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DOI: 10.1039/B606268G (Tutorial Review) Chem. Soc. Rev., 2007, 36,
15-30
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The invention of blue and purple pigments in ancient times
Heinz Berke
Institute of Inorganic Chemistry, University of Zurich,
Winterthurerstrasse 190, 8057 Zurich, Switzerland. E-mail:
hberke@aci.unizh.ch; Fax: +41 44 635 68 02; Tel: +41 44 635 46 80;
Received 2nd May 2006
First published on 12th October 2006
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Abstract
This tutorial review examines manmade blue and purple pigments
appearing in antiquity. They were obtained by chemical synthesis from
mineral starting materials and refer to chemical compounds: Egyptian
Blue (CaCuSi[4]O[10]), Han Blue (BaCuSi[4]O[10]) and Han Purple
(BaCuSi[2]O[6]), Maya Blue (x*indigo*(Mg,Al)[4]Si[8](O,OH,H[2]O)[24])
and Ultramarine Blue (Na,Ca)[8](AlSiO[12])(S, SO[4],Cl). The Egyptian
and Chinese copper-based pigments are assumed to have been developed
independently and are presumably an outcome of the historical
developments in glazing techniques. A technology transfer from Egypt
into China cannot be fully excluded but, based on the facts acquired
up to now, looks less probable.
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H. Berke received his Diploma in Chemistry at the University of
Erlangen (Germany) in 1971 and his PhD at the University of
Tubingen (Germany) in 1974. From 1974-1988 he was at the
University of Konstanz (Germany) with an intermediate stay in
the Laboratory of R. Hoffmann, Cornell University, Ithaca (USA)
in 1977. In 1981 he finished his Habilitation and in 1983 he
was awarded the Heisenberg fellowship from the "Deutsche
Forschungsgemeinschaft" and the Dozentenpreis of the Fonds der
Chemischen Industrie (Germany). In 1987 he was promoted to a C2
Professor at the University of Konstanz before he joined the
University of Zurich (Switzerland) in 1988 as a full professor
of Inorganic Chemistry. In 1991 he became director of this
Heinz institute and stayed at this position till present.
Berke
H. Berke is member of the editorial boards of the journals
Heinz Dalton Transactions and Mendeleev Communications and is
Berke presently president of the Division of Chemical Research of the
Swiss Chemical Society.
H. Berke's fundamental research activities cover various fields
of organometallic chemistry. Major efforts are devoted to the
area of transition metal hydrides, which is related to
homogeneous catalysis, in particular homogeneous hydrogenations
and hydrosilations. Metal-carbon oriented activities concern
several catalyses of C-C coupling reactions mediated by
transition metal complexes and in addition metallacumulenes,
where carbon chained units are sought to space transition metal
centers for potential use as single-electron devices. Another
research field deals with the archaeometry of ancient, manmade
blue and purple pigments.
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1. Blue and purple
1.1 Art, matter and blue in particular
Colours are an intrinsic part of human life. They produce aesthetic
stimulation and they fascinate. They are the means of expression in
art and they form part of the human culture. The blue colour,
however, differs from all the other colours. At all times in history,
people ascribed to it a special dimension.^1 Yves Klein, the "blue
entrepreneur", who in 1957 started using solely blue for his
paintings and sculptures, put it like this: Blue has no dimension, it
exceeds everything... All colours evoke associations..., whereas blue is
reminiscent of the sea and the sky, which are the most abstract parts
of the tangible and visible nature. The opinion of this art expert
has not been seconded unanimously; yet, its mere existence may
highlight the fact that blue is considered to be different from all
other colours.
The materialised forms of colour are dyes and pigments. They form the
material basis of art. Despite this fundamental role as conditio sine
qua non, the role of matter in art was due to a very rigoristic way
of thinking only marginally accepted during the European Middle Ages,
the Renaissance and still in the early 18th century.
During times of the theological hegemony and according to the
viewpoint of spirit-matter dualism, both matter as the material basis
and the craft of matter, which is in chemistry, alias alchemy was not
ascribed much importance. "Vulgar" chemistry (alchemy) was due to
people's disrespect of its ideational properties not sufficiently
recognised as a science working in favour of art. Chemistry (alchemy)
was made use of in artists' studios, but it lacked prestige, which
was also shown by its de facto exclusion from the academic world. A
simple example shows, however, to what extent chemistry (alchemy)
still managed to have an ideational influence on art: The blue
pigment lapis lazuli features, as a mineral, changing amounts of
various impurities; thus, it is usually not suitable for use in
painting without its undergoing (al)chemical separation processes.
Hence, in medieval times, there were several procedures to purify
lapis lazuli, some of which were kept secret. The use of chemistry
(alchemy) allowed improved colour saturation and brilliance, which
resulted in better expression in art.
Only toward the end of the 18th century, did science become liberated
from the theological hegemony. Science's coalition in the battle for
recognition was rewarded with a massive leap forward.^2 People grew
more prepared to make use of scientific cognition. And, as chemistry
was becoming more rational, there was a growing insight that art and
chemistry complement one another with mutual advantages for both
sides. As a consequence, the high-quality industrial pigments were
developed, some of which had been available since the beginning of
the 19th century, and could be used without restraint as of the
second part of the 19th century; this was one of the largest benefits
chemistry had on art. A silent revolution had taken place and new
possibilities for expression advanced the impressionistic style.
In the co-evolution of art and science, both sides generally reaped
more benefits from the other than they were willing to acknowledge.^3
For ancient cultures, art and science were one entity; this was
mainly due to their state of development, which was still defined
mostly by the necessities of life and their technical possibilities.
The craft of chemistry (alchemy) was, without reservation, used as
the material basis for painting. This was the case in particular for
the blue colour and the related purple colour, as the material basis
in the form of herbal and animal constituents and minerals for these
two colours were, as opposed to the elements of all other colours,
not sufficiently available. This lack of resources challenged
respective craftsmen at the time, being alchemists, to contrive
innovative ideas in many ways.
1.2 The blue minerals in ancient and in medieval times
As mentioned previously, the availability of colouring substances
played a decisive role in cultural developments and in art. In
prehistoric times, only the so-called earth colours, colours provided
by the surface soil, could be used as pigments. Blue is not an earth
colour and was therefore not available to prehistoric humans as a
pigment. Visitors of prehistoric caves, such as the caves of Altamira
in Spain or Lasceaux in France, thus notice that in the paintings on
the walls of the caves, there is no blue colour.^4 In antiquity, the
palette of available pigments could be expanded with the help of
mining. The trade of mining required specialised mineralogical
expertise and technical developments as civilizing achievements,
which had been largely unchanged up to the Middle Ages.
The gemstone lapis lazuli was the source of the blue minerals
obtained through mining in ancient and medieval times (lapis lazuli
is in mineralogy called lazurite and contains variable amounts of
calcium (Na,Ca)[8](AlSiO[12])(S,SO[4],Cl)). In ancient times, lapis
lazuli was valued for its stability and for its brilliance apparent
in very pure lapis lazuli. The almost ubiquitous, but unstable
azurite is a mineral containing copper (Cu[3](CO[3])[2](OH)[2]).
Depending on its environment, it will eventually transform into
malachite, a green pigment, and is unsuitable for outdoor use. 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). The
factual reasons are not known; however, it may be due to the fact
that the Chinese disposed of artificially manufactured blue and
azurite, the latter in almost unlimited amounts. Furthermore,
minerals containing cobalt that only after a manufacturing process in
glasses and glazes transformed into a blue colour were used.
Probably in the early 10th century AD, the blue mineral vivianite (Fe
[3](PO[4])[2]*8H[2]O) became a speciality of the European medieval
times, mined in areas north of the Alps, and was used as a pigment.^5
Vivianite is an unstable iron phosphate mineral with variable iron(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. Nowadays, this mineral
would not be used as a pigment due to its too low stability. The use
in the European Middle Ages can be explained only with the lack of
suitable alternatives. Blue vivianite may be found mainly in the
vicinity of ore deposits near the Earth's surface and in pegmatite
deposit areas if waters containing phosphates reach that area. 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).
1.3 Necessity has been the mother of invention--especially in ancient
times
The notable German art historian J. J. Winckelmann described antique
art with the phrase Noble simplicity and quiet grandeur (Edle Einfalt
und stille Grosse). He acted on the erroneous assumption that
polychromy of art objects was not of real importance for the antique
human being. The contrary was the case:^6 colour and painted objects
were valued greatly in ancient times and their frequent use
aggravated the lack of, mainly the rare, blue colour materials. Only
by the end of the 18th century was there a fundamental change in the
special situation of the scarcity of blue pigments; new blue pigments
were invented and later, at the outset of industrialisation, many
more new blue and purple materials were created chemically. Also,
known natural materials could be produced artificially on a large
scale in sufficient amounts. Necessity is the mother of invention!
According to the current state of knowledge, there were three
geographic areas in ancient times in which special blue and purple
pigments were contrived and produced (see also atlas for the
distribution of Egyptian Blue and Han Blue and Purple (Fig. 21) and
Fig. 1):
* The Mediterranean area, incl. Egypt, as of approx. 3600 BC, the
Middle East (Mesopotamia, Persia), and later also the areas of
ancient Greece and the Roman Empire, where Egyptian Blue (Fig. 1) was
produced^7 and cobalt was used to colour glasses and glazes and was
also occasionally used as a pigment in a glass-bound form (smalt).
* The area of ancient China in which, according to the current state
of knowledge, the synthetic pigments Chinese Blue and Chinese Purple
(Fig. 1), 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 (~800 BC) (Fig. 2).
* The area of Middle America with the Indian cultures, which, as of
approx. 400 AD, produced Maya Blue (Fig. 2), an intercalation
compound of indigo into the white clays of palygorskite.
Egyptian Blue (CaCuSi4O10, left), Han Blue (BaCuSi4O10, centre) and
Han Purple (BaCuSi2O6, right). Under similar conditions, the
substances Egyptian Blue and Han Blue strongly resemble one another.
The smaller the grain size, the lighter is its appearance. Egyptian
Blue is ground whereas Han Blue is granular crystalline.
Fig. 1 Egyptian Blue (CaCuSi[4]O[10], left), Han Blue (BaCuSi[4]O
[10], centre) and Han Purple (BaCuSi[2]O[6], right). Under similar
conditions, the substances Egyptian Blue and Han Blue strongly
resemble one another. The smaller the grain size, the lighter is
its appearance. Egyptian Blue is ground whereas Han Blue is
granular crystalline.
A piece of industrially produced Ultramarine Blue (left) and Maya
Blue produced from sepiolite (right). The sepiolite stems from
commercially available cat litter.
Fig. 2 A piece of industrially produced Ultramarine Blue (left) and
Maya Blue produced from sepiolite (right). The sepiolite stems from
commercially available cat litter.
It must be noted that, according to current knowledge, no
artificially produced blue pigments were used in ancient India.
Mineral pigments like azurite and lapis lazuli were used instead.
From a chemical point of view, Egyptian Blue and Han Blue and Purple
are very closely related. Being copper silicates with the alkaline
earth elements calcium and barium, they are defined chemical
compounds of the compositions CaCuSi[4]O[10] (Egyptian Blue), BaCuSi
[4]O[10] (Han Blue) und BaCuSi[2]O[6] (Han Purple). Their chemical
relationship becomes apparent on the basis of the periodic table of
the elements, according to which they differ only in the very similar
alkaline earth elements.^8-11 Ultramarine Blue, which was probably
also produced synthetically by the Chinese, possesses variable
compositions and might typically correspond to the formulation Na
[6.9](Al[5.6]Si[6.4]O[24])S[2.0].^12 Likewise, Maya Blue can also not
be assigned a defined ratio composition between the two constituents
indigo (C[16]H[10]N[2]O[2]) and the white clay mineral (palygorskite
((Mg,Al)[4]Si[8](O,OH,H[2]O)[24]) or more rarely sepiolite).^13,14
Smalt, which occurs in the form of glasses and glazes, is not a
consistently structured matter and, with the formulation Co(SiO[2])
[n], is not a defined chemical compound.
2. The chemistry of synthetic blue and purple pigments
2.1 The syntheses of Egyptian Blue, Han Blue and Purple, Ultramarine
Blue, Maya Blue and Smalt
Egyptian Blue is a defined chemical compound with the formulation
CaCuSi[4]O[10]. It is the oldest of all the above-mentioned blue and
purple pigments^11 and it can be obtained relatively easily if the
minerals lime (CaCO[3]), sand (SiO[2]) and a copper mineral (e.g.
malachite (Cu[2](CO[3])(OH)[2]) or azurite (Cu[3](CO[3])[2](OH)[2]))
or metallic copper are exposed to oxygen (O[2]) and, together with a
few percent of a flux such as potassium carbonate (K[2]CO[3]),
natrium (NaCl) or natrium sulfate (Na[2]SO[4]), are heated to
temperatures between 800 and 900 degC:
ugraphic, filename =
b606268g-t1.gif
In ancient Egypt, quite often trona, a mixture of natrium sulfate,
soda (Na[2]CO[3]) and sodium chloride, was used for the
above-mentioned flux function in syntheses. The presence of oxygen (O
[2]) from air prevents the formation of red cuprite (Cu[2]O). It
probably took generations for humans living at that time to find the
right conditions for the creation of high-quality products, including
synthesis optimisation by the addition of fluxes. This long process
required good technical abilities and expertise, e.g. for controlling
the temperature of the furnace and the addition of oxygen, in those
times done with blow tubes and later with bellows, which needed to be
passed on to future generations in accurate ways.
The chemical conditions for the preparation of Egyptian Blue also
needed to be passed on very accurately. Proof that this was done is
found in the extraordinary constancy of the chemical composition of
the Egyptian Blue elements in art objects dating back more than 2500
years, which we examined according to Table 1.
Table 1 Egyptian Blue in ancient Egyptian art, with the compositions
found^11
Composition (in
oxide percentage)
Artefact and location Dynasty Time Ca Cu Si
a Intef, General of Mentuhotep II, 11th Dynasty.
Mastaba of Mereruka, Old Kingdom 2575-2134 15.2 21.3 63.0
Saqqara, Egypt BC
Thomb of Intef,^a Theben, Middle 2040-1640 14.9 21.5 63.8
Egypt Kingdom BC
Nefertete, Berlin, Germany New Kingdom 1340 BC 17.4 30.2 52.3
Echnaton Temple, Blue of the New Kingdom 1353 BC 24.0 22.5 53.5
Talatat Stones, Amarna,
Egypt
Amulet of Bes, origin Late period 712-332 13.6 28.5 58.3
unknown BC
Mummy coffin, origin unknown Graeco-Roman 332 18.4 22.5 59.2
period BC-395 AD
Average Composition 17.3 24.4 58.4
Theoretical Composition 18.6 29.4 52.0
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The production of magnificent compact blue bodies of Egyptian Blue
(cylinder seals and amulets etc.) required specialised chemical
expertise and technical abilities. The production of such items was
achieved through various processes. The faience technique was used to
produce art objects that featured a primarily blue glazing, which
partly contained Egyptian Blue (see section 3. Examined Objects).
As mentioned above, Han Blue and Purple are compounds based on copper
silicates, as well. The production of Han Blue (BaCuSi[4]O[10]) and
Han Purple (BaCuSi[2]O[6]) is generally more difficult than the
production of Egyptian Blue. Today, it can be reconstructed easily
that, in a first step, a barium mineral (generally barite (BaSO[4])
or witherite (BaCO[3])) was exposed for several hours to quartz (SiO
[2]), a copper mineral and an essential lead salt supplement at a
temperature of 900-1000 degC. Fig. 3 shows the thermogravimetric
development of a modern synthesis on a micro scale, starting from a
mixture of BaCO[3], CuO and SiO[2.] In the case of a temperature
rise, at approx. 650 degC there is a mass loss due to the release of CO
[2] from BaCO[3]; at 800 degC, the chemical reaction starts, as seen in
the bottom DTA curve of the diagram.
Thermogravimetrics of the transformation of BaCO3, SiO2 with CuO
(top). At temperatures of approx. 650 degC, BaCO3 is decomposed and at
800 degC, the chemical reaction for the formation of Han Blue or Purple
starts. Thermogravimetrics of the decomposition of BaSO4 in the
presence of lead oxide (PbO) and quartz (SiO2) with a reference curve
for only BaSO4. The second curve shows the decomposition of the
BaSO4-SiO2 mixture. The third curve shows the accelerated
PbO-catalysed decomposition of BaSO4 according to the above-mentioned
equation (bottom).
Fig. 3 Thermogravimetrics of the transformation of BaCO[3], SiO[2]
with CuO (top). At temperatures of approx. 650 degC, BaCO[3] is
decomposed and at 800 degC, the chemical reaction for the formation
of Han Blue or Purple starts. Thermogravimetrics of the
decomposition of BaSO[4] in the presence of lead oxide (PbO) and
quartz (SiO[2]) with a reference curve for only BaSO[4]. The second
curve shows the decomposition of the BaSO[4]-SiO[2] mixture. The
third curve shows the accelerated PbO-catalysed decomposition of
BaSO[4] according to the above-mentioned equation (bottom).
For the production of larger amounts of Han Blue, these conditions
are not applicable; as mentioned previously and as shown in the
following typical equation, the temperature will be higher by approx.
100 degC.
ugraphic, filename =
b606268g-t2.gif
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.
Reaching the required temperatures was probably facilitated by
technical developments such as the invention of the twin bellows,
which were also used for other processes, e.g. in ironworks.
Furthermore, the decomposition temperature of barite (BaSO[4]) is
very high, which is why it reacts extraordinarily slow in chemical
syntheses. The use of barite leads to inferior pigment products if no
special additives are applied. Where barite was used as a starting
substance, examination of original samples reveals residual sulfur
content. The syntheses were considerably more successful when lead
salts (lead carbonates, lead oxides) were added, which proved to be
an ingenious chemical trick. All samples of Han Blue and Purple that
have so far been examined contain lead. Table 2 gives an overview of
the compositions of different objects analysed by EDX. Some samples
even revealed very high lead contents.
Table 2 Average composition of ancient objects containing Han Blue
and Purple dating from approx. 800-200 BC [weight% oxide according to
EDX]. Smaller amounts of other components add up to 100%
Sample Ca Ba Cu Si Pb S
a Not analysed due to a too high amount of lead. b See ref. 10.
Octagonal stick,^b Freer Gallery, 2.5 35.3 5.6 37.7 11.4 2.0
Washington
Octagonal stick,^b K 4069, Museum of Far 3.1 31.6 13.2 15.8 35.4 --^a
Eastern Antiquities, Stockholm
Octagonal stick,^b K 4070, Museum of Far 1.0 36.5 15.6 25.9 20.7 --
Eastern Antiquities, Stockholm
Bead 1 (Fig. 16) 1.8 6.2 2.1 25.9 61.2 1.8
Octagonal stick (Fig. 16) 1.5 32.7 6.3 19.5 33.2 2.0
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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.
ugraphic, filename =
b606268g-u1.gif
The progress of a separate decomposition experiment of barium sulfate
in the presence of lead oxide was determined with the help of a
thermogravimetric experiment and is depicted in Fig. 3. While BaSO[4]
does not undergo any changes when exposed to higher temperatures.
BaSO[4], when exposed to approx. 1000 degC in the presence of SiO[2],
eventually starts evolving gaseous products that indicate the
decomposition of BaSO[4]. If lead oxide is added to this mixture, the
reaction will undergo a considerable catalytic acceleration.
As mentioned previously, in the barium-based material system there
are several blue or possibly purple chemical compounds, contrary to
the calcium copper silicate system. Only Han Blue (BaCuSi[4]O[10]),
Han Purple (BaCuSi[2]O[6]) and another, so far unnamed, but defined,
blue compound with the formula BaCu[2]Si[2]O[7] could principally be
used as pigments owing to their facile synthetic availability. In all
syntheses, Han Purple can be produced fastest. BaCu[2]Si[2]O[7] is
found yet rarely and only in traces. BaCuSi[2]O[6] is also generated
if there is a surplus of quartz aimed at the preparation of Han Blue;
Han Blue can, under these circumstances, be obtained only after a
longer reaction time.
Recent examinations of pure Han Purple (degree of purity > 99.5%)
have shown that Han Purple is not only very difficult to obtain in a
pure state, especially using the methods of ancient times, but that
it is also, surprisingly, not purple in its pure state, but dark
blue. The purple shade of Han Purple comes from the red impurity of
copper(I) oxide, Cu[2]O, (mineral name cuprite), that is slowly
generated by the decomposition of Han Purple, which probably happens
as outlined in the following chemical equation.
ugraphic, filename =
b606268g-t3.gif
At temperatures of more than 1050 degC, 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 BaCuSi[2]O
[6] production in ancient China. In Fig. 4, the purple or rather
reddish blue shades of Han Purple have been simulated by the
admixture of copper(I) oxide to pure blue BaCuSi[2]O[6].
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 degC, comparable to the syntheses conducted
in ancient times, is, in terms of the colour shade, similar to sample
(c).
Fig. 4 Pure BaCuSi[2]O[6] (Chinese "Purple") (a), to which cuprite
(Cu[2]O) (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 degC, comparable to
the syntheses conducted in ancient times, is, in terms of the
colour shade, similar to sample (c).
Also, ancient objects treated with Han Purple contain variable
amounts of copper(I) oxide and therefore display variable purplish
shades. E. FitzHugh, the pioneer of the rediscovery of the Chinese
pigments, also highlighted this fact, which she ascribed to the
various decomposition states of the samples.^15 As will be discussed
further on, Han Purple is very unstable from a chemical point of
view; for that reason, it often shows signs of weathering on
excavated historic artefacts. While the copper(I) oxide in Han Purple
stayed stable and a decomposition of Han Purple progressed, the
purplish colour of the artefacts increased.
Artificial lapis lazuli: Ultramarine Blue. Lapis lazuli (the mineral
lazurite (Na,Ca)[8](AlSiO[12])(S,SO[4],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 our group,
it is very likely that Ultramarine Blue (typical formulation Na[6.9]
[Al[5.6]Si[6.4]O[24]]S[2.0]), 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.^12 If those conditions are not prevalent, the
elements building the alumosilicate frameworks must be available in
the form of suitable raw materials such as aluminium and silicon,
which may stem from earth minerals. In the case of the examined
objects, 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 K[2]CO[3] and soda Na[2]CO[3]).
Ultramarine Blue is generated at relatively low temperatures (400-600
degC); this has been done since the early 19th century, when the
industrial production process first came into use.^16
Maya Blue. Maya Blue was developed by the Indian cultures of Central
America on a very different chemical basis than the aforementioned
pigments.^13,14 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)[4]
Si[8](O,OH,H[2]O)[24]) or sepiolite), for which temperatures between
150 and 200 degC were necessary.
Smalt. As mentioned previously, smalt is a substance with a variable
chemical composition, a solution of cobalt in glass with the
formulation Co(SiO[2])[n]. Smalt, which was in ancient times used
only moderately, was used much more frequently as a pigment by the
end of medieval times.^17 It was produced in a similar way to glass
and glazes and thus relatively low temperatures were needed (approx.
600 degC). In the presence of aluminium ions, a spinel compound with
the defined composition CoAl[2]O[4] may be generated (known as
Thenards Blue as of 1802). It is yet unclear if this chemical
compound was also used as a pigment in ancient times.
2.2 Chemical structures and properties of synthetic blue and purple
pigments
The microscopic structure of ancient pigments provides information on
their chemical and physical features as well as the process by which
they were produced.
Copper silicate pigments. The two chemically very closely related
compositions CaCuSi[4]O[10] (Egyptian Blue) and BaCuSi[4]O[10] (Han
Blue) differ only in the way they exchange the earth alkali element.^
8-11 This is a minor variation from a chemical point of view. 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. With this infinite connection, a grid of four-ring units
and eight-ring units is generated that displays a puckering in the
eight-ring units so that two opposite four-ring units make up the
"lower" or "upper" sides of the eight-ring units. Opposite four-ring
units become so close that their terminal Si-O^- groups take up a
copper ion in a square planar arrangement (Fig. 5).
Schematic depiction of an isolated layer of MCuSi4O10 (M = Ca, Ba)
with the Cu2+ ions (blue) in a square planar complex. The
coordination of the M2+ ions (yellow) is through the bond, with
adjoining layers complemented to an eightfold coordination (O red, Si
dark green).
Fig. 5 Schematic depiction of an isolated layer of MCuSi[4]O[10] (M
= Ca, Ba) with the Cu^2+ ions (blue) in a square planar complex.
The coordination of the M^2+ ions (yellow) is through the bond,
with adjoining layers complemented to an eightfold coordination (O
red, Si dark green).
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. In contrast
to copper, the calcium and barium ions act as relatively independent
counterions, positioned between the layers, and therefore do not have
only small effects on the colour properties. 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 (Fig. 1).
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.^8-11 This results in the formation of an
infinite arrangement of Cu[2] units (Fig. 6).
Schematic depiction of the layered structure of Han Purple,
BaCuSi2O6. The Cu2 units (blue) are perpendicular to the plane of the
layer. They are held together by the four bridging SiO2 units of the
silicate four-ring units (Ba yellow, Si dark green).
Fig. 6 Schematic depiction of the layered structure of Han Purple,
BaCuSi[2]O[6]. The Cu[2] units (blue) are perpendicular to the
plane of the layer. They are held together by the four bridging SiO
[2] units of the silicate four-ring units (Ba yellow, Si dark
green).
From a chemical point of view, this copper-copper bond is a very
uncommon feature. Metal-metal bonds are in general chemical
curiosities if they appear in materials other than metals. The
crystals of Han Purple are also dichroitic. As a consequence, the
substance's appearance gets lighter when ground and shows otherwise,
as a barium-copper-silicate compound, great similarity in various
physical properties to Han Blue. Thus, the two substances can be
mixed easily, leading to blue/purple colour shades.
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^11 and lead to destruction of pigment
layers of paintings containing Han Purple.
Regarding the formation of these compounds in real preparations, a
model of progressive silicate condensation has been developed.
Starting from the basic structural element SiO[4]^4- (orthosilicate),
the Si[2]O[7]^6- ion (disilicate) is generated at first. It is
represented in the stable BaCu[2]Si[2]O[7] phase. In the next step,
the above-mentioned four-ring unit Si[4]O[12]^8-
(cyclo-tetra-silicate) is generated by advancing condensation; this
forms the basic silicate unit of Han Purple. The Si[4]O[12]^8- may in
turn condense, further building the infinitely connected layers of
puckered Si[8]O[20] eight-membered ring units and planar
four-membered rings of Egyptian and Han Blue. The reaction scheme of
Fig. 7 stresses the fact that, along the pathway of silicate
condensations to eventually yield Han Blue, Han Purple is generated
as an intermediate product. It is therefore expected to be seen as a
first product formed after short reaction times. An "Egyptian Purple"
(CaCuSi[2]O[6]) is not yet known as a chemical compound.
Schematic depiction of the silicate condensation process during the
synthesis of Egyptian Blue, Han Blue and Purple.
Fig. 7 Schematic depiction of the silicate condensation process
during the synthesis of Egyptian Blue, Han Blue and Purple.
Maya Blue. The chemical structure of Maya Blue was only recently
determined.^13,14 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 (Fig. 8) 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.
Schematic depiction of the structure of Maya Blue. In the tubular
channels of the structural framework of the fibrous clay compound
palygorskite ((Mg,Al)4Si8(O,OH,H2O)24), indigo molecules are
statistically arranged. The water molecules have been removed from
the depiction (Mg, Al grey, Si dark green, O red, N light blue,
carbon colourless).
Fig. 8 Schematic depiction of the structure of Maya Blue. In the
tubular channels of the structural framework of the fibrous clay
compound palygorskite ((Mg,Al)[4]Si[8](O,OH,H[2]O)[24]), indigo
molecules are statistically arranged. The water molecules have been
removed from the depiction (Mg, Al grey, Si dark green, O red, N
light blue, carbon colourless).
Ultramarine blue. The chromophore of Ultramarine Blue and lapis
lazuli is the blue S[3]^- radical ion incorporated in sodalite cage
structures ([Al[6]Si[6]O[24]]^6-)^12 (Fig. 9). As is the case with
Maya Blue, 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 S[3]^-
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 S
[2]^- radical is also unavoidably incorporated during any kind of
preparation, as is the case with natural lapis lazuli. A higher ratio
of S[2]^- 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.
Schematic depiction of part of the structure of Ultramarine Blue. In
the sodalite cages ([Na8Al6Si6O24]2+)--here depicted with sodium ions
within the walls of the cage--Cl- ions are enclosed within the
sodalite. In the case of Ultramarine Blue, these are replaced by S3-
radical ions (Na grey, Si dark green, Al blue, O red, S yellow).
Fig. 9 Schematic depiction of part of the structure of Ultramarine
Blue. In the sodalite cages ([Na[8]Al[6]Si[6]O[24]]^2+)--here
depicted with sodium ions within the walls of the cage--Cl^- ions
are enclosed within the sodalite. In the case of Ultramarine Blue,
these are replaced by S[3]^- radical ions (Na grey, Si dark green,
Al blue, O red, S yellow).
Smalt. In ancient times, smalt, ground cobalt glass (Co(SiO[2])[n]),
was barely used as a pigment in painting; but as of the end of the
medieval times, its use increased. Its structure is that of a typical
glass matrix in which tetrahedral holes are partly filled with Co^2+
ions and which is, in some of those places, unstable in its
coordination form. Forming thus practically colourless "octahedral
cobalt", the smalt pigment tends to be susceptible to bleaching.^17
"Tetrahedral cobalt" appears to be more stable in alkaline glazes.
The blue glazes of the tiles of the Ishtar gate in Babylon, for
instance, lasted well (however, besides Co^2+ they also contain small
amounts of Cu^2+ ions as chromophores).^18 If prevalent in glazes,
the blue Co^2+ colour is usually not denoted as smalt even though the
chemical relationship cannot be denied.
3. Examined objects
In the following, a number of examined original objects, that are
exemplary for the respective synthetic pigments, will be presented.
Objects containing Egyptian Blue. As mentioned previously, Egyptian
Blue was used widely in the Mediterranean area and in the Middle East
from pre-dynastic Egyptian times on till the end of the Roman Empire;
its widespread use was probably not only due to trade, but also due
to "technology transfer", the dissemination of the knowledge of its
production. Hence, also in Mesopotamia, compact blue bodies such as
building blocks, parts of ornaments or amulets were found that had
been produced in complicated processes. They were probably produced
on location as of 1500 BC, which implies that there was a previous
dissemination of the knowledge of its production.
As a pigment, Egyptian Blue is contained above all in historical
paintings, in compact blue art objects and blue Egyptian faience.
Amongst the many Egyptian pigment samples examined by our group and
partly listed in Table 1, some stemmed from the paint on mummy
coffins.^9 In Old Egypt, mummy coffins were decorated in exactly the
same technical way over thousands of years.^19 On the wood of which
the coffin is made, there is a layer of Nile mud, which was possibly
applied to serve as a binder and filler. Over the layer of Nile mud,
the (blue) pigment layer consisting of recarbonated lime was applied,
in which the blue platelet-like crystals of Egyptian Blue were
incorporated.
A sample of the crown of the famous Bust of Queen Nefertete
(Agyptisches Museum der Staatlichen Museen, Berlin) was examined (
Fig. 10). It is dated back to 1340 BC (New Kingdom). The analysis
reveals very pure Egyptian Blue with only a few impurities stemming
from the sand, lime and flux used for the synthesis.
Picture of the Bust of Nefertete (Egyptian Museum of the State
Museum, Berlin).
Fig. 10 Picture of the Bust of Nefertete (Egyptian Museum of the
State Museum, Berlin).
The late Egyptian period is documented by the Amulet of the dwarf god
Bes (712-332 BC), which was supposed to protect those who wore it (
Fig. 11). Judging from how it was produced, it is part of the class
of blue compact bodies. The production was probably conducted in two
stages. In the first stage, Egyptian Blue was synthesised and in the
second stage it was, after being moulded in a dry-press procedure,
compacted in another firing with the help of a binder (e.g. gelatine
or wax). One stage production would have had the disadvantage of the
flux's tendency to migrate to the surface of the formed compact body
and thus stop the chemical reaction in the core. In that case, the
body would not be Egyptian Blue throughout. The sintering process in
the two-stage production process was notably improved by the addition
of a small amount of glass powder, which led to the creation of more
solid objects.^20 It can, with a high probability, be excluded that,
in Old Egypt, gypsum was used for the "cold" moulding of blue
objects.
Amulet of the Egyptian dwarf god Bes (712-323 BC). Produced as a
compact Egyptian Blue body in a two-stage process (see text).
Fig. 11 Amulet of the Egyptian dwarf god Bes (712-323 BC). Produced
as a compact Egyptian Blue body in a two-stage process (see text).
Another widely used technique of ancient Egypt based on Egyptian Blue
is the faience technique, which is one of the oldest techniques,
especially for the blue colourisation of art objects. With the
faience technique, moulded bodies are glazed in a procedure that has
been reconstructed very accurately in several modern experiments.^
21,22 An impressive modern demonstration of one of the blue faience
techniques is shown in Fig. 12. The dried object formed from dry
quartz is immersed in the aqueous sludge of burnt cementation powder,
which consists of sodium carbonate, calcite, quartz and small amounts
of copper oxide and sodium chloride. After the sludge layer has
dried, it is burnt for 5 hours at approx. 950 degC. After cooling, a
porous outer layer can be crumbled. The quartz body is covered in
blue glazing on all sides (Fig. 12).
Production of blue faience according to the process of wet
classification.21
Fig. 12 Production of blue faience according to the process of wet
classification.^21
On a microscopic level, the blue faience glazing usually consists not
only of coloured glass, but also of blue, spicular, incorporated
crystals; the latter do not have exactly the same composition as
Egyptian Blue, as was shown in exemplary experiments on the
incorporation of Egyptian Blue.^23 A microscopic view of needles of
"Egyptian Blue" embedded in glass is shown in Fig. 13.
Microscopic view of needles of "Egyptian Blue" embedded in glass.
Fig. 13 Microscopic view of needles of "Egyptian Blue" embedded in
glass.
The glass paste also known as glass frits, which was often used on
mummy coffins as a coloured coating incorporated in different
binders, is from a chemical point of view very similar to the faience
glazings.
In the following, further objects containing Egyptian Blue found in
the area of Mesopotamia shall be discussed. The blue building block
from Nimrud (1300-700 BC) in present-day Iraq (British Museum,
London), (Fig. 14) which in addition to Egyptian Blue also contains
large amounts of quartz, is blue throughout and was probably produced
just like compact bodies of Egyptian Blue in a two-stage production
process, in which, during the second firing, quartz was added. The
added quartz content is probably responsible for the higher solidity
necessary for the body to be suitable for use as construction
material. Further Egyptian Blue samples were investigated from
compact blue beads and from cylindrical seals from Nuzi in
present-day Iraq.
Brick Nimrud. A Mesopotamian building block made from Egyptian Blue
and quartz, presumably along the lines of the "compact body
procedure" (see text). (c) Copyright the Trustees of The British
Museum.
Fig. 14 Brick Nimrud. A Mesopotamian building block made from
Egyptian Blue and quartz, presumably along the lines of the
"compact body procedure" (see text). (c) Copyright the Trustees of
The British Museum.
With most Egyptian Blue objects from Mesopotamia it is difficult to
prove whether they had been imported from Egypt or whether they were
produced on location. The import hypothesis is supported by some
objects that are not typical for Mesopotamia, such as amulets
depicting the Egyptian god Bes that were found frequently, even
though Bes had no divine significance in Mesopotamia. Domestic
production of such artefacts implies technology transfer, which may
have taken place as of 1500 BC.
Many Egyptian Blue artefacts from the Roman period were found in
Europe, north of the Alps,^7 including the areas south of Hadrian's
Wall. They were indeed of Roman origin and naturally any such finds
were from times earlier than the fall of the Roman Empire.
Nevertheless, there are, up to now, two exceptions, where wall
paintings were dated to the 9th century AD. One comes from
Switzerland, which caught our special interest. The Monastery of
Mustair has, in its main church hall, frescoes dated to 860 AD, which
indeed contain Egyptian Blue^24 (vide infra). It is unknown where
this material came from, whether it was a left-over from Roman times
or was produced on the spot (Fig. 15). The latter would have required
a way to hand down the description of the preparation.
Picture of the Monastery of Mustair, Switzerland (certified World
Cultural Heritage of UNESCO) (top) and part of the lowest layer
fresco of the southern wall of the main church of the Monastery
(bottom). Copyright by Oskar Emmenegger, Stocklistrasse, 7205 Zizers,
Switzerland.
Fig. 15 Picture of the Monastery of Mustair, Switzerland (certified
World Cultural Heritage of UNESCO) (top) and part of the lowest
layer fresco of the southern wall of the main church of the
Monastery (bottom). Copyright by Oskar Emmenegger, Stocklistrasse,
7205 Zizers, Switzerland.
Objects containing Han Blue and Purple and Ultramarine Blue. The Han
Blue or Purple pigments so far identified in compact bodies or in
paint layers stemmed from the time of the late Western Zhou period
(1207-771 BC), the Eastern Zhou period (770-221 BC) and the Qin and
Han periods (220-207 BC and 206 BC-220 AD). The early objects
examined by us were glazed decoration objects such as beads and
earrings that contained Han Blue and Purple and others, even
Ultramarine Blue (Fig. 16).
Bead 1 (top left corner), with a faience layer, contains Han Purple
and Ultramarine Blue and has a white core. It dates from 777-766
BC.35 Bead 2 (top right corner), with a faience pigment layer,
contains Han Blue and Ultramarine Blue and has a coloured core. It
dates from the 8th-6th century BC. Origin: the archaeological
excavation site Li County (Northwestern China). Bead 3 (bottom left
corner) is composed of a heterogeneous, compact blue body (Han Blue)
which is part of the class of the sinter minerals that are rich in
lead and barium. Excavated and dated: spring and autumn period
(770-476 BC).36 The octagonal stick (bottom right corner), dates from
5th-3rd century BC37,38 and is composed of equally coloured sinter
material rich in lead and barium, partly crystallised and partly
glassy with a decomposed, partly whitish surface.26
Fig. 16 Bead 1 (top left corner), with a faience layer, contains
Han Purple and Ultramarine Blue and has a white core. It dates from
777-766 BC.^35 Bead 2 (top right corner), with a faience pigment
layer, contains Han Blue and Ultramarine Blue and has a coloured
core. It dates from the 8th-6th century BC. Origin: the
archaeological excavation site Li County (Northwestern China). Bead
3 (bottom left corner) is composed of a heterogeneous, compact blue
body (Han Blue) which is part of the class of the sinter minerals
that are rich in lead and barium. Excavated and dated: spring and
autumn period (770-476 BC).^36 The octagonal stick (bottom right
corner), dates from 5th-3rd century BC^37,38 and is composed of
equally coloured sinter material rich in lead and barium, partly
crystallised and partly glassy with a decomposed, partly whitish
surface.^26
In one bead (approx. 800 BC), all three pigments were identified. It
is, so far, the earliest occurrence of a copper silicate pigment.^
25,26 The synthetic source of Ultramarine Blue could be presumed but
not proven. In the case of Han Purple, there was plenty of proof of
its occurrence in the period of the Warring States.^10 Judging from
the artefacts that have been examined so far, a trend can be
established, according to which Han Blue was preferred in the early
times, whereas later, probably due to a different fashion, more
artefacts were made from the pigment Han Purple. From the Eastern
Zhou, the Qin and the Han periods, many octagonal sticks were found
that were compact bodies made throughout from the same material (Fig.
16). They contain both Han Blue and Han Purple. The Qin and Han
periods were the first to produce paints in which Han Blue and Purple
could be verified. One example is the Terracotta Army found in the
tomb of the first Chinese Emperor Qin Shihuan; amongst other colours,
it had also been painted with Han Purple^11 (Fig. 17); so far, no Han
Blue has been found in the pigment layers of the Terracotta Warriors.
The blue colour on the Terracotta Army was, according to the current
state of knowledge, rendered mainly through the use of azurite. While
traces of Han Blue have been found to accompany polychromic pigment
layers of the Han period,^27 no accurately dated objects have yet
been identified containing Han Blue alone as the blue component. The
use of the synthetic barium copper silicate pigments apparently ended
with the end of the Han period. A mural painting in the tomb of Bin
Wang, in the area of Xian, that had been painted with Han Purple
during the Eastern Han period is one of the last pieces of evidence.^
28
Fragments of the trousers of a Warrior of the Terracotta Army, Xian,
China, painted purple (fragments 003-92) (bottom right) and a
microscopic cross section through a pigment layer of one of the
above-mentioned fragments (top right). Purple fragments correspond
with Han Purple. Red particles are composed of vermilion. Horizontal
extent: 22 mm. Under the pigment layer, there is a layer of varnish
and farther below there is clay.
Fig. 17 Fragments of the trousers of a Warrior of the Terracotta
Army, Xian, China, painted purple (fragments 003-92) (bottom right)
and a microscopic cross section through a pigment layer of one of
the above-mentioned fragments (top right). Purple fragments
correspond with Han Purple. Red particles are composed of
vermilion. Horizontal extent: 22 mm. Under the pigment layer, there
is a layer of varnish and farther below there is clay.
4. Archaeometry of the pigments of objects with Egyptian Blue, Han
Blue and Purple and Ultramarine Blue
In the last two decades, micro-Raman spectroscopy, scanning electron
microscopy (SEM and EDX) and X-ray fluorescence spectroscopy have
proved to be the best tools for analysing blue pigments in mixtures.
EDX delivers micro-element analytical data as part of SEM and X-ray
fluorescence, whereas Raman spectroscopy and X-ray powder
diffractometry help to conduct a phase analysis. SEM additionally
provides information on the surface structure and the heterogeneity
of the samples. The sensitivities of all these methods have increased
significantly in the last few years. Therefore, it can be assumed
that there is a quasi destruction-free situation in archaeometric
examinations due to the small sample quantities. All of the
above-mentioned analytical methods can be used in a non-destructive
way; however, the size and the immobility of the objects often make
it impossible to examine them with those tools.
The most significant progress in the area of archaeometry in the last
few years has been made in Raman spectroscopy, with an enormous
increase in the sensitivity of the technology.^29 Raman spectroscopy
is the most appropriate technique to identify blue pigments.^30 All
copper silicate pigments and Ultramarine Blue can easily be
identified by a characteristic "fingerprint pattern" in the spectrum,
which makes it possible to perform a phase analysis (see spectra in
Fig. 18 as examples of Egyptian Blue from the Bust of Queen Nefertete
and the fresco in Mustair (Switzerland)).
Raman spectrum of virtually pure Egyptian Blue from the crown of the
Bust of Nefertete (a) and from a sample of the fresco in the church
of the Monastery of Mustair, Switzerland (b) in the range of 200-1400
cm-1 (514 nm). Spectrum (b) also contains bands for SO42- (presumably
gypsum) and for CO32- (dolomite). A band at 1118 cm-1 could not yet
be assigned.
Fig. 18 Raman spectrum of virtually pure Egyptian Blue from the
crown of the Bust of Nefertete (a) and from a sample of the fresco
in the church of the Monastery of Mustair, Switzerland (b) in the
range of 200-1400 cm^-1 (514 nm). Spectrum (b) also contains bands
for SO[4]^2- (presumably gypsum) and for CO[3]^2- (dolomite). A
band at 1118 cm^-1 could not yet be assigned.
Examples of the identification of the Chinese pigments are the Raman
spectra of Han Blue and Purple and Ultramarine Blue found from
various ornamental objects (Fig. 19) as well as those of Han Purple
found from samples of the Terracotta Army of the Qin period and found
on the tomb of Bin Wang of the Eastern Han period (Fig. 20).
Raman spectra of bead 1 (a) of Han Blue and of bead 3 (b) and of Han
Purple of the octagonal stick (c) in the range 1200-200 cm-1.26
Excitation laser 514 nm.
Fig. 19 Raman spectra of bead 1 (a) of Han Blue and of bead 3 (b)
and of Han Purple of the octagonal stick (c) in the range 1200-200
cm^-1.^26 Excitation laser 514 nm.
Raman spectra of Han Purple from a sample of the pigment layer of the
Terracotta Army in Xian, China (a), and from a sample of the pigment
layer of the frescoes from the tomb of Bin Wang (b). Spectral range
1400-400 cm-1 (514 nm).
Fig. 20 Raman spectra of Han Purple from a sample of the pigment
layer of the Terracotta Army in Xian, China (a), and from a sample
of the pigment layer of the frescoes from the tomb of Bin Wang (b).
Spectral range 1400-400 cm^-1 (514 nm).
5. The dissemination of the blue pigments and technology transfer
The various blue pigments that humans have produced give rise to
thoughts about whether these developments have taken place
independently from one another or whether knowledge and technology
transfer have furthered or even made possible some of the
achievements. With regard to Maya Blue, there are no indications that
e.g. far eastern expertise on indigo processing may have influenced
the Indian developments. In the case of Egyptian Blue and the
pigments, however, it may not only be conjectured how and when the
production of the Chinese pigments may have started, but also whether
their chemical similarity and the significantly earlier production of
Egyptian Blue give enough indications about whether the Chinese
pigments were produced on the basis of Egyptian Blue.
The dissemination of knowledge on Egyptian Blue might have sparked
the Chinese developments by means of an ancient "technology
transfer". The distribution of Egyptian Blue into the east has,
through findings in that area, been verified as far as to the regions
of present-day Persia (Fig. 21).
Atlas showing the approximate ancient distribution of Egyptian Blue
(blue) and Han Blue and Purple (pink). Red lines indicate the many
ways of the silk roads, along which not only trading occurred but
also exchange of ideas.
Fig. 21 Atlas showing the approximate ancient distribution of
Egyptian Blue (blue) and Han Blue and Purple (pink). Red lines
indicate the many ways of the silk roads, along which not only
trading occurred but also exchange of ideas.
However, it is unclear whether the knowledge about Egyptian Blue
really reached Central Asia; further archaeological findings and
archaeometric studies will be necessary to answer this question. A
geographical overlap of the distribution of the production of
Egyptian Blue with the Chinese locations for the barium copper
silicate pigments would support, from a chemical point of view,
assertion of an ancient technology transfer between the western and
eastern worlds; according to current knowledge, however, it cannot
conclusively be affirmed.
Therefore, the possibility of a technology transfer seems, from the
present-day view, not very likely. It is clear that early inventions,
such as the invention of the copper silicate pigments, could not have
been sudden discoveries, but requested preceding evolutionary
developments. On the basis of an evolutionary process, it can
feasibly be demonstrated that the Chinese developments as of approx.
1100 BC were taking place independently from the knowledge about
Egyptian Blue. Though, the two historical developments were in many
ways taking place similarly. Fig. 22 shows the two independent
developments of Egyptian and Han Blue in a flowchart. In both the
eastern (China) and the western (Mediterranean area, Egypt,
Mesopotamia, Persia) hemispheres, the techniques of glazing stone and
clay objects were the first step of the developments. The western
hemisphere started out earlier, at first with glazes rich in alkali
metal, and later with glazes containing elevated calcium contents.
Based on these glazing techniques for the blue colour, the faience
and frit techniques were developed and Egyptian Blue emerged and was
used in crystalline form.^31 In the eastern hemisphere, the glazing
techniques were introduced much later, probably even on the basis of
transmissions from the west (Fig. 22). As of 1100 BC, however, heavy
metal glazes (Pb, Ba) were developed in the east, which eventually
led to the creation of early heavy metal glasses and to the
production of barium copper silicate pigments.^32,33 With regard to
the copper silicate pigments, it is probable that they were produced
independently from one another as parallel developments in the
eastern and western hemispheres.
Flowchart showing the development of alkaline earth copper silicate
pigments: Egyptian Blue and Han Blue and Purple.
Fig. 22 Flowchart showing the development of alkaline earth copper
silicate pigments: Egyptian Blue and Han Blue and Purple.
6. Summary of historical considerations
As mentioned previously, the blue pigments were invented from
necessity. Humans did not have unlimited access to blue as a pigment,
as blue is not an earth colour. About 5500 years ago, with the
development of important civilisations, the use of minerals and their
chemical transformation probably started. The first synthetic pigment
was Egyptian Blue, which the Egyptians, along with the knowledge
about its production, transmitted to many cultures in the
Mediterranean area and beyond. The Romans were the last to produce
Egyptian Blue industrially in factories. With the downfall of the
Roman Empire, Egyptian Blue was not passed on any longer and the use
of the pigment came to an end.
Today, it is assumed that the pigments Han Blue and Purple were used
at least since 800 BC probably mostly locally in the smaller
north-western Chinese areas. Those are the mineral-rich areas with
large copper, barium and lead deposits. As examinations have shown so
far, at first Han Blue was used in preference for ornaments, whereas,
as of approx. 400 BC, Han Purple replaced it as the preferred
pigment. The Terracotta Army of the first Emperor of China, Qin
Shihuang, stemming from approx. 220 BC, was painted largely with Han
Purple.
It is assumed today that the use of Han Blue and Purple ended in the
Han period (220 AD) at the same time as the Chinese Empire was again
split; it is probable that, as was the case with Egyptian Blue,
political changes stopped the dissemination of the Chinese pigments.
Yet, the investigation of the Chinese pigments is far from finished.
It is most probable that valuable archaeometric findings will follow
and, just as archaeology uncovers more artefacts, shed new light on
ancient times, probably above all on the time between 800 BC and 200
AD
In this context, the question arises about the production and
dissemination of synthetic Ultramarine Blue produced in ancient
times. It is possible that Ultramarine Blue was not only produced in
the geographical area of ancient China, but also in other empires
such as ancient Mesopotamia, where natural lapis lazuli was
frequently used and where there was a high potential for chemical
activity. The introduction of Egyptian Blue might have helped to
overcome the deficiency of blue pigments and might have stopped any
further activities toward the creation of blue pigments. Time will
tell if Ultramarine Blue still occurs in archaeological findings of
that area.
The historical findings regarding Maya Blue have so far also been too
scarce for the scientists to reach definite conclusions. Neither the
earliest occurrence of the pigment, nor the time spans in which it
was used or disseminated have been confirmed.
The copper silicate pigments have also played a role in recent
history. Table 3 provides information on the most important data of
the last two centuries, starting with the rediscovery of Egyptian
Blue through Napoleon's scientific squad on the Egyptian Expedition.
Table 3 Chronology of Egyptian and Han Blue and Purple in the 19th
and 20th centuries.
1809 M. Chaptal, first examination of Egyptian Blue
1814 Sir Humphrey Davy, identification of Egyptian Blue in samples
from Pompeii
1874 Fontenay, first synthesis of Egyptian Blue
1889 Fouqui, formulation as (SiO[2])[4]CaOCuO
1900 Le Chatelier, patent specification no. 112761, Patent Office
Berlin, patent for Han Blue and Purple developed from Egyptian
Blue
1916 Bock, industrial production of Egyptian Blue
1959 Pabst, X-ray structure of Egyptian and Han Blue, synthesis of
Han Blue
1983 FitzHugh, first evidence of Han Blue in ancient artefacts
1989 Finger, synthesis and X-ray structure of Han Purple
1992 FitzHugh, first evidence of Han Purple in ancient Chinese
samples
1992 Janczak, correct X-ray structure of Han Purple
1996 Rhone Poulenc filed a patent for the sol-gel method to produce
Han Blue and Purple
2004 Jaime, Han Purple for the "spin computer"
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The actual scientific investigations did not start until 1809. In
1874, the first synthesis of Egyptian Blue was made and in 1889
Egyptian Blue was defined as an independent compound. The structural
determination of Egyptian Blue in 1959 by Pabst is another notable
achievement. Han Blue and Purple were, for the first time, recognised
as ancient synthetic pigments in the year 1983, when E. FitzHugh and
staff members identified Han Blue and Purple in original samples. The
"re-invention" of Han Blue and Purple by Le Chatelier in 1900, which
was independent from the ancient Chinese knowledge, was another
remarkable milestone. In 1900, Le Chatelier filed an application for
a patent based on Egyptian Blue with the Patent Office in Berlin.
Le Chatelier unknowingly repeated history with his plan to make new
blue and purple pigments available. Necessity is the mother of
invention, even though the necessity in this case was not vital but
driven rather by a "gap in the market". As a further milestone in the
modern development of Han Blue, Le Chatelier's plan was, in 1996,
behind a purely commercial background, improved and optimised with
modern methods by the company Rhone Poulenc, which then filed a
patent application for its new achievements. The latest cognition
regarding the Chinese pigments concerns Han Purple and is still being
investigated. Han Purple may have special magnetic properties on an
atomic level. The authors of an original publication^34 hope that the
Cu[2] units described might, with their electron spins, be of use in
virtual addressing in sub-nano-chips in the so-called "spin
computer", which would lead to an enormous increase in the
performance of computers. Hence, the properties of these ancient
chemical compounds might continue to be useful for humanity and will
further be accompanying human innovations.
7. Chemistry and opportunity
All the pigments described previously have one thing in common: they
were all created by human inventive talent, which is apparently
strongest when there are situations of deficiency, even if the
invention does not necessarily serve vital needs. Colour is an
intrinsic part of human life. The human being strives for expression
and sensation through colour and thus needs a suitable material
basis. Part of this material basis is the pigments, some of which
humans created with chemical processes. Chemistry was used as an
opportunity. Chemistry was not only an opportunity for the people in
ancient times and at the age of industrialisation, but still is an
opportunity for the people of today. Seizing opportunities with
chemistry means to use silent force and reach copious abundance,
according to the notable German chemist Justus von Liebig. In this
matter, the ancient civilisations are our role models. Nowadays,
however, chemistry is also responsible for its risks.
Acknowledgements
First of all, I would like to thank Dr Hans-Georg Wiedemann, Stafa,
Switzerland, who has worked with me on many scientific papers on this
subject. I would also like to thank him for providing several
illustrations. Furthermore, I am indebted to the many people
providing samples who cannot be listed here individually by name, but
who are listed in the original publications. With their samples, they
provided my group with the essentials to conduct our archaeometric
studies. I am particularly indebted to the Egyptian Museum of the
State Museum Berlin, Prussian Cultural Heritage, for the sample of
the Bust of Queen Nefertete, on which the Raman spectrum was
conducted, and to Dr Q. Ma, China National Institute of Cultural
Property, Beijing, China, who provided the early Chinese samples and
examined them in Zurich. The exact source can be found in the quoted
original publications. Furthermore, I would like to thank Ms C.
Blansdorf, Bavarian State Office for the Preservation of Ancient
Monuments, Munich, for providing samples, the photographs of the
Terracotta Army and the microscopic view. I am also indebted to Dr
Susanne Greiff, Roman Germanic Central Museum, Mainz, Germany, for
providing a Han Purple sample of the Bin Wang tomb in Baizi, China.
We are also grateful to Dr Jurg Goll, Institut fur Denkmalpflege ETH
Zurich, Switzerland for providing an Egyptian Blue sample of the
frescoes in the church of the Monastery Mustair, Switzerland.
Finally, I thank the colleagues R. W. P. Wild, A. Portmann, S.
Bouherour and T. Corbiere for their deep commitment to the projects,
on which this paper is based. Their names are also listed in the
references. Special thanks go to Dr Ferdinand Wild, whose advice and
support significantly contributed to the work on this paper.
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