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This remarkable artificial yellow pigment was discovered by Fischer. It is a compound of the nitrites of cobalt and potassium. Usually it is free from water, but it sometimes contains three molecules, and is then represented by the formula K6Co2(NO2)123H2O. Other proportions of water also occur; but when the compound contains four molecules, its hue is somewhat greenish. The dry or anhydrous variety is best made by mixing a solution of a cobaltous salt, strongly acidified with acetic acid, with a concentrated solution of potassium nitrite, and keeping the mixture warm. Perhaps a pigment of finer hue is obtained by passing a stream of nitric oxide gas mixed with air into a solution containing nitrate of cobalt and a little acetate of potassium; from time to time a little potassium carbonate is added.
Another method of preparing a variety of aureolin having a singularly bright yellow hue consists in adding a solution of sodium cobaltinitrite acidified with acetic acid to a dilute solution of potassium acetate or nitrate. I have tried the experiment in accordance with the instructions given by Messrs. Adie and Wood (in the 'Transactions of the Chemical Society,' vol. lxxvii., 1900, p. 1076), but I have used sometimes other cobalt salts instead of the acetate, and have also so arranged the constituents of the two solutions that the precipitate of aureolin is intimately associated with barium sulphate precipitated at the same time. By this means an opaque pigment of bright and light yellow hue is obtained. The pure aureolin obtained by this process contains sodium as well as potassium, and is represented by the formula K2NaCo(NO2)6H2O. Its value as a pigment is at least equal to that of the better-known varieties of aureolin. The pure pigment prepared in the way indicated, without any suggestion of its use in painting, by Messrs. Adie and Wood, requires less oil than usual and dries well; in water-colour painting it shows one distinct advantage over the older varieties of aureolin, for it is less soluble in water and does not sink into the paper.
The several varieties of aureolin are not much affected by caustic potash solution or by dilute hydrochloric or nitric acid, and are very slowly attacked and blackened by solution of sulphuretted hydrogen, but are at once destroyed by ammonium sulphide. Ordinary aureolin is slightly soluble in cold water.
Aureolin is of a pure yellow colour, and is almost transparent whether used in water or oil painting. In oil some samples dry with great difficulty, and become very dirty if exposed to the air during the progress of desiccation; other samples dry and harden too quickly - the exact cause of this difference of deportment has not been ascertained. Moreover, as ordinarily ground in oil, some varieties require a very large proportion of the medium. These defects may be easily remedied by heating the slow-drying variety of the ground pigment to 212° F. immediately before the addition of the oil, and by using, instead of raw linseed oil, the siccative linseed oil, prepared by means of borate of manganese. Thus prepared, aureolin not only dries quickly, but it retains its purity of hue; moreover, a surface of the dried oil pigment will yield nothing to a wet cloth passed over it instead of staining it yellow. The reason why this staining occurs with ordinary aureolin ground in oil, even when it has at last become dry, is that the oil does not suffice to protect the particles of pigment from the solvent action of moisture.
The quick-drying variety of aureolin should be ground in poppy oil.
I find that the variety of aureolin which contains sodium as well as potassium (see above) dries perfectly well when ground in purified linseed oil, even when the latter has not been made siccative by special treatment.
Aureolin properly prepared in oil, as described above, does not fade by exposure to sunlight, nor does it darken, except so far as the admixed oil is concerned. As a water-colour aureolin is practically permanent, even in sunlight, as the following figures show:
In the Burlington Club trials it was found that aureolin stood perfectly for four years when exposed in an ordinary frame or in air kept dry, but that it lost somewhat by exposure, during the same time, in a hermetically sealed tube in the presence of ordinary moist air.
The fading of fugacious organic pigments, such as the lakes from cochineal, is accelerated by their commixture with aureolin, which particularly hastens the destruction of indigo, even in oil. The aureolin cannot so act without being itself likewise affected; it generally becomes, under such circumstances, of a brownish hue.
It is easy to learn whether a sample of aureolin is free from combined water by heating a small portion somewhat strongly in a long test-tube; dew will condense upon the upper part of the tube if water be present in the pigment. The presence of yellow organic matters in imitative or adulterated aureolins may generally be detected by mixing some powder of the suspected sample with spirits of wine and a few drops of strong ammonia; the liquid becomes red orange, or yellow if the aureolin be not pure. Aureolin containing chrome yellow is blackened by a solution of sulphuretted hydrogen, Aureolin is the first pigment described (in the present manual) which illustrates the remarkable colouring power of the element cobalt. The hues derived from this metal acting as a chromogen range from yellow to green, blue and violet or purple; there is also a rose cobalt. Various oxides, themselves colourless, serve as chromophores.
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