Han blue

Han purple and Han blue (also called Chinese purple and Chinese blue) are synthetic barium copper silicate pigments that were developed in China in ancient times.

Han purple and Han blue were used in ancient and imperial China from the Western Zhou period (1045–771 BC) until the end of the Han dynasty (c. 220 AD).[1][2]

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.[3]

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 electric 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 theory, i.e. a non-spectral 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).[2] The decomposition of Han purple to form copper (I) oxide is[4]

3 BaCuSi2O6 → BaCuSi4O10 + 2 BaSiO3 + 2 CuO

Above 1050 °C, the CuO copper (II) oxide breaks down to copper (I) oxide:[4]

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).[2][5]

Han blue

Han blue has the chemical formula BaCuSi4O10. In 1993, it was discovered to occur naturally as the rare mineral effenbergerite.[6]

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.[7]
  • Each 4-ring silicate is linked to four others in the adjacent level, in a zig zag pattern.[5]
  • The copper ions are very strongly contained within the stable silicate structure.[2]

Chemical and physical properties

Han purple and blue are similar in many of their physical properties, which allows them to be mixed, but they differ in their chemical properties.[2]

Han purple

Han purple is chemically and thermally less stable than Han blue. It fades and decomposes in dilute acid.[5][8][9] Han purple starts to decompose at temperatures more than 1050–1100 °C and forms a green-black glass at around 1200 °C.[2][8]

It becomes more purplish when ground.[7]

Han blue

Han blue is more chemically and thermally stable. It does not break down in dilute acids [5][8] and becomes more bluish when ground.[7]

Manufacture

Manufacturing depends on the raw materials, their ratios, fluxes, temperature, atmosphere and reaction time.[5]

Production seems to have been focused in northern China, around 200–300 km north of the city of Xian. This is the area with large deposits of raw materials.[2] There are no written records about the production of Han purple or Han blue and so information about manufacture has been achieved through experimentation.[7]

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.[7]

The barium source was either witherite (BaCO3) or baryte (BaSO4).[9] The rarity of witherite may favor baryte as the most likely source.[7] Baryte has a slower decomposition rate and so favors Han blue production. Witherite conversely favors Han purple.[8] In the use of baryte, lead salts (lead carbonate or lead oxide) would have been needed to increase yield.[9] Lead has been detected in association with Han purple and Han blue [2][10][11]

Lead acts as a catalyst in the decomposition of barium minerals and as a flux.[2] The amount of lead is important. Experiments show that too much lead (more than 5%) causes partial melting and glass formation above 1000 °C.[8]

The role of lead is[2]

BaSO4 + PbO ↔ PbSO4 + BaO

The manufacturing process

The production for Han blue using witherite is[2]

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

The solid state reaction to create barium copper silicates starts at roughly 900 °C.[8] Han purple is formed fastest.[2][5] Han blue forms when there is an excess of silica and a longer reaction time allowed for.[2] Early Chinese manufacture generally produced a mixture of Han blue and Han purple particles in various ratios, but pure colors were sometimes manufactured.[11] 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.[4]

Prolonged firing causes Han purple to break down and form Han blue:[4]

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.[2][9] Han purple is thermally sensitive and so temperature control for producing Han purple needed to be fairly constant (±50 °C).[4] Han blue is thermally less sensitive.[7] 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.[4]

Temperature would have been controlled by testing of firing materials, the size, shape, and material of the kiln, and the control of the environment.[7] Technology for achieving and maintaining high temperatures would have been known from metal and ceramic production[1][7] e.g. the potential use of twin bellows as used in metal production.[2]

Comparison

Han purple Han blue
Chemical formula BaCuSi2O6 BaCuSi4O10
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.[2] The difference is that Egyptian blue (CaCuSi4O10) has calcium in the position of Han blue's barium (BaCuSi4O10). The similarities lead some to suggest that Han blue was based on Egyptian Blue knowledge, which had travelled east along the Silk Road.[9] Independent innovation in China would still have been needed to replace calcium with barium[9] (the Han pigments start to form at 100–200 °C higher than Egyptian blue[1]).

The 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.[2]
  • Alternatively, that examples of Han blue predate the official Silk Road and therefore that development was completely independent.[1]

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.[1]

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,[10] and to argue for independent Chinese invention.[1] It has been argued that Taoist alchemists developed Han purple from their knowledge of glass making.[1]

The increase and decrease of barium glasses, and Han purple and Han blue, follow similar patterns. Both peaked in the Han dynasty, declining afterwards.[1] Pre-Han to Tang dynasties see a shift from lead-barium-silicate type glass to lead-soda-lime glass.[12] The reason for decline is debatable. Liu et al.[1] attribute the decline to the decline of Taoism when Confucianism was introduced, since they link pigment manufacture to the ideology of Taoism. Berke (2007)[2] 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 favored in earlier (Zhou) periods, and Han purple in later periods (c. 400 BC).[2]

The Han pigments consist of varying combinations of blue, purple and colorless components.[11] The grinding together of Han purple and Han blue would have allowed a variety of blue-purple shades.[7]

The pigments were used for

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.[2]

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.[10] It is thought that they were pigment sticks which were traded then ground to be used as pigment bases in paints.[4][9] It has also been suggested that they are of importance themselves, as ceremonial or bureaucratic items of importance.[10]

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.[3] Han purple seems to have mostly been used on the trousers (pants) of the warriors.[3] The pigment was bound to the terracotta surface with lacquer.[13] 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.[1] There is no evidence of Han blue being used for the warriors (azurite was used for the blue).[2][3]

Painted pottery figurines

Smaller painted pottery figurines have been found e.g. the Western Han dynasty Chu Tombs, Xuzhou, Jiangsu Province[14] and in the Han dynasty Yangling tombs of Emperor Liuqi and his Empress (156–141 BC).[15]

Ceramic vessels

Han blue and Han purple were used to decorate Han dynasty Hu dark grey pottery vessels.[10][11]

Metal objects

Bronze vessels in the Han dynasty, e.g. a bowl and top of a steamer were decorated with Han purple.[11]

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.[10]
  • 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).[14]

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 so its purple color increases with time.[2]

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.[11] In addition, Han blue has fungicidal properties and so preserves better. Han purple reacts with oxalic acid to form BaCu(C2O4)2. 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.[4]

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)[4]

See also

References

External links

  • Raiders of the Lost Dimension (Magnet Lab, FSU) May 21, 2006
  • Microscopic image of Han Purple (credit: Marcelo Jaime of MST-NHMFL)
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