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 per cent. magnesia, 187 per cent. water, and 78.7 per cent. 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.

Mars Yellow: Mars Orange - Artificial Ochre - Jaune de Mars.

This pigment is a kind of yellow ochre prepared artificially. It may be made by precipitating a salt of iron mixed with alum by means of caustic soda, or potash, or lime. The salts of iron used are either green vitriol (ferrous sulphate) or the ferric chloride. If green vitriol be employed the precipitate formed gradually becomes yellow on exposure to the air. Upon the proportion of alum mixed with the iron salt depends the depth of the yellow colour in the product, for the alumina precipitated with the iron hydrate acts as a diluent of the colour. When lime is used as a precipitant for the iron compound (if this be green vitriol or ferric sulphate), calcium sulphate, that is, gypsum, comes down along with the ferric hydrate and basic ferric sulphate, and serves to lighten the colour.

By submitting the different varieties of Mars yellow to various degrees of heat, with or without a little nitre, a number of products of different hues are obtained, including Mars orange, Mars red, Mars brown, and Mars violet. All these preparations require very thorough washing to fit them for use on the palette of the artist.

The Mars colours are permanent when carefully prepared and thoroughly purified from soluble salts. They seem sometimes to have a slightly injurious effect upon a few of the best semi-permanent pigments of organic origin, such as the madder colours. This action may be due to the ferric hydrate in them combining with the colouring matter, and displacing some of the alumina previously united with it. In this direction it is probable that Mars yellow will be more active than the deeper-coloured pigments produced by calcining it at various temperatures.

Naples Yellow: Jaune de Naples - Jaune d'Antimoine - Neapelgelb - Giallo di Napoli.

Under this name three different substances are included. The pigment generally sold in England as 'Naples yellow' is an excellent imitation made by mixing cadmium yellow or deep cadmium with a white, preferably a zinc white. But a true Naples yellow, which is a basic lead antimoniate, is still procurable from some artists' colourmen. This preparation is sometimes made by heating together for two hours a mixture of 1 part tartar emetic, 2 parts nitrate of lead, and 5 parts common salt, all the ingredients being of the purest quality, and the heat not exceeding that at which common salt fuses. A more recent process, in which zinc oxide is introduced among the materials which are heated together, yields a paler but excellent product. A bright pale variety of yellow ochre seems to have formerly gone under the name of Naples yellow.

This antimonial yellow has been known from very early times as an enamel colour. It has been found upon Babylonian bricks at least 2,500 years old. Persian pottery as early as the thirteenth century of our era is occasionally decorated with antimonial yellow.

In oil the genuine and the imitative Naples yellows are quite permanent, so far as light is concerned, but the genuine kind is liable to be darkened, like other lead compounds, by air containing sulphuretted hydrogen. In water-colour painting genuine Naples yellow is quite inadmissible, for it blackens rapidly, but irregularly, in the presence of mere traces of sulphur compounds. This blackening, like that of lead white under similar conditions, is much more marked in darkness than in light.

Naples yellow, in contact with metallic iron, tin, pewter, zinc, and several other metals, is discoloured and blackened. An ivory instead of a steel spatula, or palette knife, should be used with this pigment. The darkening in question is due in part to attrition, owing to the extreme hardness of the particles of the lead antimoniate, however finely the material may have been ground, and partly to the reducing effect of the above-named metals upon this antimoniate. Iron in the form of its oxide or hydrate (as in light red or yellow ochre), or in complex combinations (such as Prussian blue), does not exert any effect upon Naples yellow. A statement to the contrary effect has crept into a large number of technical manuals, but I have been unable to discover the slightest experimental evidence in favour of such a view. Naples yellow, however, is injured by and does injure some of the organic pigments, such as the cochineal reds and the numerous yellow lakes. But as Naples yellow cannot be used as a water colour, and as the above-named organic pigments ought to be entirely excluded from the palettes of all artists, the action in question is of little importance. Naples yellow acts upon indigo also.

Indigo, however, is a pigment, to which a very high degree of permanence cannot be assigned; there is, moreover, no reason why it should be associated with Naples yellow, as other yellow pigments may be safely used to modify its hue.

Another pigment also is sold as jaune d'antimoine. It is a mixture of the oxychlorides of bismuth and lead with lead antimoniate. When carefully prepared it yields a rich paint of good body, but its use cannot be recommended to artists.