Saltpetre as a substance: Tao Hongjing's flame test
Saltpetre, potassium nitrate, looks much like other white salts, and before anything can be made from it one has to be able to tell it apart. Around 492 the Chinese physician and alchemist Tao Hongjing, in a commentary on the classical pharmacopoeia, described two of its signs: the violet flame that potassium gives, and a violent flare on glowing charcoal.
According to Needham, from this point Chinese alchemists and pharmacists gradually learn to isolate and purify inorganic salts, and by the middle of the 9th century they have saltpetre pure enough for the first gunpowder mixtures.
Saltpetre is the main part of a gunpowder mixture, and whether the mixture burns at all depends on its purity. So the history of gunpowder begins not with a weapon but with the skill of recognising and purifying one salt.
Why this date
reliable within a few years. Needham dates Tao Hongjing's commentary on the classical pharmacopoeia to 492; the closely related compilation Ming yi bie lu belongs to 510. Needham himself allows that the criteria Tao describes may go back to the 3rd century.
Node sources
- Needham J., Ho Ping-Yü, Lu Gwei-Djen, Wang Ling. «Science and Civilisation in China», vol. 5, pt. 7: «Military Technology: The Gunpowder Epic», Cambridge University Press, 1986, p. 97 (1986), 97
But the turning-point is reached in +492, when Thao Hung-Ching