It is known that calcined gypsum, after being moistened with a solution of alum and again burnt, acquires much greater hardness and solidity. Another process is to make a solution of one pound of borax in nine pounds of water, which is formed over the calcined fragments of gypsum. They are then kept at a strong red heat for six hours, ground to a powder and worked. The effect is said to be much better if a pound of tartar and twice the quantity of wp-ter are added to the solution.
This article was originally published with the title "How to Harden Gypsum"