Gypsum

Gypsum subjected to a temperature of 250- 280 degrees C loses its water completely, and thus its binding capacity.
Gypsum lacks opaque properties and therefore is not used in painting. It is used primarily for gesso on wood.
When used in grounds, it surpasses the value of gesso, as it is more flexible than it.
Like chalk, it serves as a matrix in the production of dyes (such as seal wax), and as a filler in forgery. Dyes adulterated with gypsum are not suitable for the fresco technique, as they badly affect the proper formation of the calcium carbonate film. They also affect the tone of the dye.
Recognition:
Dissolves easily in dilute hydrochloric acid when heated, does not agitate when dissolved.