Improvements in the Manufacture of Pearl Hardening or Satin Spar

1577. Brothers, W., and Brothers, W. M. Jan. 20. Calcium sulphate.-Gypsum or other natural or waste calcium sulphate is treated by boiling with hydrochloric acid either in an open vessel or under pressure. Finely-crystalline calcium sulphate, such as pearl hardening or satin spar, such as is used in finishing paper, textile fabrics, or other articles, is removed from the hot solution by filtering, the clear solution being allowed to cool, when the rest of the calcium sulphate crystallizes out. Only enough calcium sulphate may be added to the acid to form a clear solution. In the case of impure calcium sulphate, it is washed and treated with sulphuric acid before being placed in the hydrochloric acid.