The chrome yellow thus produced is said to be chemically pure, and of great covering power, equal to the best chrome yellow in the market.
The process is very simple, and the crude lead ore is transformed into chrome yellow in from three to four days.
Characters.—Pure chromate of lead has an orange-yellow colour, and in whatever manner it be made it always has this tint. Commercially, chromes are made of a great variety of tints, from a pale-lemon chrome to a deep scarlet, through all the intermediate shades of yellows and oranges; in fact, most colour makers produce not less than eight, and some more shades of chromes, whence it is obvious that they cannot be chemically pure, but must be mixed with some other bodies.
In commerce, chromes are distinguished as “pure” and “common”: the distinction between them is that the “pure” chromes are made from a lead base, and consist of chromate of lead mixed to a larger or smaller amount with sulphate of lead, the paler shades containing most of the latter; while the “common” chromes are mixed with china clay, barytes, gypsum, or similar bodies.
To distinguish the two kinds of chromes, treat with boiling hydrochloric acid. If pure, the chrome will completely dissolve, the solution usually having a green colour. On cooling, crystals of lead chloride separate out. The liquor will give a white precipitate with barium chloride.
By taking a weighed quantity of chrome, dissolving in hydrochloric acid, adding excess of barium chloride, filtering, thoroughly washing the precipitate with boiling water, drying and weighing it, and making the necessary calculations, every one part of the precipitate being equal to 1·3 parts of sulphate of lead, the quantity of the latter in the chrome is obtained.
The “common” chromes, which mostly contain barytes, are not completely dissolved on boiling in hydrochloric acid, the barytes they hold being left as an insoluble residue. By taking a weighed quantity of the chrome, and filtering off, drying and weighing the residue, the amount of barytes present can be ascertained; the solution can be tested for sulphate of lead, which they sometimes contain, as described above.
Chrome yellows and oranges should be assayed for colour and tint, care being taken to compare them with a thoroughly reliable standard sample. Their colouring power and body should also be assayed. Another property which should be tested is the colour that they yield on mixing with Prussian blue. A great deal of chrome is used for making greens by mixing with Prussian blue, and as there is a very considerable difference between them in the shade of green they give, it is rather important to test this property, which can readily be done by mixing 100 grains of the chrome with 10 grains of Prussian blue, grinding them together in a mortar, and observing the shade of the green which is produced thereby; if the green is not bright and pure, the chrome is not fit to be made into greens, and should be rejected for that purpose.
Iron Chromate.—If a solution of chloride of iron acidulated with hydrochloric acid be added to neutral chromate of lead, a light orange powder is precipitated, which is chromate of iron. Dried at 104° F., it is found to consist of 65 to 65·11 chromic acid, and 34·58 to 34·78 oxide of iron. Chromate of iron is insoluble in water, dissolves easily in hydrochloric, nitric, and sulphuric acids, and decomposes when mixed with soda lye. Under strong heat, it melts into a brownish mass. It may be used in painting in oils, as a substitute for chromate of lead. Although inferior to the latter in brilliancy, it has certain advantages over it—it does not blacken with exposure to sulphuretted hydrogen, is not injurious to the user, and withal is cheaper.
Zinc Chromates.—The before-quoted writer in the Chemical Trade Journal, speaking of zinc chromates, says that although it is possible to prepare lead chromates having shades varying imperceptibly from the palest lemon to a deep granite red, in the case of the zinc compounds scarcely any variation from the normal is possible, this being, however, different from anything obtainable with lead. Zinc yellows fall considerably below the ordinary chromes in their colouring power, but they are faster in light and are less poisonous. More than 80 per cent. of the amount annually made in Germany is used for the production of zinc green by mixing with Prussian blues, of which substance Holland, Switzerland, and Hungary are the greatest consumers. The zinc yellows met with in commerce vary in their constitution considerably, being usually acid chromates of zinc and potassium, basic zinc chromates being rare. Ordinary salts of zinc invariably contain small amounts of iron, which must be removed before they are used in the manufacture of colours. The simplest method is to heat them with the quantity of permanganate theoretically necessary to convert all the iron present into the ferric state, adding zinc hydrate, which need not be free from iron; after thorough stirring, the whole is allowed to settle, and filtered, when the liquid will be found to contain not a trace of iron.