Scientific American 25, 20.6.1863
Chromium is a metal which forms several oxides that are much used in the practical arts. Its two native combinations are the chromate of lead and the chromite of iron - a compound of the oxides of chromium and iron. Metallic chromium may be obtained by igniting its oxide intensely with about one-tenth of its weight of charcoa. Its color resembles that of platinum; it scratches glass and takes a good polish. There are four oxides of chromium, namely: Protoxide, CrO; a sesquioxide, Cr2O3; an intermediate oxide, Cr O, Cr2O3, and chromic acid Cr O3. The sesquioxide of chromium (Cr2 O3) is obtained by heating chromate of mercury or chromate of ammonia to a dull redness; and it is also formed by the action of a red heat upon bichromate of potassa. In this case neutral chromate of potassa is formed, which may be removed by washing the product. This oxide is of a green color, is not changed by heat and is much used in enamel and porcelain painting, also as a pigment for printing on paper. Chromic acid forms carmine-colored needle-shaped crystals, rapidly deliquescing in the air, and dissolving very easily in water, forming a dark reddish brown or lemon yellow solution. The concentrated acid corrodes paper like oil of vitriol, and, when in dilute solution, it likewise attacks paper and other kinds of organic matter, especially when aided by light or heat, becoming converted into the brown oxide or the green sesquioxide, according to the extent of the decomposition. The chromate of potash is manufactured by heating pulverized chrome iron-ore with carbonate of potassa and a little niter in a reverberatory furnace - stirring the mixture to absorb oxygen. The product thus obtained is digested in water, and the solution obtained is super-saturated with nitric acid, which precipitates silica and leaves the chromate in solution. The bichromate of potash is much used in the arts of dyeing and printing cotton and woolen fabrics; also in photography. It is obtained by adding a sufficient quantity of sulphuric acid to a solution of the chromate until its taste becomes sour, then setting it aside for a few days, when deep orange crystals are formed, constituting the bichromate. The crystals are anhydrous prisms, and are soluble in hot water. Solutions of these chromates are very injurious to the skin, causing sores which are difficult to heal. Ppaer impregnated with a solution of the bichromate possesses photographic powers. It has remarkable coloring properties, as one art of it will impart a yellow color to 40,000 parts of water.
The great use of bichromate of potash is photographic operations in due to the ready way in which it parts with some of its oxygen to organic matter under the influence of light. Bichromate of potash may be looked upon as containing one part of chromic acid in the free state. Now chromic acid has a large quantity of oxygen locked up in it (threeequivalents of oxygen to one equivalent of chromium) and it is somewhat of an unstable compound, having a tendency to give up oxygen and pass to a lower state of oxidation at the slightest opportunity. When placed in contact with an organic substance, such as paper, gelatine, leather, horn, parchment, the skin, &c., the oxygen of the chromic acid is ready to unite with the carbon and hydrogen in those bodies. The change goes on, however, very slowly in the dark, requiring the stimulating action of light to set it up, but under this influence it speedily passes to the state of brown oxide, Cr O2, and then to the sesquioxide Cr2O3.
This formation of brown chromate of oxide of chromium is the first result of the action of light upon a mixture of bichromate of potash and organic matter, and occasions the brown tint left behind in the paper. If the action of light is allowed to proceed further, the deoxidation ultimately proceeds to the greatest possible extent, and the chromic acid is entirely reduced to the state of sesquioxide of chromium. This has a green color, and its presence may often be observed in photographs printed in this manner. The reduced brown oxide of chromium reacts in several ways, like a combination of an acid and a base, and when washed with metallic and other solutions, give rise to other insoluble metallic compounds of various colors, by a process of double decomposition. Hence the numerous bichromate of potash printing processes in which variously-colored positives are produced. Upon gelatine, in its numerous forms of gelatine, isinglass, glue and the allied bodies, gum, &c., another action takes plave at the same time. The reduction of chromic acid is effected in the same manner, but the oxygen which it loses attacs the gelatine and converts it into a slightly different chemical substance, rendering it partially or entirely insoluble in water.
The bichromate of potash is employed as a mordant to prepare woolen goods to be dyed black with logwood. The goods are simply boiled first in a weak solution of it, then in an extract of logwood. It is also employed as a mordant for brown colors on wool. With a preparation of the acetate of nitrate of lead, cotton cloth is dyed yellow by subsequent immersion in a solution of chrome; and, if immersed afterwards in warm lime-water, it will change into orange. The chromate of lead is employed as a yellow paint, and is applicable to both oil and water colors. The sulphate of chromium combines with the sulphate of potassa, forming a beautiful double salt, which crystalizes in green and purple octahedra, and is called chrome alum. A small quantity of the metal chromium will unite with steel, forming an alloy suitable for the finest cutlery instruments.
Chrome ore is obtained in great abundance in hills near Baltimore, Md., where the manufacture of the bichromate of potash is carried on extensively.