have been found about the production of Han purple or Han blue, so information about manufacture has been achieved through experimentation. 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. The barium source was either witherite (BaCO3) or baryte (BaSO4). The rarity of witherite may favor baryte as the most likely source. Baryte has a slower decomposition rate and so favors Han blue production. Witherite conversely favors Han purple. In the use of baryte, lead salts (lead carbonate or lead oxide) would have been needed to increase yield. Lead has been detected in association with Han purple and Han blue. Lead acts as a catalyst in the decomposition of barium minerals and as a flux. The amount of lead is important. Too much lead (more than 5%) causes partial melting and glass formation above 1000 °C. The role of lead is: BaSO4 + PbO 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: 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 °C. Han purple is formed fastest. Han blue forms when an excess of silica is present and a longer reaction time is allowed. Early Chinese manufacture generally produced a mixture of Han blue and Han purple particles in various ratios, but pure colors were sometimes manufactured. 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. Prolonged firing causes Han purple to break down and form Han blue: 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. Han purple is thermally sensitive, so temperature control for producing Han purple needed to be fairly constant . Han blue is thermally less sensitive. 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. Temperature would have been