Copperas, a name given to green vitriol, particularly to that of iron. It is purified and prepared in the same manner as alum and saltpetre, being passed through several lixivia, till it is wholly reduced to crystal. It is used in dyeing wool and hats black, in making ink, tanning leather, and in pre-paring a kind of Spanish brown for painters.

A patent was granted in May 1791 to Mr. Wm. MuRdock, of Redruth, Cornwall, for a method of making (from the same materials, and from processes entirely new) copperas, vitriol, and different sorts of dye, or dying stuff, paints, and colours.

The patentee directs any quantity of what remains after the calcination of mundic, or such other ores as contain sulphur, arsenic, and zinc, to be taken, and washed in water which is to be placed on the top, or on any other part of the kiln, house, or oven, while the mundic or other ores are burning; the heat of which will cause the water to evaporate ; or the water may be evaporated to a crystallizing point, by exposing it to the heat of the sun, after which it should be suffered to stand for 24 hours, or longer, when crystals of copperas, or green vitriol, will be produced. From this process arises a considerable saving ; as. the ores remaining after fusion, may be ap-plied to various chemical purposes.