Copperas is generally prepared from the soft, white variety of iron
pyrites, frequently found to a great extent in the coal measures.
These, on exposure to air and moisture, decompose the latter, taking up
oxygen, and are thus converted into sulphate of iron. Copperas forms
pale greenish blue semi-transparent crystals, containing forty-five
per cent. of water. If this be expelled, there remains a dull whitish
powder. The crystals dissolve readily in one and one-half times their
weight of cold water, and less than half their weight of boiling water.
The direct uses of copperas have very much diminished in feather
dyeing; as for dyeing black in conjunction with logwood it has been
almost entirely superseded by bichromate of potash. In drabs and in
saddening down light colors it is, to a certain extent, still used.
It is used in quantities so small, however, that there is no serious
results to be feared, as it must be used in quantity to injure the
fibre.