On Chemical Reactions of Tibetan Colour Materials

【以下の文は本年7月に開催された2001北京蔵学討論会 (蔵学研究中心, 中華人民共和国)に於ける発表の要旨です】

The colour composition system or the technique of mixing colours has a long theoretical tradition in Tibet. Among those theories, there has been a quite interesting discussion not only on Tibetological interest but also from the point of view of the scientific history.

Several Tibetan artists wrote that "ba bla" (orpiment yellow) should not be mixed with "spang ma" (malachite green). For example, Mi pham rgya mtsho (1846-1912) says,

"ljang gu dang ba bla 'dres na tshon rul ba ste mi legs/ gcig gi steng du gcig phog na yang nag por 'gro/"[Mi pham: 86]If orpiment and green are mixed the colour will change chemically. If they come into contact with each other it can darken [and spoil another colour].

And for the similar kind of chemical reaction, we have to be carefull when "li khri" (minium orange) is mixed with "mtshal" (vermilion red).

li khri mang po la mtshal nyung du btab kyang rul ba sogs shes par bya'o/ [Mi pham: 86]You shoud know that on much minium if even a little vermilion is mixed, then a chemical change occurs.

Here we are informed two kinds of chemical reactions of Tibetan Colour materials. One is "ba bla" with "spang ma" and the other is "li khri" with "mtshal."

First case, the chemical reaction when the colour material "ba bla" (orpiment yellow): a natural yellow trisulfide of arsenic, As2S3 (三硫化砒素), and "spang ma (malachite green)" a basic carbonate of copper, CuCO3・Cu(OH)2 (塩基性炭酸銅) can be analysed as following chemical formula:

CuCO3・Cu(OH)2 + As2S3 → Cu2S (copper sulfide: 硫化銅=硫銅鉱=輝銅鉱)

The substance of this copper sulfide apears black colour.

The case of when "li khri" (minium orange: the synthetic tetraoxide of lead, Pb3O4 : 四三酸化鉛) is mixed with "mtshal" (vermilion red: mercury sulfide: 硫化水銀) can be analysed as following chemical reaction formula:

Pb3O4 + HgS → PbS (lead sulfide: 硫化鉛)

This substance also apears black colour. So we can possible to prove that Mi pham rgya mtsho's argument has a chemical rationality.