Han purple and Han blue

{{Short description|Pigments developed in ancient China}} [[File:Eastern Han Luoyang Mural of Liubo players.jpg|thumb|right|330px|Detail of a mural from an Eastern Han tomb near Luoyang, Henan showing a pair of Liubo players, containing both Han blue and Han purple pigments]]

'''Han purple''' and '''Han blue''' (also called '''Chinese purple''' and '''Chinese blue''') are synthetic barium copper silicate pigments developed in China and used in ancient and imperial China from the Western Zhou period (1045–771 BC) until the end of the Han dynasty ({{Circa|220 AD}}).

==Color== Azurite was the only natural blue pigment used in early China. Early China seems not to have used a natural purple pigment and was the first to develop a synthetic one.<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>

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. It is a purple in the way the term is used in colloquial 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 science, i.e. a nonspectral color between red and 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 (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 (Cu₂O) 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<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>

:3 BaCuSi₂O₆ → BaCuSi₄O₁₀ + 2 BaSiO₃ + 2 CuO

Above 1050&nbsp;°C, the CuO copper (II) oxide breaks down to copper (I) oxide:<ref name="Wiedemann and Berke"/>

:4 CuO → 2 Cu₂O + O₂

==Chemistry== Both Han purple and Han blue are barium copper silicates (containing barium, copper, silicon, 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 BaCuSi₂O₆.

Han purple has a layered structure with isolated 4-ring silicates, and contains a copper-copper bond which makes the 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 important 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 BaCuSi₄O₁₀. In 1993, it was discovered to occur naturally as the rare mineral effenbergerite.<ref name="Effenbergerite">[http://www.mindat.org/min-6911.html Effenbergerite mineral information. Mindat. Accessed September 23, 2008]"</ref>

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">{{cite journal | last1 = Berke | first1 = H. | last2 = Wiedemann | first2 = H. G. | year = 2000 | title = The Chemistry and Fabrication of the Anthropogenic Pigments Chinese Blue and Purple in Ancient China | journal = East Asian Science, Technology, and Medicine | volume = 17 | pages = 94–120 | doi = 10.1163/26669323-01701006 }}</ref> *Each four-ring silicate is 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 blue are similar in many of their physical properties, which allow them to be mixed, but they differ in their chemical properties.<ref name="Berke 2007"/>

====Exotic properties and applications to superconductivity and quantum computing research ====

In 2006 scientists at Stanford, Los Alamos National Laboratory and the Institute for Solid State Physics (University of Tokyo), showed that Han purple "loses a dimension" under suitable conditions when it enters a new state, as a Bose-Einstein Condensate. The researchers noted that

<blockquote>"We have shown, for the first time, that the collective behavior in a bulk three-dimensional material can actually occur in just two dimensions. Low dimensionality is a key ingredient in many exotic theories that purport to account for various poorly understood phenomena, including high-temperature superconductivity, but until now there were no clear examples of 'dimensional reduction' in real materials," said Ian Fisher</blockquote>

Other research team members alluded to potential applications to quantum computing. In conventional computers, electron charges transport information, but electron spin might in the future play a similar role in "spintronic" devices:

<blockquote>"Spin currents are capable of carrying far more information than a conventional charge current—which makes them the ideal vehicle for information transport in future applications such as quantum computing," stated first author Suchitra Sebastian. Noted Fisher: "Our research group focuses on new materials with unconventional magnetic and electronic properties. Han Purple was first synthesized over 2,500 years ago, but we have only recently discovered how exotic its magnetic behavior is. It makes you wonder what other materials are out there that we haven't yet even begun to explore."<ref>[http://news.stanford.edu/news/2006/june7/flat-060706.html 3-D insulator called Han Purple loses a dimension to enter magnetic 'Flatland'] ''Dye first made 2,500 years ago is focus of quantum spin study''. Stanford University News, June 2, 2006</ref><ref>{{cite web|url=https://www.sciencedaily.com/releases/2006/07/060712075724.htm|title=Purple Haze: Ancient Pigment Reveals Secrets About Unusual State Of Matter}}</ref><ref>[https://www.nsf.gov/mobile/discoveries/disc_summ.jsp?cntn_id=107087 Purple Haze] {{Webarchive|url=https://web.archive.org/web/20170107011358/https://www.nsf.gov/mobile/discoveries/disc_summ.jsp?cntn_id=107087 |date=2017-01-07 }} Ancient pigment reveals secrets about unusual state of matter. National Science Foundation, July 11,2006</ref></blockquote>

====Han purple==== Han purple is chemically and thermally less stable than Han blue. It fades and decomposes in dilute acid.<ref name="Wiedemann et al. 1998"/><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="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&nbsp;°C and forms a green-black glass at around 1200°C.<ref name="Berke 2007"/><ref name="Wiedemann and Bayer"/> It 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"/> and 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 {{convert|200|–|300|km|abbr=on}} north of the city of Xi'an. This is the area with large deposits of raw materials.<ref name="Berke 2007"/> No written records have been found about the production of Han purple or Han blue, 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 treated, though no evidence exists as yet of treatment.<ref name="Berke and Wiedemann"/>

The barium source was either witherite (BaCO₃) or baryte (BaSO₄).<ref name="Berke 2002"/> The rarity of witherite may favor baryte as the most likely source.<ref name="Berke and Wiedemann"/> Baryte has a slower decomposition rate and so favors Han blue production. Witherite conversely favors 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="Berke 2007"/><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>

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. Too much lead (more than 5%) causes partial melting and glass formation above 1000&nbsp;°C.<ref name="Wiedemann and Bayer"/>

The role of lead is:<ref name="Berke 2007"/> :BaSO₄ + PbO {{eqm}} PbSO₄ + BaO

===The manufacturing process=== The preparation of Han blue using malachite, silica and witherite as raw minerals also releases carbon dioxide and water vapor as by-products according to the following reaction:<ref name="Berke 2007"/>

:Cu₂(CO₃)(OH)₂ + 8 SiO₂ + 2 BaCO₃ → 2 BaCuSi₄O₁₀ + 3 CO₂ + H₂O

The solid-state reaction to produce barium copper silicates starts at roughly 900&nbsp;°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 an excess of silica is present and a longer reaction time is allowed.<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:<ref name="Wiedemann and Berke"/>

:3 BaCuSi₂O₆ → BaCuSi₄O₁₀ + 2 BaSiO₃ + 2 CuO

The temperature needed to be high (around 900–1000 °C) and kept at that temperature for long periods.<ref name="Berke 2007"/><ref name="Berke 2002"/> Han purple is thermally sensitive, so temperature control for producing Han purple needed to be fairly constant {{nobr|(± 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="Liu">{{cite journal|doi=10.1016/j.jas.2007.01.005|title=Influence of Taoism on the invention of the purple pigment used on the Qin terracotta warriors|year=2007|last1=Liu|first1=Z.|last2=Mehta|first2=A.|last3=Tamura|first3=N.|last4=Pickard|first4=D.|last5=Rong|first5=B.|last6=Zhou|first6=T.|last7=Pianetta|first7=P.|journal=Journal of Archaeological Science|volume=34|pages=1878|issue=11 |bibcode=2007JArSc..34.1878L |citeseerx=10.1.1.381.8552|s2cid=17797649 }}</ref><ref name="Berke 2007">{{cite journal|doi=10.1002/chin.200719227|title=The Invention of Blue and Purple Pigments in Ancient Times|year=2007|last1=Berke|first1=Heinz|journal=ChemInform|volume=38|issue=19}}</ref><ref name="Berke and Wiedemann"/> e.g. the potential use of twin bellows as used in metal production.<ref name="Berke 2007"/>

==Comparison==

{| class="wikitable" |- ! Characteristics ! Han purple ! Han blue |- | Chemical formula<br /> {{small|(Oxide notation)}} | BaCuSi₂O₆ <br /> {{small|(BaO·CuO·2SiO₂)}} | BaCuSi₄O₁₀ <br /> {{small|(BaO·CuO·4SiO₂)}} |- | Minimum temperature for production | 900 – 1000&nbsp;°C | ''circa'' 1000&nbsp;°C |- | Manufacture time | 10 – 24 hours | 20 – 48 hours |- | Decomposition temperature | 1050 – 1100&nbsp;°C | >1200&nbsp;°C |- | Thermally stable? | No | Yes |- | Stable in acid? | No | Yes |- | Color increase when ground? | Yes | Yes |}

==History== ===Hypothesis on origin=== Han blue and Egyptian blue have the same basic structure and have very similar properties.<ref name="Berke 2007"/> The main difference is that Egyptian blue (CaCuSi₄O₁₀) has calcium in the position of Han blue's barium (BaCuSi₄O₁₀). The similarity led some to suggest that Han blue was based on Egyptian blue knowledge, which had traveled 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 the Egyptian blue).<ref name="Liu"/>

The two hypotheses underlying the speculations about the exact chronology of the invention of these blue pigments can be summarized as follows: * 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"/>

===Chinese invention=== 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"/><ref>{{cite web|url= https://www-ssrl.slac.stanford.edu/content/science/highlight/2007-03-30/ancient-warriors-and-origin-chinese-purple |title= Ancient Warriors and the Origin of Chinese Purple |website=Stanford University |date= 30 March 2007 }}</ref>

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 glassmaking and the manufacture of pigments,<ref name="FitzHugh and Zycherman 1983"/> and to argue for independent Chinese invention.<ref name="Liu"/> Taoist alchemists may have developed Han purple from their knowledge of glassmaking.<ref name="Liu"/>

The lead is used by pigment maker to lower the melting point of the barium in Han Purple.<ref>{{cite web|url= https://hyperallergic.com/165493/a-lost-purple-pigment-where-quantum-physics-and-the-terracotta-warriors-collide/ |title= A Lost Purple Pigment, Where Quantum Physics and the Terracotta Warriors Collide|date= 18 December 2014}}</ref>

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 dynasties see a shift from lead-barium-silicate type glass to lead-soda-lime glass.<ref name="Seligman">{{cite journal|doi=10.1038/138721a0|title=Early Chinese Glass from Pre-Han to Tang Times|year=1936|last1=Seligman|first1=C. G.|last2=Ritchie|first2=P. D.|last3=Beck|first3=H. C.|journal=Nature|volume=138|pages=721|issue=3495|bibcode=1936Natur.138..721S |s2cid=4097744}}</ref> The reason for decline is debatable. Liu ''et al.''<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.{{fact|date=November 2014}}

==Uses in cultural contexts==

Han blue seems to have been favored in earlier (Zhou) periods, and Han purple in later periods (''circa'' 400 BC).<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 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 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 due to varying proportions of Han blue, Han purple, and colorless material.<ref name="FitzHugh and Zycherman 1983"/> They are thought to be pigment sticks which were traded then ground to be used as pigment bases in paints.<ref name="Wiedemann and Berke"/><ref name="Berke 2002"/> They may have been 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 emphasized 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 needed for the manufacture of Han purple ({{convert|950|–|1050|C|disp=sqbr}}), so the same kilns may have been used for both processes.<ref name="Liu"/> No evidence indicates 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">{{cite journal|doi=10.1002/jrs.1766|title=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|year=2007|last1=Cheng|first1=Xiaolin|last2=Xia|first2=Yin|last3=Ma|first3=Yanru|last4=Lei|first4=Yong|journal=Journal of Raman Spectroscopy|volume=38|pages=1274|issue=10|bibcode=2007JRSp...38.1274C }}</ref> and in the Han dynasty Yangling tombs of Emperor Liuqi and his Empress (156–141 BC).<ref name="Zuo">{{cite journal|doi=10.1002/jrs.963|title=Analysis of the pigments on painted pottery figurines from the Han Dynasty's Yangling Tombs by Raman microscopy|year=2003|last1=Zuo|first1=Jian|last2=Zhao|first2=Xichen|last3=Wu|first3=Ruo|last4=Du|first4=Guangfen|last5=Xu|first5=Cunyi|last6=Wang|first6=Changsui|journal=Journal of Raman Spectroscopy|volume=34|pages=121|issue=2|bibcode=2003JRSp...34..121Z }}</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 Luoyang 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 Xi'an area is one of the last examples of the use of synthetic barium copper silicate pigments (Han purple).<ref name="Cheng"/>

==Preservation== Due to the instability of Han purple, it shows significant signs of weathering on archaeologically excavated artifacts. The copper(I) oxide formed in the decomposition of Han purple (see section on color) remains stable, but Han purple continues to deteriorate, and 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, so preserves better. Han purple reacts with oxalic acid to form BaCu(C₂O₄)₂. The light blue color of this coordination polymer 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. They are *BaCu₂Si₂O₇ (blue color) *Ba₂CuSi₂O₇ (light blue color)<ref name="Wiedemann and Berke"/>

==See also== *Blue pigments *{{annotated link|Cobalt blue}} *{{annotated link|Egyptian blue}} *{{annotated link|Maya blue}} *{{annotated link|Persian blue}} *{{annotated link|Prussian blue}} *{{annotated link|Ancient Chinese glass}} *{{annotated link|List of colors}} *{{annotated link|List of inorganic pigments}}

==References== {{reflist|30em}}

==External links== *[http://www.Magnet.FSU.edu/mediacenter/news/pressreleases/2006may31.html Raiders of the Lost Dimension] {{Webarchive|url=https://web.archive.org/web/20141225132115/http://www.magnet.fsu.edu/mediacenter/news/pressreleases/2006may31.html |date=2014-12-25 }} (Magnet Lab, FSU) May 21, 2006 *[https://web.archive.org/web/20140810151358/http://www.spacedaily.com/reports/Raiders_Of_The_Lost_Dimension.html Microscopic image of Han Purple] (credit: Marcelo Jaime of MST-NHMFL)

{{DEFAULTSORT:Han Purple And Han Blue}} Category:Shades of blue Category:Shades of violet Category:Shades of purple Category:Pigments Category:Inorganic pigments Category:Barium compounds Category:Copper(II) compounds Category:Oxides Category:Silicates Category:Ancient China