SALTS OF IRON.
208. GREEN VITRIOL, IRON VITRIOL, or COPPERAS, is a mineral salt formed on a decomposition of pyrites ([236]) by the moisture of the atmosphere. It is also called SULPHAT OF IRON.
Its colour is bright green, and its taste very astringent; a solution of it in water dropped on oak bark instantly produces a black spot.
Although copperas is occasionally found in grottoes, caverns, the galleries of mines, and other places; yet, being much in request by dyers, tanners, and the manufacturers of ink, it is artificially prepared from pyrites. This mineral being moistened and exposed to the air, a crust is formed upon it, which is afterwards dissolved in water; and from this the crystals of vitriol are obtained by evaporation.
The principal use of vitriol is in dyeing woollen articles, hats, and other manufactures, black. It is the basis of ink, and is used in the manufacture of Prussian blue. If it be reduced to powder by the action of fire in a crucible, and mixed with powder of galls, it forms a dry portable ink. Sulphuric acid ([24]) may be obtained from this kind of vitriol by distillation. The residue, after the process is completed, is used as a red paint; and when washed, is employed for the polishing of steel.
SALTS OF COPPER.
209. BLUE VITRIOL, or SULPHAT OF COPPER, is a blue salt formed by a combination of copper with sulphuric acid ([24]).
This substance, though sometimes found in a state of concretion, or in the form of powder disseminated over the surface of stones that have been in contact with water impregnated with it, is more frequently an artificial preparation obtained from evaporating the water which runs through copper mines. In the mines of Neussol, in Hungary, at the depth of 380 feet beneath the surface of the ground, are several vats, placed at different distances, for the purpose of collecting the water impregnated with copper, and which flows into them through a kind of gallery above. From this water the vitriol is afterwards separated by evaporation. A process somewhat similar is pursued in our own country.
In the principal blue vitriol manufactories established in France, the operation is thus carried on. Pieces of copper are first dipped into water, and their surface, while wet, is covered with a stratum of powdered sulphur. The copper thus prepared is put into an oven, and heated to redness. After some time, it is taken out, and, while hot, is plunged into a vessel filled with water. These operations are repeated several times, till the whole of the copper is dissolved, and the water becomes loaded with vitriol. Thus saturated, the water is placed over a fire till all the fluid particles are dissipated, and the vitriol alone is left.
Blue vitriol is used by artists and manufacturers in various ways. It is employed in dyeing: and enters into the composition of black colours, to which it gives depth and solidity. Blue feathers are stained by plunging them into a hot solution of it. The beautiful grass-green colour of the shops, called mineral green, is made from blue vitriol; and fowling-pieces and tea-urns are browned by washing them with a preparation of it.