Han Purple and Han Blue
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{{Infobox Color|title=Han Purple|textcolor=white|
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'''Han Purple''' and '''Han Blue''' (also called Chinese Purple and Chinese Blue) are synthetic barium copper silicate [[pigments]] that were developed in [[China]] at least 2,000 years ago.
Han Purple and Han Blue were used in ancient and imperial China from the [[Western Zhou]] period (1207-771 BC) until the end of the [[Han dynasty]] (c. 220 AD). <ref name="Liu">Liu, Z., Mehta, A., Tamura, N., Pickard, D., Rong, B., Zhou, T. and Pianetta, P. 2007. Influence of Taoism on the invention of the purple pigment used on the Qin terracotta warriors. ''Journal of Archaeological Science'' 34, 1878-1883.</ref> <ref name="Berke 2007">Berke, H. 2007. The invention of blue and purple pigments in ancient times. ''Chemical Society Review'' 36, 15-30.</ref>
==Color==
[[Azurite]] was the only natural blue pigment used in early China. <ref name="Thieme">Thieme, C. 2001. (translated by M. Will) Paint Layers and Pigments on the Terracotta Army: A Comparison with Other Cultures of Antiquity. In: W. Yongqi, Z. Tinghao, M. Petzet, E. Emmerling and C. Blänsdorf (eds.) ''The Polychromy of Antique Sculptures and the Terracotta Army of the First Chinese Emperor: Studies on Materials, Painting Techniques and Conservation.'' Monuments and Sites III. Paris: ICOMOS, 52-57.</ref> Early China seems not to have used a natural purple pigment and was the first to develop a synthetic one. <ref name="Thieme"/>
Han Blue in its pure form is, as the name suggests, blue.
Han Purple in its pure form is actually a dark blue, that is close to [[Indigo|electric indigo]]. It is a [[purple]] in the way the term is used in [[colloquial]] [[English language|English]], i.e., it is a color between [[red]] and [[blue]]. It is not, however, a purple in the way the term is used in [[color theory]], i.e. a non-[[Spectrum|spectral]] color between [[red]] and [[violet (color)|violet]] on the ''line of purples'' on the [[CIE chromaticity diagram]]. Perhaps the most accurate designation for the color would be to call it ''Han Indigo'', although it could also be regarded as a bright shade of [[Ultramarine|ultramarine]] (classifying ''ultramarine'' as a color and not a pigment).
The purple color seen in samples of Han Purple is created by the presence of red [[copper (I) oxide]] (Cu<sub>2</sub>O<sub>2</sub>) which is formed when Han Purple decomposes (the red and blue making purple).<ref name="Berke 2007"/> The decomposition of Han Purple to form copper (I) oxide is:
3BaCuSi<sub>2</sub>O<sub>6</sub> → BaCuSi<sub>4</sub>O<sub>10</sub> + 2BaSiO<sub>3</sub> + 2CuO <ref name="Wiedemann and Berke">Wiedemann, H. G. and Berke, H. 2001. Chemical and Physical Investigations of Egyptian and Chinese Blue and Purple. In: W. Yongqi, Z. Tinghao, M. Petzet, E. Emmerling and C. Blänsdorf (eds.) ''The Polychromy of Antique Sculptures and the Terracotta Army of the First Chinese Emperor: Studies on Materials, Painting Techniques and Conservation.'' Monuments and Sites III. Paris: ICOMOS, 154-169.</ref>
Above 1050 °C, the CuO [[copper (II) oxide]] breaks down to copper (I) oxide:
2CuO → Cu<sub>2</sub>O + 1/2O<sub>2</sub> <ref name="Wiedemann and Berke"/>
==Chemistry==
Both Han Purple and Han Blue are barium copper silicates (containing [[barium]], [[copper]], [[silica]], and [[oxygen]]). However, they differ in their formula, structure and chemical properties.
===Chemical Formula and Molecular Structure===
====Han Purple====
Han Purple has the [[chemical formula]] BaCuSi<sub>2</sub>O<sub>6</sub>.
Han Purple has a layered [[chemical structure|structure]] with isolated 4-ring [[silicates]], and contains a copper-copper bond which makes the [[chemical compound|compound]] more unstable than Han Blue (metal-metal bonds are rare)<ref name="Berke 2007"/> <ref name="Wiedemann et al 1998">Wiedemann, H. G. Bayer, G. and Reller, A. 1998. Egyptian blue and Chinese blue. Production technologies and applications of two historically importan blue pigments. In: S. Colinart and M. Menu (eds.) ''La couleur dans la peinture et l'émaillage de l'Égypte ancienne''. Actes de la Table Ronde Ravello, 20-22 mars 1997. Bari: Edipuglia, 195-203.</ref>
====Han Blue====
Han Blue has the chemical formula BaCuSi<sub>4</sub>O<sub>10</sub>.
Han Blue, like Han Purple, has a layered structure with silicate forming the structural framework. However, Han Blue is more stable because of structural features such as:
* It is more silica-rich <ref name="Berke and Wiedemann">Berke, H. and Wiedemann, H. G. 2000. The Chemistry and Fabrication of the Anthropogenic Pigments Chinese Blue and Purple in Ancient China. ''East Asian Science, Technology and Medicine (EASTM)'' 17, 94-120.</ref>
* Each 4-ring silicate if linked to four others in the adjacent level, in a zig zag pattern <ref name="Wiedemann et al 1998"/>.
* The copper [[ions]] are very strongly contained within the stable silicate structure <ref name="Berke 2007"/>.
===Chemical and Physical Properties===
Han Purple and Han Blue are similar in many of their physical properties, which allows them to be mixed, but they differ in their chemical properties <ref name="Berke 2007"/>.
====Han Purple====
Han Purple is [[chemical stability|chemically]] and thermally less stable than Han Blue. It fades and decomposes in dilute [[acid]]. <ref name="Wiedemann and Bayer">Wiedemann, H. G. and Bayer, G. 1997. Formation and Stability of Chinese Barium Copper-Silicate Pigments. In: N. Agnew (ed.) ''Conservation of Ancient Sites on the Silk Road: Proceedings of an International Conference on the Conservation of Grotto sites''. Los Angeles: The Getty Conservation Institute, 379-387.</ref> <ref name="Wiedemann et al 1998"/> <ref name="Berke 2002">Berke, H. 2002. Chemistry in Ancient Times: The Development of Blue and Purple Pigments. ''Angewandte Chemie International Edition'' 41/14, 2483-2487.</ref> Han Purple starts to decompose at temperatures more than 1050 - 1100 °C and forms a green-black glass at around 1200 °C <ref name="Berke 2007"/> <ref name="Wiedemann and Bayer"/>.
Becomes more purplish when ground <ref name="Berke and Wiedemann"/>.
====Han Blue====
Han Blue is more chemically and thermally stable. It does not break down in dilute acids <ref name="Wiedemann et al 1998"/> <ref name="Wiedemann and Bayer"/>.
Becomes more bluish when ground <ref name="Berke and Wiedemann"/>.
==Manufacture==
Manufacturing depends on the raw materials, their ratios, fluxes, temperature, atmosphere and reaction time.<ref name="Wiedemann et al 1998"/>
Production seems to have been focused in northern China, around 200-300km north of the city of [[Xian]]. This is the area with large deposits of raw materials<ref name="Berke 2007"/>. There are no written records about the production of Han Purple or Han Blue and so information about manufacture has been achieved through experimentation<ref name="Berke and Wiedemann"/>.
===Raw Materials===
The raw materials needed are a barium mineral, [[quartz]], a copper mineral and a lead salt. It is unknown whether minerals were used in their natural form or were pre-treated, though there is no evidence as yet of pre-treatment<ref name="Berke and Wiedemann"/>.
The barium source was either [[witherite]] (BaCO<sub>3</sub>) or [[baryte]] (BaSO<sub>4</sub>) <ref name="Berke 2002"/>. The rarity of witherite may favour baryte as the most likely source<ref name="Berke and Wiedemann"/>. Baryte has a slower decomposition rate and so favours Han Blue production<ref name="Wiedemann and Bayer"/>. Witherite conversely favours Han Purple<ref name="Wiedemann and Bayer"/>. In the use of baryte, lead salts ([[lead carbonate]] or [[lead oxide]]) would have been needed to increase yield<ref name="Berke 2002"/>. Lead has been detected in association with Han Purple and Han Blue <ref name="FitzHugh and Zycherman 1983">FitzHugh, E. W. and Zycherman, L. A. 1983. An Early Man-Made Blue Pigment from China: Barium Copper Silicate. ''Studies in Conservation'' 28/1, 15-23.</ref> <ref name="FitzHugh and Zycherman 1992">FitzHugh, E. W. and Zycherman, L. A. 1992. A Purple Barium Copper Silicate Pigment from Early China. ''Studies in Conservation'' 28/1, 15-23.</ref> <ref name="Berke 2007"/>
Lead acts as a [[catalyst]] in the decomposition of barium minerals and as a [[flux]] <ref name="Berke 2007"/>. The amount of lead is important. Experiments show that too much lead (more than 5%) causes partial melting and glass formation above 1000 °C.<ref name="Wiedemann and Bayer"/>
The role of lead is:
BaSO<sub>4</sub> + PbO ↔ PbSO<sub>4</sub> + BaO <ref name="Berke 2007"/>
===The Manufacturing Process===
The production for Han Blue using [[witherite]] is:
Cu<sub>2</sub>(CO<sub>3</sub>)(OH)<sub>2</sub> + 8SiO<sub>2</sub> + 2BaCO<sub>3</sub> → 2BaCuSi<sub>4</sub>O<sub>10</sub> + 3CO<sub>2</sub> + H<sub>2</sub>O <ref name="Berke 2007"/>
The solid state reaction to create barium copper silicates starts at roughly 900 °C<ref name="Wiedemann and Bayer"/>. Han Purple is formed fastest<ref name="Berke 2007"/> <ref name="Wiedemann et al 1998"/>. Han Blue forms when there is an excess of silica and a longer reaction time allowed for<ref name="Berke 2007"/>. Early Chinese manufacture generally produced a mixture of Han Blue and Han Purple particles in various ratios, but pure colors were sometimes manufactured<ref name="FitzHugh and Zycherman 1992"/>. Han Blue could have been brought to a melt, but Han Purple does not form a [[homogeneous]] melt so it would have had to use a [[sintering]] process<ref name="Wiedemann and Berke"/>.
Prolonged firing causes Han Purple to break down and form Han Blue:
3BaCuSi<sub>2</sub>O<sub>6</sub> → BaCuSi<sub>4</sub>O<sub>10</sub> + 2BaSiO<sub>3</sub> + 2CuO <ref name="Wiedemann and Berke"/>
The temperature needed to be high (sround 900-1000 °C) and kept at that temperature for long periods<ref name="Berke 2002"/><ref name="Berke 2007"/>. Han Purple is thermally sensitive and so temperature control for producing Han Purple needed to be fairly constant (±50 °C)<ref name="Wiedemann and Berke"/>. Han Blue is thermally less sensitive<ref name="Berke and Wiedemann"/>.
Under the right conditions, the manufacture of Han Purple would have taken around 10-24 hours, while Han Blue would have taken twice as long<ref name="Wiedemann and Berke"/>.
Temperature would have been controlled by testing of firing materials, the size, shape and material of the [[kiln]], and the control of the environment<ref name="Berke and Wiedemann"/>. Technology for achieving and maintaining high temperatures would have been known from metal and ceramic production<ref name="Berke and Wiedemann"/> <ref name="Liu"/> e.g. the potential use of twin bellows as used in metal production<ref name="Berke 2007"/>.
==Han Purple and Han Blue: A Comparison==
{| class="wikitable"
|-
!
! Han Purple
! Han Blue
|-
| Chemical formula
| BaCuSi<sub>2</sub>O<sub>6</sub>
| BaCuSi<sub>4</sub>O<sub>10</sub>
|-
| Minimum temperature for production
| 900-1000 °C
| c. 1000 °C
|-
| Manufacture time
| 10-24 hours
| 20-48 hours
|-
| Decomposition temperature
| 1050-1100 °C
| >1200 °C
|-
| Thermally stable?
| No
| Yes
|-
| Stable in acid?
| No
| Yes
|-
| Color increase when ground?
| Yes
| Yes
|}
==Origins==
===Han Blue and Egyptian Blue===
Han Blue and Egyptian Blue have the same basic structure and have very similar properties<ref name="Berke 2007"/>. The difference is that Egyptian Blue (CaCuSi<sub>4</sub>O<sub>10</sub>) has [[calcium]] in the position of Han Blue's barium (BaCuSi<sub>4</sub>O<sub>10</sub>). The similarties lead some to suggest that Han Blue was based on Egyptian Blue knowledge, which had travelled east along the [[Silk Road]] <ref name="Berke 2002"/>. Independent innovation in China would still have been needed to replace calcium with barium<ref name="Berke 2002"/> (the Han pigments start to form at 100-200 °C higher than Egyptian Blue<ref name="Liu"/>).
The alternative suggestions are:
* That earlier alkali metal glazing techniques were based on knowledge from Egypt, but that the copper silicate pigments (Egyptian Blue and Han Blue) developed from these glazes in two independent areas: Egypt and China<ref name="Berke 2007"/>.
* Alternatively, that examples of Han Blue predate the official Silk Road and therefore that development was completely independent<ref name="Liu"/>.
===A Chinese invention? Links with glass making===
The case against links with Egyptian Blue includes the absence of lead in Egyptian Blue and the lack of examples of Egyptian Blue in China<ref name="Liu"/>.
The use of quartz, barium and lead components in [[ancient Chinese glass]] and Han Purple and Han Blue has been used to suggest a connection between glass making and the manufacture of pigments<ref name="FitzHugh and Zycherman 1983"/>, and to argue for independent Chinese invention<ref name="Liu"/>. It has been argued that [[Taoist]] [[alchemy|alchemists]] developed Han Purple from their knowledge of glass making. <ref name="Liu"/>
The increase and decrease of barium glasses, and Han Purple and Han Blue, follow similar patterns. Both peaked in the [[Han dynasty]], declining afterwards<ref name="Liu"/>. Pre-Han to [[Tang dynasty|Tang]] dynasties see a shift from lead-barium-silicate type glass to lead-soda-lime glass <ref name="Seligman">Seligman, C. G., Ritchie, P. D. and Beck, H. C. 1936. Early Chinese Glass from Pre-Han to T'ang Times. ''Nature'' 138, 721.</ref>. The reason for decline is debatable. Liu et al. (2007)<ref name="Liu"/> attribute the decline to the decline of Taoism when [[Confucianism]] was introduced, since they link pigment manufacture to the ideology of Taoism. Berke (2007)<ref name="Berke 2007"/> believes that political changes stopped the distribution of the pigments as the Chinese Empire was split at the end of the Han period.
==Uses in Cultural Contexts==
Han Blue seems to have been favoured in earlier ([[Zhou Dynasty|Zhou]]) periods, and Han Purple in later periods (c. 400BC)<ref name="Berke 2007"/>.
The Han pigments consist of varying combinations of blue, purple and colorless components<ref name="FitzHugh and Zycherman 1992"/>. The grinding together of Han Purple and Han Blue would have allowed a variety of blue-purple shades<ref name="Berke and Wiedemann"/>.
The pigments were used for:
* Beads (from late [[Zhou Dynasty|Western Zhou period]] (1201-771 BC) <ref name="Berke 2007"/>)
* Octagonal sticks (from [[Warring States period]]<ref name="FitzHugh and Zycherman 1983"/>)
* The [[Terracotta Army]] ([[Qin dynasty]])
* Painted figurines ([[Han dynasty]])
* Ceramic vessels (Han dynasty)
* Metal objects (Han dynasty)
* Wall paintings (Han dynasty)
====Beads====
Some of the earliest examples of the use of the Han pigments are beads which date back to the [[Zhou Dynasty|Western Zhou]] period. The pigments are either present as compact bodies or in glazed layers<ref name="Berke 2007"/>.
====Octagonal sticks====
These are compact bodies (solid sticks/rods) with shades ranging from light blue to dark purple. The range of colors is dues to varying proportions of Han Blue, Han Purple and [[colorless]] material <ref name="FitzHugh and Zycherman 1983"/>. It is thought that they were pigment sticks which were traded then ground to be used as pigment bases in paints<ref name="Berke 2002"/> <ref name="Wiedemann and Berke"/>. It has also been suggested that they are of importance themselves, as [[ceremonial]] or [[bureaucratic]] items of importance<ref name="FitzHugh and Zycherman 1983"/>.
====Terracotta Army====
Han Purple and Han Blue were first used in paints in the [[Qin dynasty]]. Han Purple was used for the [[Terracotta Army]] in the tomb of Emperor [[Qin Shi Huang]]- the expense of producing Han Purple and other pigments in such large quantities would have emphasised luxury and status<ref name="Thieme"/>. Han Purple seems to have mostly been used on the trousers (pants) of the warriors<ref name="Thieme"/>. The pigment was bound to the [[terracotta]] surface with [[lacquer]]<ref name="Rogner">Rogner, I. 2001. New Methods to Characterise and to Consolidate the Polychrome Qi-lacquer of the Terracotta Army. In: W. Yongqi, Z. Tinghao, M. Petzet, E. Emmerling and C. Blänsdorf (eds.) ''The Polychromy of Antique Sculptures and the Terracotta Army of the First Chinese Emperor: Studies on Materials, Painting Techniques and Conservation''. Monuments and Sites III. Paris:ICOMOS, 46-51.</ref>.
The warriors were fired at the same temperature as that need for the manufacture of Han Purple (950-1050 °C) and so the same kilns may have been used for both processes<ref name="Liu"/>.
There is no evidence of Han Blue being used for the warriors ([[azurite]] was used for the blue)<ref name="Berke 2007"/> <ref name="Thieme"/>.
====Painted pottery figurines====
Smaller painted pottery figurines have been found e.g. the Western Han dynasty Chu Tombs, [[Xuzhou]], [[Jiangsu]] Province<ref name="Cheng">Cheng, X., XIa, Y., Ma, Y., and Lei, Y. 2007. Three fabricated pigments (Han purple, indigo and emerald green) in ancient Chinese artefacts studied by Raman microscopy, energy-dispersive X-ray spectrometry and polarized light microscopy. ''Journal of Raman Spectroscopy'' 38, 1274-1279.</ref> and in the Han dynasty Yangling tombs of [[Emperor Jing of Han|Emperor Liuqi]] and his Empress (156-141 BC)<ref name="Zuo">Zuo, J., Zhao, X., Wu, R., Du, G., Xu, C. and Wang, C. 2003. Analysis of the pigments on painted pottery figurines from the Han Dynasty's Yangling Tombs by Raman microscopy. ''Journal of Raman Spectroscopy'' 34, 121-125.</ref>.
====Ceramic vessels====
Han Blue and Han Purple were used to decorate Han dynasty ''Hu'' dark grey pottery vessels<ref name="FitzHugh and Zycherman 1983"/> <ref name="FitzHugh and Zycherman 1992"/>.
====Metal objects====
Bronze vessels in the Han dynasty e.g. a bowl and top of a steamer were decorated with Han Purple<ref name="FitzHugh and Zycherman 1992"/>.
====Wall paintings====
* A [[lintel]] and [[pediment]] from a Han dynasty tomb near [[Loyang]] were painted with a light blue pigment consisting of blue, purple and colorless components<ref name="FitzHugh and Zycherman 1983"/>.
* An Eastern Han period tomb mural painting in the [[Xian]] area is one of the last examples of the use of synthetic barium copper silicate pigments (Han Purple)<ref name="Greiff">Greiff et al. 2002 cited in Cheng, X., Xia, Y., Ma, Y. and Lei, Y. 2007. Three fabricated pigments (Han purple, indigo and emerald green) in ancient Chinese artifacts studied by Raman microscopy, energy-dispersive x-ray spectrometry and polarized light microscopy. ''Journal of Raman Spectroscopy'' 38, 1274-1279.</ref>.
==Preservation==
Due to the instability of Han Purple, it shows significant signs of weathering on archaeologically excavated artefacts. The copper (I) oxide formed in the decomposition of Han Purple (see section on color) remains stable, but Han Purple continues to deteriorate and so its purple color increases with time<ref name="Berke 2007"/>.
Han Purple fades in acid, so colorless particles found in pigments containing Han Blue and Han Purple may be particles which were originally purple but which faded in acidic conditions in burial<ref name="FitzHugh and Zycherman 1992"/>.
In addition, Han Blue has fungicidal properties and so preserves better. Han Purple reacts with [[oxalic acid]] to form BaCu(C<sub>2</sub>O<sub>4</sub>)<sub>2</sub>. The light blue color of this salt may explain the light blue color of some of the Terracotta Warriors' trousers - the color resulting from the presence of [[oxalate]]-excreting lichens<ref name="Wiedemann and Berke"/>.
==Notes==
Two other synthetic blue barium copper silicate compounds have been found in trace amounts, but are as yet unnamed. These are:
* BaCu<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> (blue color)
* Ba<sub>2</sub>CuSi<sub>2</sub>O<sub>7</sub> (light blue color)<ref name="Wiedemann and Berke"/>
==See also==
* [[Egyptian Blue]]
* [[List of colors]]
* [[Maya Blue]]
* [[Prussian Blue]]
* [[Ancient Chinese glass]]
==References==
{{reflist|2}}
==External links==
*[http://www.sciencedaily.com/releases/2006/06/060601211745.htm Raiders Of The Lost Dimension: Understanding The Quantum Mechanics Of The Universe] (ScienceDaily) June 2, 2006
*[http://images.google.com/imgres?imgurl=http://www.spacedaily.com/images/microscopic-han-purple-sm.jpg&imgrefurl=http://www.einsteindaily.com/&h=80&w=100&sz=3&hl=en&start=12&tbnid=Bkb5C7Yoza2HyM:&tbnh=66&tbnw=82&prev=/images%3Fq%3D%2522Han%2Bpurple%2522%26gbv%3D2%26ndsp%3D20%26svnum%3D10%26hl%3Den%26safe%3Doff%26sa%3DN\ Raiders of the Lost Dimension (Article has color sample of Han Purple):]
[[Category:Pigments]]
[[Category:Barium compounds]]
[[Category:Silicon compounds]]
[[Category:Copper compounds]]
[[Category:Oxides]]
{{Category:Shades of blue}}
{{Category:Shades of violet}}