lime-plaster

In the coal district of Ou-ma-too, a manufacture of sulphate of iron, or green vitriol, is established. A quantity of hepatic iron pyrites, mixed with an equal amount of coal, both being broken into small similar-sized fragments, is accumulated into a pyramidal form, and coated carefully over with lime-plaster. By this process much heat is generated and extricated, and the heap remains untouched until the smoke has totally subsided. The mixture is then removed, thrown into water, and submitted to heat, when crystals of sulphate of iron are obtained by evaporation.

1800, George Newenham Wright et al., China..., page 65