By: Emma (Y13)
Introduction
Egyptian Blue is well known for its advanced technology and has been suggested to be one of the earliest forms chemical synthesisation. However, its Chinese counter parts of Chinese Purple and Blue (also known as Han Purple and Han Blue), which came almost 500 years after Egyptian Blue, are less well known but possible more impressive.
When Chinese farmers were preparing their land for irrigation in 1974, the last thing they expected to find were pieces of terracotta with facial features sculpted on their surface. This led to the uncovering of China’s first emperor’s, Qin Shi Huang, Terracotta Army. Over 8 000 soldiers, 600 horses and nearly 40 000 weapons were found in three separates pits on an area of 38 square miles. The soldiers were crafted in order to protect the emperor on his journey to and during the afterlife. It is believed that more soldiers were intended to be crafted, due to the empty bays that had been dug out, but were never filled as Huang died much early than expected at age 48.
Analysis conducted but Art Historians found unusual traces of purple and blue pigments on the surface of the warriors but were unable to retrieve a sample before it vanished as it reacted with the air. The use of purple and blue became a great point of curiosity as pigments were previously crafted from natural dyes of the earth, or earth colours, however blue is not an earth colour. Egyptian Blue was the only form of blue pigment previously found but did not react with the air in the same way this pigment did, but why?
Chemical Composition
Chinese Purple, as this pigment came to be known, is formed from baryte (BaSO4) and a mixture of quartz and copper minerals (Berke, 2002), creating a resulting chemical formula of: BaCuSi2O6. This differs greatly from the structure of Egyptian blue, which is calcium based rather than barium based, suggesting that the Chinese were unaware of the Egyptian Blue compound as calcium was much more stable and plentiful.
In its pure state, Chinese Purple is actually a dark blue as the copper contaminates are what produce the red colouring (Liu et al., 2007). It is this copper found within Chinese Purple that has caused the biggest curiosity among chemists and art historians as it contains a copper-to-copper bond (Cu-Cu), a structure most closely related to that of copper acetate, but the pigment is much more diamagnetic (Berke, 2002), further highlighting the rarity and uniqueness of Chinese Purple.
The Cu2 units within the pigment create a structure that has significant difference to its sister Chinese Blue and the Egyptian Blue and most likely has resulted in the unstable nature of Chinese Purple as it will react with both mineral and oxalic acids (Berke, 2002), resulting in the fast decay of the pigment when exposed to the air after so long underground, as well as microorganisms that secrete these types of acid. Some scientists have suggested that this compound is the first to contain a metal-to-metal bond, though many have discredited this theory after looking at it in greater depth.
As the copper ions are fixed within the compound, light emission is affected. Both pigments produce a dual band pattern when looked at through a fluorescence spectrometer, but unlike Chinese Blue with both wavelengths being equal in length, the purple compound has a short wavelength and a longer one with the red (allowing green light to be absorbed) being the shorter of the two and blue (absorbing orange) the longer one (Berke and Wiedemann, 2000).
With the chemical properties explaining both the colour and instability of Chinese Purple, how were chemists able produce such large quantities of the pigment to glaze over 8 000 life sized warriors?
Production of Chinese Purple
Chinese Purple is produced as a kinetic product of Chinese Blue taking around 12-24 hours to produce with the blue pigment taking around double to synthesise at temperatures of 1 560°C. This became problematic as Chinese Purple begins to decompose at around 1 000°C which is very far from the temperature needed to produce Chinese Blue (Berke, 2002) as one could not be made in sufficient quantities without the other. The answer to this problem came from the process of bronze casting during the Shang Dynasty, 1600 BC to 1027 BC.
In order to achieve the finest of details on large scale castings, metallurgists discovered that adding small amounts of lead (most commonly in the form of PbO) not only reduced the viscosity of the bronze but its melting temperature (Liu et al., 2007). This was a well-known process during the Qin Dynasty, suggesting that the use of lead oxides in the production was deliberate to lower the melting temperature of the baryte and allow for Chinese Blue to be produced at a temperature much closer to that of Chinese Purple (Liu et al., 2007).
There is also evidence to suggest that as the Terracotta Warriors were being fired at temperatures in the range of 950 -1 050, meaning the craftsmen had unintentionally received technology to produce the pigments at the required temperature. Furthermore, this explains why slightly different shades of Chinese Blue and Purple are found throughout the Qin and Hang Dynasties, based on the temperatures that were controlled in the pottery kilns (Liu et al., 2007), with more blue than purple being found towards the end of the dynasties. This is likely to relate to the thermal instability of Chinese Purple and its low thermal decomposition temperature (Berke and Wiedemann, 2000) and the use of different temperatures within the kilns during the different time periods and dynasties.
Historical and Social Context of Chinese Purple
Despite many historians originally believing that the Chinese learnt about Egyptian Blue through the Silk Road, more and more research is suggesting that the Chinese created this technology entirely of their own accord, without knowledge of the Egyptian technology. The biggest indicator of this outside of the different chemical bases would be the production of barium-lead-glass by Tao Alchemists during the Warring States Period and the Qin and Hang Dynasties.
The refractive index of barium glass is much bigger than that of normal glass which is what contributes to its jade-like appearance and therefore its appeal to Taoist alchemists and monks. Ancient Chinese texts have referred to the different colours the glass could appear depending on what was added to it and how ‘the Taoist monks used to make five-coloured jade with five stones.’ (Lun Heng: Wang 27-97). Jade was a precious material within the Tao religion, with many properties such as those to preserve the human body and spirit as well as provide physical immortality, so it was only natural that they wanted to synthesise this precious material and make more of it.
Emperor Qin Shi Huang was notorious for his fear of death and became very interested in Taoism during his hunt for an elixir of life. A connection can be made between these two as Huang began to drink an elixir, but it contained lead, so he ended up killing himself faster instead. It does not seem unreasonable that the elixir he was drinking contained synthesised jade.
This information further supports the idea put forward by (Liu et al., 2007) that Taoism heavily influenced the creation of Chinese Purple, and their production of barium-lead-glass is what ultimately lead to the creation of Chinese Purple.
Summary
Perhaps what makes Chinese Purple so special, even more so than Egyptian and Chinese Blue, is its unstable nature and how craftsmen during the Qin and Han dynasties were able to combat this with the resources that were available to them at the time. Or maybe it is how the decline of Taoism lead to the forgetting of the barium-lead-glass production process (Liu et al., 2007) until the 19th century. However, it is undebatable that there is a certain art to the creation of the pigment that we in the 21st century will never be able to quite replicate with all the advance equipment that is available to use as we lack the perfect balance of control of quantities and coincidence of kiln temperatures.
References:
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