, the water and flux (soda ash) are drawn to the surface. When fired the glaze forms
only at the surface, where both silica and flux are found.
Plaster Questions
1. What is plaster?
Plaster is an ambiguous term with at least two different meanings. It can refer to either lime
plaster or plaster of Paris. The two plasters have chemically distinct compositions.
2. What is plaster of Paris?
Plaster of Paris is calcium sulfate hemihydrate, CaSO4 • ½ H2O.
3. What is the oxidation state of calcium in plaster of Paris?
Calcium (II) = Ca2+
4. Know how to draw Lewis structure for sulfate.
O
O S O
O
S 1 x 6 e¯ = 6
O 4 x 6 e¯ = 24
+ dianion 2 e¯ = 2
valence e¯
32
5. What is the “• ½ H2O” in CaSO4 • ½ H2O?
The “dot water” refers to the “waters of hydration”, or the amount of water in the crystal. In
this case there is “half a water molecule” for every molecule of calcium sulfate. It makes
more sense to think of this as two molecules of calcium sulfate sharing one water molecule in
its crystal.
6. How is plaster of Paris made?
Plaster of Paris is made from by heating a mineral called gypsum, CaSO4 • 2 H2O, to about
150˚C Some, but not all, of the water is driven off:
CaSO4 • 2 H2O
150˚C
CaSO4 • 1/2 H2O
(At higher temperatures it is possible to drive off all the water and create anhydrous calcium
sulfate, but this material is not as useful as a plaster ingredient.)
(A large deposit of gypsum is found in Montmartre in Paris, hence the our name plaster of
Paris.)
7. How is plaster of Paris used?
Plaster of Paris is a dry powder. After water is added, it liberates heat and becomes a
moldable (“plastic”) material that hardens slowly into gypsum, which can be further worked
by carving or sanding because it is a relatively soft material. In effect this completes the
cycle of gypsum to plaster of Paris to gypsum:
CaSO4 • 2 H2O
gypsum
calcium sulfate
dihydrate
150˚C
CaSO4 • 1/2 H2O
plaster of Paris
calcium sulfate
hemihydrate
H2O
CaSO4 • 2 H2O
gypsum
calcium sulfate
dihydrate
Typically, plaster of Paris is mixed with various amounts of silica (SiO2) to give it more
strength.
8. What is lime plaster?
Lime plaster is “slaked” lime with silica added to give it strength. The chemical formula
for slaked lime is Ca(OH)2, also known as calcium hydroxide.
9. Where does lime plaster come from?
Slaked lime, or lime plaster, is made from a mineral called limestone, which chemist call
calcium carbonate, CaCO3.
10. Know the Lewis structure for carbonate.
O
O
C
O
C 1 x 4 e¯ = 4
O 3 x 6 e¯ = 18
+ dianion 2 e¯ = 2
valence e¯
22
11. How is the limestone processed to give lime plaster?
Limestone is first calcined (roasted) at 850˚C to drive off CO2 giving lime (sometimes
called quick lime), which chemists call calcium oxide.
850˚C
CaCO3
calcium
carbonate
CaO
+
CO2
calcium
oxide
The calcium oxide is unstable in the presence of moisture. It reacts with water to give
hydrated, or slaked, lime:
H2O
CaO
Ca(OH)2
calcium
oxide
"quick lime"
(+
heat )
calcium
hydroxide
"slaked lime"
12. How does slaked lime react with the air?
Slaked lime is unstable in air and slowly reacts with the carbon dioxide in the air to form
calcium carbonate,
CO2
Ca(OH)2
calcium
hydroxide
"slaked lime"
CaCO3
calcium
carbonate
"limestone"
thus completing the cycle from limestone to quick lime to slaked lime back to limestone:
CaCO3
850˚C
"limestone"
CaO
"quick lime"
H2O
Ca(OH)2
"slaked lime"
CO2
CaCO3
"limestone"
13. What is the point of converting limestone to limestone?
The slaked lime plaster is a “plastic” material, one that can be molded and shaped. Then,
with time, it slowly converts to a hard, durable mineral.
14. How is slaked lime used?
Slaked lime has many uses. When mixed with water it forms a spreadable plaster that can be
used in art, architecture, and building. Slaked lime is almost always admixed with a variety
of materials to make it stronger. When mixed with course sand, one gets mortar, a strong
“cement” used by bricklayers. When mixed with finely ground sand (called “flint” by
potters), one gets the material used to plaster a wall, which can be adorned with a fresco
painting.
India Ink Questions
1. What is India ink? What color is it?
India ink is a black, aqueous suspension of soot. It is also known as lamp black.
2. How is soot made?
Soot is the product of incomplete combustion. Most organic materials burn completely in
oxygen to give carbon dioxide and water (and nitrogen oxides, if nitrogen is present).
However, if the burning material does not mix thoroughly with oxygen, the carbon is not
oxidized all the way to carbon dioxide, but rather to fine, black, graphite–like material
(mostly carbon).
3. What is gum arabic and what is its function in India ink?
Gum arabic is a starchy material that comes from the Acacia tree in Ethiopia. It dissolves
readily in water. Soot particles, which are not water soluble, are suspended in gum arabic.
Thus, the function of the gum arabic is to bring the black pigment into solution.
Paper/Papyrus Questions
1. Why does water have such a high boiling point compared to molecules of similar weight?
The attractive forces between molecules of, say, methane (MW = 16 g/mole) are very weak
compared to the attractive forces in water (MW=18). It is easy to over the attractive forces
between methane molecules, which happens at its boiling point of –162 ˚C, but much are
harder to overcome in water, which has a boiling point of +100˚C.
2. What intermolecular force is holding the water molecules together?
Hydrogen bonding.
3. What is hydrogen bonding?
Hydrogen bonding is a special intermolecular force between electronegative atoms and
hydrogens atoms on electronegative atoms, for example, between lone-pair on O and the H–
O bond. It is often drawn as a dipole–dipole attraction:
H
O
••••• H O
H
H
hydrogen
bond
4. How strong are hydrogen bonds?
Relatively weak, about 1-5 kcal/mol. Covalent bonds are typically much stronger, 80-100
kcal/mol. However, many hydrogen bonds taken together can be quite strong.
5. What is the primary chemical component of paper and papyrus?
The main chemical component in paper and papyrus in cellulose.
6. What is cellulose?
Cellulose is a strong rigid polymer made from monomeric glucose units:
7. What holds the strands of cellulose together in paper and papyrus?
A network of hydrogen bonds.