Indian Yellow

Synonyms:

Indian Yellow, Piuri, Purree, Peori, Jaune Indien, Indischgelb

This remarkable pigment is obtained at Monghyr, a town in Bengal, from the urine of cows which have been fed upon mango-leaves. It generally occurs in the bazaars of the Panjáb in the form of large balls, having an offensive urinous odour.

Indian yellow is an impure magnesium salt of euxanthic acid. The essential part of it is a compound containing 4.5 percent magnesia, 187 percent water, and 78.7 percent euxanthic anhydride; but this substance is always associated, even in the most carefully purified samples of prepared Indian yellow, with various impurities both mineral and organic. The pure magnesium euxanthate is represented by the formula C19H16MgO11, 5H2O.

For artistic purposes the crude imported Indian yellow is thoroughly powdered, and then washed with boiling water, until the liquid filtered from it is no longer coloured; a brown impurity, and much of the evil smell, are thus removed. The colour of the washed product is enriched by leaving it in contact for a day or two with a saturated solution of sal-ammoniac, and then repeating the treatment with hot water.

Thus purified, this pigment presents a translucent orange-yellow colour of great depth and beauty. Ground in oil, some specimens are practically unchanged, even after long exposure to sunlight, any darkening they show being due either to imperfect purification, or to the change of the associated oil. Such change is reduced to a minimum if poppy oil be substituted for linseed oil, or if the latter be previously treated with manganese borate. On the other hand, I have met with specimens of Indian yellow ground in oil which, after five years' exposure, have lost nearly one-third of their original depth, and have, at the same time, become rather reddish-brown in hue. As a water-colour, Indian yellow retains its hue unimpaired when exposed to diffused daylight; sunlight very slowly bleaches it, the hue it acquires being somewhat brownish. The rate of alteration and of reduction in force caused by sunlight may be approximately represented by these figures:

When this water-colour pigment is exposed to sunlight in the presence of air maintained in a state of perfect dryness it loses its colour much more rapidly than under ordinary conditions. For this reason it may be advisable to incorporate an extra proportion of glycerin with Indian yellow when prepared as a water-colour.

As a general rule, Indian yellow suffers no change by admixture with any pigment itself permanent, nor is it affected by sulphur compounds. True Naples yellow, however, most of the chromates, and probably aureolin also, tend to embrown it to some extent.

Indian yellow which has been adulterated with lead chromate (chrome yellow) becomes dark-brown when moistened with ammonium sulphide.

A fine yellow pigment may be prepared from the euxanthic acid, which is the characteristic constituent of Indian yellow, by throwing it down in combination with the two bases - alumina and magnesia. The following directions may be followed: Dissolve 1 part of pure euxanthic acid in just sufficient dilute ammonia. Pour the solution into a liquid prepared by dissolving 45 parts of potash-alum, 15 parts Epsom salts, and 6 parts salammoniac in 250 parts of water. Now cautiously add dilute ammonia to the mixture, stirring all the time, and avoiding any excess of ammonia. The precipitated pigment is to be thoroughly washed, and then pressed, dried, and ground.