Gli egiziani iniziarono una produzione seria del
colore da circa il 4000 aC. Hanno introdotto il
lavaggio dei pigmenti per aumentare la loro forza
e purezza. Hanno anche introdotto nuovi
materiali, il più famoso dei quali era il Blu Egizio –
il primo blu prodotto intorno al 3000 a. C.
l pigmento è un silicato di calcio e rame
(CaCuSi4O10) ottenuto mescolando una
sale di calcio (carbonato, solfato o idrossido),
un composto di rame (ossido o malachite) e
sabbia (silice).
Questo veniva riscaldato per produrre un vetro
colorato ridotto in polvere per l'uso. Le vernici
erano fatte utilizzando il pigmento di base con
gomme o colla animale, che lo rendeva
lavorabile e lo fissava alla superficie da
decorare.
Può essere impiegato nell’affresco. Sconsigliato
nella tempera, nell’olio e nell’encausto
Si può ottenere in modo relativamente
semplice se la calce minerale (CaCO3), la
sabbia (SiO2) e un minerale del rame (per
es. malachite (Cu2(CO3)(OH)2) o azzurrite
(Cu3(CO3)2(OH)2)) oppure del rame
metallico vengono esposti all’ossigeno
(O2) insieme con una piccola percentuale
di un fondente come carbonato di
potassio (K2CO3), sale (NaCl) o solfato di
sodio(Na2SO4), vengono riscaldati a
temperature tra 800 e 900 °C:
Nell'antico Egitto, molto spesso si usava come fondente il trona, una miscela di solfato di
sodio, soda (Na2CO3) e cloruro di sodio, è stato utilizzato nella sintesi. La presenza di
ossigeno (O2) dell’aria evita la formazione di rosso cuprite (Cu2O).
N.B. Le condizioni chimiche per la preparazione del Blu Egizio sono state
trasmesse con estrema precisione. La prova che questo è stato fatto si trova nella
straordinaria costanza della composizione chimica degli elementi Blu egizio in
oggetti d'arte risalenti a più di 2500 anni,
Gli Egizi usavano anche la malachite,
probabilmente il più vecchio pigmento verde
conosciuto, e l’azzurrite, un pigmento blu
verdastro. I due pigmenti sono chimicamente
simili comprendendo di base un carbonato di
rame (2CuCO3 • Cu(OH)2), e si trovano come
minerali naturali che gli Egizi convertivano in
pigmenti mediante frantumazione e lavaggio.
Sia l’azzurrite che la malachite
appartengono alla classe dei
carbonati di rame contenenti
idrossido. I minerali di carbonato
costituiscono una classe in cui gli
anioni (CO3)2- sono legati da vari
cationi nella cella elementare. In
particolare I carbonati di rame sono
carbonati minerali idrati che
contengono sia il catione rame (Cu2+)
che gli anioni ossidrilici (OH-).
L’Azzurrite ha formula chimica
Azurite: Cu3(CO3)2(OH)2 or Cu(OH)2 • 2(CuCO3)
Cu3(CO3)2(OH)2; la Malachite è
Malachite: Cu2CO3(OH)2 or Cu(OH)2 • (CuCO3)
Cu2(CO3)(OH)2.
Cella elementare
Azzurrite
Cella elementare
Malachite
I brillanti colori verdi e azzurri esposti da questi
due minerali sono dovuti all'inclusione del
rame, che è un agente pigmentante molto
efficace, tra i costituenti chimici. Questo tipo di
colorazione è chiamato idiocromatica.
La Malachite possiede un rapporto di 1:1
Cu(OH)2 : CuCO3, mentre l’azzurrite possiede
un rapporto di 1:2 per le stesse sostanze.
Poichè Cu(OH)2 è più ossidato rispetto CuCO3 e
la malachite possiede il più alto rapporto di
Quando sottoposta a lento riscaldamento, la
questo catione, la malachite si ossida più
malachite finemente triturata dà acqua e
facilmente dell’azzurrite. L'ossidazione più
frequente della malachite è responsabile del anidride carbonica (CO2), diventando alla
fine ossido rameico nero (CuO). Si scioglie in
suo colore verde brillante rispetto al blu
soluzione acida e rilascia CO2, ma rimane di
profondo dell’azzurrite.
colore verde. Mentre, reagendo con NaOH
caldo forma ossido rameico sulla superficie.
Tuttavia il pigmento non reagisce con NaOH
a freddo.
Malchite in polvere sottile viene lentamente
scurita anche dall’acido solfidrico (H2S).
Perugino made a
malachite green shirt on
a green earth
background.
Perugino, 1503, Museo Pinacoteca di
S.Francesco, Montefalco, Italy
Raphael, Madonna and Child Enthroned with Saints,
The Metropolitan Museum of Art
The azurite blue of the Virgin's mantle has
darkened due to its degradation into green
malachite and now this mantle looks greenish.
La
era la fonte dei minerali blu
ottenute attraverso l'estrazione in epoca antica e
medievale (lapis lazuli è chiamato in mineralogia
lazurite e contiene quantità variabili di calcio
(Na,Ca)8(AlSiO12)(S,SO4,Cl)). Nei tempi antichi, i
lapislazzulo è stato valutato per la sua stabilità e per
la sua brillantezza appariscente nei lapislazzuli
purissimi.
Due to its expense, ultramarine was often
reserved for iconographically significant figures
such as the Virgin Mary or Christ. To reduce
the amount of pigment used, and thus the
cost, ultramarine was often underpainted with
carbon black, azurite, indigo, or green
pigments such as green earth.
Lapis lazuli was in ancient times
mined in only one location, situated
in the area of present-day
Afghanistan (Badakhshan). It should
be mentioned that in ancient China,
the use of natural lapis lazuli was
not as common as in other cultures
(Persia, Mesopotamia, Egypt).
Lapis lazuli contains the mineral lazurite
(Na8[Al6Si6O24]Sn), which is responsible for its
bright blue color along with a variety
of accessory minerals including: calcite, pyrite,
amphibole, colorless pyroxene (often
diopside), haüyne, sodalite, forsterite,
muscovite, nosean, phlogopite, and
wollastonite.
The intense blue color is due to the presence of the S3radical anion in the crystal that partially replacing
sulfate or chloride anions in cages within the
tetrahedral aluminous-silicate framework. An electronic
excitation of one electron from the highest doubly
filled molecular orbital into the lowest singly occupied
orbital results in a very intense absorption at
λmax~617 nm.
The almost ubiquitous, but unstable
azurite is a mineral containing copper
(Cu3(CO3)2(OH)2). Depending on its
environment, it will eventually
transform into malachite, a green
pigment, and is unsuitable for outdoor
use.
In the traditional method of producing
pigment from lapis lazuli, as outlined by
Cennini, these minerals are removed by a
lengthy purification process which, when
repeated several times, results in several
grades of pigment, each less saturated in color
than the one before. The last, crudest, grade is
typically referred to as ultramarine ash.
Calcite generally remains present, even
following extraction by the Cennini method
Lazurite is easily identified by Raman
spectroscopy by the presence of a strong
band centered near 549 cm-1 (the S3symmetric stretching mode) and a weaker
band centered between 582 and 586 cm-1
(the S2- symmetric stretching mode).
Raman spectrum acquired from a 14th century
Italian illuminated manuscript, from the
Laudario of Sant’Agnese by Pacino di
Bonaguida; 785 nm excitation, 1.25 mW/μm2,
10 s acquisition. The starred peak is the main
diagnostic feature for ultramarine pigments,
and the band at 1049 cm-1 is likely due to the
presence of the pigment lead white (2PbCO3 ·
Pb(OH)2) mixed with the ultramarine.
Lapis lazuli (the mineral lazurite
(Na,Ca)8(AlSiO12)(S,SO4,Cl)) was in ancient
times the only stable and durable mineral
blue. Azurite, which is unstable, was often
used as a chemical substitute. Based on
findings of some groups, it is very likely
that Ultramarine Blue (typical formulation
Na6.9[Al5.6Si6.4O24]S2.0), which is chemically
very closely related to the mineral
lazurite, was artificially produced in
ancient China.
In the early 10th century AD, the blue mineral
vivianite (Fe3(PO4)2•8H2O) became a speciality
of the European medieval times, mined in
areas north of the Alps, and was used as a
pigment.
is an unstable iron phosphate mineral
with variable Fe(II/III) content that is
completely colourless if kept in an oxygen-free
environment (solely contains iron(II)) and that,
if exposed to oxygen, will over time oxidise
into blue and eventually brown compounds
with a higher iron(III) content.
Vivianite owes its diagnostic indigo-blue color to
intervalence charge transfer (IVCT) between iron ions
filling dimers of edge-sharing octahedra in the crystal
structure. IVCT indicates: the excitation of an electron from
a cation of low oxidation state and subsequent transfer to a
neighboring higher oxidation state cation. It is often
induced by visible light with a suitable wavelength and
produces a characteristic color. For the process to occur,
cation electron orbitals should overlap so the electrons can
hop back and forth.
However, the aging of blue pigments based on
vivianite in old paintings has lately become a
serious concern.
The mineral oxidation product of vivianite is
the brown santabarbaraite pseudomorphus, in
which half of the iron atoms are iron(II) and
the other half iron(III).
Cornelisz van Poelenburgh (1586-1667),
Adoration of the Shepherds; (Fig. 1, left).
in oil paint a color loss due to a
Fe2+→Fe3+
oxidation is also possible without
destabilization of the vivianite structure.
The robe of Virgin Mary is bleached by an alteration
of vivianite and ultramarine pigments. The cross
section (Fig. 2, right) shows the paintning`s
stratigraphy in the affected area (two underlayers of
altered, changed vivianite and an upper blue
ultramarine layer).
The area of ancient China in which, according
to the current state of knowledge, the
synthetic pigments Chinese Blue and Chinese
Purple, also called Han Blue and Han Purple,
were produced; the area extends on a
relatively limited territory about 200–300 km
north of the ancient city of Xian. Today it is
thought that, in this area in northern China,
Ultramarine Blue (‘‘artificial lapis lazuli’’) might
also have been produced (y≈800 BC)
Han Blue and Purple are compounds based on copper silicates, as well. The production of
Han Blue (BaCuSi4O10) and Han Purple (BaCuSi2O6) is generally more difficult than the
production of Egyptian Blue.
In a first step, a barium mineral
(generally barite (BaSO4) or witherite
(BaCO3)) was exposed for several
hours to quartz (SiO2), a copper
mineral and an essential lead salt
supplement at a temperature of 900–
1000 °C.
For the production of larger amounts of Han
Blue, these conditions are not applicable as
shown in the following typical equation, the
temperature will be higher by approx. 100 °C.
The limited availability and the high stability of
the rare barium minerals had a restrictive
effect on these syntheses.
Thus, it was necessary to reach relatively high
temperatures for the synthesis.
Lead salt additives serve a chemical double
function: on the one hand they assist the
catalytic decomposition of barium minerals at
lower temperatures and on the other they
serve as fluxes in a similar way to the additives
in the preparatives of Egyptian Blue.
The purple shade of Han Purple comes
from the red impurity of copper(I) oxide, Cu2O,
(mineral name cuprite), that is slowly
generated by the decomposition of Han
Purple, which probably happens as outlined in
the following chemical equation.
At temperatures of more than 1050 °C, this decomposition
takes place at a quite fast rate. The production of copper(I)
oxide depends on the conditions of preparation but, based on
the traced ancient synthetic procedures, this problem could not
be avoided in BaCuSi2O6 production in ancient China.
Han Purple is very unstable from a chemical
point of view. While the copper(I) oxide in Han
Purple stayed stable and a decomposition of
Han Purple progressed, the purplish colour of
the artefacts increased.
Pure BaCuSi2O6 (Chinese ‘‘Purple’’) (a), to
which cuprite (Cu2O) (g) was gradually added.
The samples (b–f) contain increasing
amounts of cuprite. According to the
increasing amount of cuprite, the
shade becomes more reddish. ‘‘Normal’’ Han
Purple, which is produced in a synthesis at
approx. 1000 °C, comparable to the
syntheses conducted in ancient times, is, in
terms of the colour shade, similar to sample
(c).
The two chemically very closely related compositions CaCuSi4O10 (Egyptian Blue) and
BaCuSi4O10 (Han Blue) differ only in the way they exchange the earth alkali element.
The two compounds have the same basic structure and very similar properties. They both
have layered structures with (SiO)4 silicate squares forming the structural framework. Four
of those (SiO)4 four-ring units form new (SiO)8 four-ring units through condensation and
new connections.
Opposite four-ring units become so close that their terminal Si–O- groups take up a copper
ion in a square planar arrangement.
These copper ions are the colouring
agent (chromophore).
They are very tightly bound in the
stable silicate matrix and cannot be
removed easily by chemical and
physical means. This tight binding is
the key to the high stability of
Egyptian and Han Blue.
Heat, strong acids and light cannot
harm these two pigments.
The chromophore copper is not a very efficient colouring agent, however;
consequently, it needs quite a fair amount of material to reach a certain
intensity in colour. The layered structure leads to the formation of platelet
crystals that feature anisotropic interaction with light. The crystals show
dichroism, which can be seen in the substance’s lighter appearance when
grinding is applied. Different shades of darker or lighter blue can thus be
produced
Han Purple features a layered
structure, as well, but its framework
differs greatly from the structure of
Han Blue. Its basic units are isolated
(SiO)4 four-ring units, whose terminal
oxygen atoms bind two connected
copper atoms. This results in the
formation of an infinite arrangement
of Cu2 units
Owing to its Cu–Cu bond structure, Han Purple
has a low chemical stability. Even weak acids
wear and bleach it; hence, a light-blue mixture
of Ba/Cu oxalate is formed under the influence
of oxalic acid that may occur under natural
circumstances, for instance by excretion of
certain micro-organisms and lead to
destruction of pigment layers of paintings
containing Han Purple.
Artificial lapis lazuli: Ultramarine Blue.
Lapis lazuli (the mineral lazurite
(Na,Ca)8(AlSiO12)(S,SO4,Cl) in altri
termini è un calcare mineralizzato
contenente dei cristalli cubici di lazurite)
was in ancient times the only stable
and durable mineral blue. Azurite,
which is unstable, was often used as a
chemical substitute. Based on findings
of our group, it is very likely that
Ultramarine Blue (typical formulation
Na6.9[Al5.6Si6.4O24]S2.0), which is
chemically very closely related to the
mineral lazurite, was artificially
produced in ancient China.
Nowadays, Ultramarine Blue can be very easily
obtained in the presence of sodium salts
(sodium carbonate), sulfur compounds and in
alkaline conditions compounds and in alkaline
conditions.
The blue colouring of sulfur radical ions was
probably produced by the reduction of the
existing sulfate with spectroscopically detected
carbon particles, which were contained in the
plant ashes (basically potash K2CO3 and soda
Na2CO3). Ultramarine Blue is generated at
relatively low temperatures (400–600 °C); this
has been done since the early 19th century,
when the industrial production process first
came into use.
The chromophore of Ultramarine Blue and lapis lazuli is the blue S3-2 radical ion
incorporated in sodalite cage structures ([Al6Si6O24]-6). This naturally very unstable
ion becomes very stable in its incorporated form by chemical protection in a solid
matrix. Due to its chemical similarity, Ultramarine Blue is also denominated
‘‘artificial lapis lazuli’’. The surrounding matrix does not significantly influence
the essential colour properties of the S3- ion.
Ultramarine Blue and lapis lazuli feature a
significantly higher degree of light absorption
than copper silicate pigments. The blue tone is
very intense, even if prevalent only in small
amounts in pigment mixtures. However, it must
be highlighted that the yellow S2- radical is also
unavoidably incorporated during any kind of
preparation, as is the case with natural lapis
lazuli. A higher ratio of S2- radicals ‘‘dilutes’’ and
lessens the blue pigment properties or may, in
still higher concentrations, lead to a greenish
tone due to the mixture of blue and yellow or
even to a green pigment that previously has not
been used. The rare natural green tone of
mineral lapis lazuli leads to a considerable
enhancement in value.
Maya Blue was developed by the Indian
cultures of Central America on a very
different chemical basis than the
aforementioned pigments. It is derived from
the colour indigo, which was already
known in ancient times. By means
of a ‘‘high-tech’’ process, the Indians
embedded indigo in white clays
(palygorskite ((Mg,Al)4Si8(O,OH,H2O)24) or
sepiolite), for which temperatures between
150 and 200 °C were necessary.
Indigo
The pigment was made by burning
incense made from tree resin and
using the heat to cook a mixture of
indigo plants and a type of clay called
palygorskite. As the mixture was
heated, indigo molecules filled a
network of tiny channels inside the
clay. Some of these bits of indigo
plugged the pores on the surface,
preventing the color from escaping
over time.
The clay, in turn, protects the indigo
from the environment. Harsh
chemicals can destroy a sensitive bond
within indigo molecules -- changing
the color from blue to yellow. Like the
double-parked car that prevents you
from opening the driver-side door to
your own car, the clay channels take
up the space around the bond,
blocking these chemicals.
Modular structure of Palygorskite (left) and
Sepiolite (right) viewed along the c-axis. H2O in
the channels is not shown.
In tube-like channels of
palygorskite or sepiolite clays,
indigo molecules are
intercalated. The guest
molecule indigo is the
chromophore. It has long
been argued whether the
indigo molecules are located
in the core of the clay or
whether they are adsorbed
tightly at the clay’s surface.
Thorough investigations have
shown that the indigo
molecules are clearly
incorporated in the clay.
The channels which take these molecules up provide much more efficient chemical
protection than residing on the surface. In contrast to ‘‘free’’ indigo, intercalated indigo
thus does not fade, even under harsh conditions. For instance, the photochemical
properties of the guest indigo had improved to such an extent that Maya Blue became a
valuable pigment suitable for outdoor use. Some of those stable indigo clays are even
more brilliant in appearance than natural indigo, which is considered to be due to the
presence of iron nanoparticles formed during processing from the indigo plant raw
materials.
Thee fading of indigo paint is caused by the
chemical breakdown of the component:
indigotin. The extent to which this process
takes place depends on a variety of factors.
Frans Hals, St. George civic guard
1627, Frans Hals Museum. The
indigo has faded to a greyish light
blue.
Frans Hals, St. Adrian civic guard,
1627, Frans Hals Museum. The indigo
blue colour of the sashes has been
well preserved.