A name apparently given by persons ignorant of its true nature, to the sulphate of iron, obtained by the decomposition of iron pyrites. The English copperas, or green vitriol, as it is also called, is made from the natural combination of iron with sulphur, known in the various places where they are found, by the several names of copperas stones, horse gold, gold stones, etc. These are collected in great quantities, and laid in heaps about two feet thick, upon a clay floor, surrounded by boards, that direct the rain water that falls upon them to flow into a cistern. The clay floors at some works are 100 feet long, 15 broad at top, but narrowing to 12 at bottom, as they shelve gradually to allow the water to run off easier. The cistern usually contains about 100 tons of water. The copperas stones lie 5 or 6 years before they yield a strong liquor. The sun and rain are found to be the proper agents; and that other water, although prepared by exposure to the sun, and sprinkled on them, only retards the produce.
When a new bed is made, the work is hastened by mixing a good quantity of the old fermented earth with the new stones; and when this has come to perfection, it is refreshed every fourth year by a layer of fresh copperas stones on the top. The copperas water is considered rich if it weigh fourteen-eightieths more than water. The copperas liquor is boiled in leaden vessels, containing about twelve tons. About a cwt. of old iron is put into it at first, and more added as it dissolves, amounting, in one boiling, to about 15 cwt. Fresh water from another boiler is added to supply the loss from evaporation. When the boiling is finished, (which is determined by the deposition of crystals in a little earthen vessel, after a small quantity has been allowed to cool therein,) the liquor is drawn off into a tarras cistern 20 feet long, 5 feet deep, 9 feet over at top, but tapering towards the bottom, where, being left for a fortnight to cool and crystallize, the remaining liquor is drawn off, and is reserved for boiling with new liquor. At the bottom and sides of the cistern or coolers there is usually a crystalline deposit 5 inches thick, - that at the sides being of a bright colour, while that at the bottom is foul and dirty.
At some works, instead of putting all the iron into the boiler, a portion of it is added to the liquor in the cistern before boiling. When the pyrites is too abundant in sulphur, and does not alter by exposure to the weather, it is usual to roast it in piles to drive off the superabundance; and when the sulphur is required as a separate product, to distil the pyrites in close vessels. Copperas is also formed by simply exposing some kinds of bituminous earth to air and moisture, from which the crystals are afterwards obtained by washing. The French also manufacture copperas by dissolving old iron in weak sulphuric acid, and crystallizing the solution. Copperas crystallizes in the form of rhomboidal prisms, which are transparent and of a beautiful green colour. Its taste is harsh and styptic; it reddens vegetable blues; two-thirds of cold water, or three-fourths of boiling water, dissolve it. A moderate heat calcines and whitens copperas, by driving off the water of crystallization; a greater heat expels the sulphuric acid. Its constituents, according to Berzelius, are 28.9 acid, 28.3 protoxide, and 43 water; according to Mr. Perret, 1 prime acid, 2 oxide, and 7 water.