Chromium (sym. Cr, atom. weight, 52.5) is a metal, so called (chrōma, 'colour') from the many-coloured compounds it produces. It was discovered by Vauquelin in 1797, in the chromate of lead, , one of its rarer ores. As chrome iron ore, , it is extensively distributed in America, Sweden, Hungary, &c.
The metal has been obtained in several modifications, one of which is so refractory as to be infusible at a temperature sufficient to volatilise platinum, while it may be heated to redness without oxidation, and resists the action of most acids. Another variety is a powder which burns brilliantly when heated in air, and is readily dissolved by acids. The metal itself has not been employed in the arts, but many of the chromates are much used in painting and colouring. It has of late years been found useful for combining with the iron of projectiles. Chromium forms four compounds with oxygen, of which the chief are chromic oxide, , and chromic acid, .
Chromic oxide (chrome green) possesses a bright-green colour, and is the colouring ingredient in the emerald. Owing to its indestructibility by heat, it is used in porcelain-painting; while, being non-poisonous, it is now substituted for arsenical green pigments in wall-papers.
Chromic acid or anhydride, , forms dark-red crystals, containing no water. When strongly heated it becomes incandescent, and is converted into chromic oxide. It forms several classes of salts: the Chromates, such as chromate of lead, ; the Bichromates, of which bichromate of potash, , or , is an example; and the trichromates and tetrachromates, which are unimportant.
Chromate of Lead, , is well known to artists as 'chrome yellow.' It is readily prepared by mixing a solution of acetate of lead with one of chromate of potash. When boiled with lime, its bright-yellow colour disappears, and a scarlet basic chromate is obtained, which is used in the dyeing of calico.
Bichromate of Potash, , is prepared from chrome ironstone, by calcination with chalk and carbonate of potash, and subsequent treatment with nitric acid. It forms large red crystals, and has many uses in the arts. When added to a solution of gelatine and allowed to dry, it is found that on exposure to light the gelatin becomes insoluble, and a process based on this property has been used to some extent in photography. As an oxidising agent in galvanic batteries it is very effective, while mixed with sulphuric acid it is used in the bleaching of oils.
Chromic acid and its salts are all more or less poisonous, owing to their corrosive and oxidising action on organic tissues. Chrome Steel containing a small percentage of chromium is valued for its toughness and strength; and chromium is now generally used with iron for projectiles.