CHROMIUM, (Chrome.) A metal dis covered by Vauquelin in 1797. It exists chiefly in two native compounds • the one formerly called red lead of Siberia, which is a chromate of lead ; the other, a compound of the oxides of chromium and iron. Chromium is a whitish, brit tle, and very infusible metal ; sp. gr. 5.5. When with nitre, it is converted into chromic acid. Its equivalent num ber is 28. It forms two compounds with oxygen,—a green oxide, and a red per oxide ; the latter being sour, and com bining with salefiable bases, is called chromtc acid. The oxide consists of 28 chromium 12 oxygen ; and chromic acid of 28 chromium 24 oxygen. Chro mic acid is of a red color, and forms a variety of colored compounds, some of which are much used in the arts ; such as the chromate and bichromate of potash, largely manufactured for the use of cali co-printers, and the chromatee of lead, employed as yellow and red dyes and paints. The oxide of chrome is green, and furnishes a valuable color forporce lain and in enamel. Chromic acid gives color to the ruby, and the green of the emerald is due to oxide of chrome. Chrome iron ore is found in abund ance distributed over the United States. In Maryland, at the Bare Hills near Bal timore • and in Delaware county, Pa., it is found very plentifully. In that locality, one firm has upwards of 100 hands em ployed, and are daily shipping the mi neral to Baltimore. The proprietors of farms upon which it is found, receive $3 per ton for washed chrome—and in the rook state it is sometimes worth $5 per ton. Mr. Wood's chrome iron ore mine, on the River Ortorara, separating Chester and Lancaster counties, Pa., is probably the most extensive chrome mine in the world, being 170 feet deep-200 feet long and 30 feet broad : the ore yields 93 per cent. of oxide of chrome. The mineral is also found in great abundance at vari ous points east of the Mine Ridge, in Lancaster, Chester, and Delaware coun ties, Pa., and is all, or nearly all, shipped to Baltimore, whence it is exported large.
ly to Europe. This ore forms the basis of many of the colored preparations of chrome, for which see DYEING.
The chief application of this ore is to the production of chromate of potash, from which salt the various other prepa rations of this metal used in the arts are obtained. The ore, freed, as well as pos sible, from its gangue, is reduced to a fine powder, by being ground in a mill under ponderous edge-wheels, and sifted. It is then mixed with one-third or one half its weight of coarsely bruised nitre, and exposed to a powerful heat, for seve ral hours, on a reverberatory hearth, where it is stirred about occasionally. In the large manufactories of this country, the ignition of the above mixture in pots is laid aside, as too operose and expen sive. The calcined matter is raked out, and lixiviatecl with water. The bright yellow solution is then evaporated brisk ly, and the chromate ofpotash falls down in the form of a granular salt, which is lifted out from time to tinle from the bottom with a large ladle, perforated with small holes, and thrown into a draining box. This saline powder may be formed into regular crystals of neutral chromate of potash, by solution in water and slow evaporation; or it may be converted into a more beautiful crystalline body, the bichromate of potash, by treating its con centrated solution with nitric, muriatic, sulphuric, or acetic acid, or, indeed, any acid a stronger affinity for the second atom of the potash than the chro mic acid does.
Dichromate of potash, by evaporation of the above solution, and slow cooling, may be obtained in the form of square tables, with bevelled edges, or flat four sided prisms. They are permanent in the air, have a metallic and bitter taste, and dissolve in about one-tenth of their weight of water, at 60° F. ; but in one half of their weight of boiling water. They consist of chromic acid 13, potash 6; or, in 100 parts, 68.4+ 31.6. This salt is much employed in calico-printing and in dyeing ; which see.