A cheap, but not particularly bright product, can be obtained from old printer’s type. The metal, which is an alloy of antimony and lead, is powdered, mixed with 3 parts of saltpetre and 4 parts of common salt, melted, and the mass washed out with water.
Other formulæ which are said to yield a good result are as follows: 12 parts of white lead, 3 parts of antimony oxide, 1 part of ammonium chloride, 1 part of alum. Or: 16 parts of stibnite, 24 parts of lead, 1 part of common salt and 1 part of ammonium chloride. The intimate mixture of these materials is first gently heated with access of air, then more strongly, and the mass extracted with water. There are many other recipes for the preparation of Naples yellow, the majority of which are distinguished by an apparently arbitrary arrangement of the materials; for there is no scientific reason. If it were possible to accurately obtain any desired high temperature in a furnace, the manufacture of Naples yellow would no longer be a matter of skill, but the same product could be obtained at every attempt. Since this is not yet the case, the exact procedure for the preparation of this colour can only be found by careful experiments.
Naples yellow is, as has been said, a handsome colour, and offers a great resistance to varied reagents. It is only changed by one of them, sulphuretted hydrogen, by the prolonged action of which it is turned completely black.
Antimony Yellow is very similar in composition to Naples yellow. It consists of a mixture of lead antimoniate with the oxides of lead and bismuth. It is prepared by the process recommended by Meromé by intimately mixing 3 parts of finely powdered bismuth with 24 parts of powdered stibnite and 64 parts of saltpetre, melting the mixture and shaking it whilst molten into water. The brittle mass is finely powdered, washed and dried, then melted with 128 parts of litharge and 8 parts of sal ammoniac. The mass obtained has a fine pale yellow colour; when powdered it is antimony yellow. This pigment has almost fallen into disuse because of its instability and the high price of bismuth.
Calcium Chrome Yellow.—Calcium forms a yellow pigment with chromic acid, which, although far surpassed by the lead chromes in fineness of shade, has the advantage over them of greater stability and cheapness. For purposes for which cheap and at the same time permanent colours are required calcium chrome yellow can be recommended. It is most simply prepared from potassium chromate and calcium chloride, which, as a by-product of many chemical operations, is obtainable at very low prices. The deepest pigment is obtained when the precipitation is done with a boiling solution of the chromate. Calcium chromate, in addition to its use alone, may be employed instead of white pigments to produce pale shades from deep lead chromes. This addition should not be carried to a great extent or the chrome will be made too light, since calcium chromate has a much lower specific gravity than lead chromate.
Barium Yellow, Yellow Ultramarine or Permanent Yellow.—This pigment consists of barium chromate. The finest product is obtained when a solution of a barium salt, generally barium chloride, is precipitated boiling by a solution of potassium chromate. The very finely divided precipitate has a pale yellow colour very similar to that of pale lead chromes. This handsome pigment is distinguished by the valuable property of being practically unaltered by the atmosphere; it is only attacked by strong acids and alkalis. By long heating, the colour of this compound is gradually changed to a handsome green, which consists of a compound of barium and chromic oxides, and has occasional use as an artists’ colour. In order to obtain this pigment, the heating must be intense and long continued. According to the author’s experiments, it is not sufficient to heat for a short time to a very high temperature; in that way a mass is obtained of very unequal colour. The best result was obtained by spreading barium yellow in a thin, even layer in a flat porcelain dish and heating to whiteness for 10 hours.
Zinc Chrome Yellow.—Zinc chromate is inferior to lead chromate in beauty, but has the advantage of permanence. It does not blacken in an atmosphere of pure sulphuretted hydrogen, and resists very well the action of other agents. Zinc yellow may be prepared by the immediate precipitation of a solution of zinc sulphate by a solution of potassium chromate, both being boiling, but the very bright precipitate obtained in this way is not stable; on washing, it gives up chromic acid continually to the wash water, and only a pale yellow residue remains. A very fine colour is obtained in the following manner: zinc sulphate is dissolved in water and boiled for half an hour with 1 per cent. of white zinc whilst stirring. This operation effects the separation of iron oxide and the neutralisation of the free acid generally present in commercial zinc sulphate. When the solution has cleared by standing, it is precipitated by a solution of potassium chromate, the precipitate collected on a filter and allowed to drain completely; it is then washed with very small quantities of water and finally dried. A pure yellow precipitate is only obtained when all the iron oxide has been removed by boiling the zinc sulphate solution with white zinc; if the liquid contains only a very small quantity of iron, it has yet a very considerable influence on the colour, the yellow is not pure, but has a brownish tinge. Zinc yellow is used alone, and mixed with other pigments. Chrome yellows of all possible shades may be obtained in this way. Chrome yellows are often found in commerce which consist essentially of zinc chromate.
Cadmium Chrome Yellow.—When a solution of cadmium sulphate, or any other cadmium salt, is mixed with a solution of potassium chromate, a precipitate of cadmium chromate, CdCrO₄, is formed. This pigment has a beautiful, deep yellow colour, in no way inferior in shade to the finest lead chromes, and having the great advantage over the latter of being entirely unaltered by the atmosphere; it is thus to be highly recommended for artistic purposes. The high price at which it is sold prevents its general use, though now that cadmium compounds are to be obtained at so much lower prices than formerly, the price of cadmium chromate appears to be excessively high.
Cadmium Yellow is cadmium sulphide, CdS; in nature it occurs as the somewhat rare mineral greenockite. Cadmium yellow is obtained by dissolving metallic cadmium in sulphuric acid, and precipitating the solution with sulphuretted hydrogen. The solution of cadmium sulphate must be digested for some time with excess of cadmium, in order to separate the foreign metals present as impurities; the colour is not so fine when a quite pure cadmium solution is not used.
Cadmium yellow is a very bright yellow. Several shades are obtained according as the solution of cadmium sulphate used in its precipitation is neutral or acid. The reason of this difference in shade lies apparently in the different size of the crystals of which the precipitate is composed. The deep, pure yellow colour becomes still deeper by fusion, which takes place at a white heat. Weak alkalis, acids and sulphuretted hydrogen do not alter cadmium yellow; it is thus to be regarded as a durable artists’ colour. It can be mixed with ultramarine without decomposition, when a fine green is formed; but mixed colours cannot be made from cadmium yellow and blue copper pigments, since these would blacken in the light.