30. GALLSTONE
Is a deep-toned gorgeous yellow, affording richer tints than most other yellows, but it cannot be depended on for permanency, and therefore is seldom employed. Its colour is soon changed and destroyed by strong light, though not subject to alteration by impure air. In oil it is ineligible. A true gallstone is an animal calculus formed in the gall-bladder, chiefly of oxen; but the pigment sold under that name is often replaced by a substitute, resembling the original in colour, but of greater stability.
31. GAMBOGE,
Sometimes designated Drop Gum, and variously written Gamboge, Camboge, Gamboage, Cambogia, Cambadium, Cambogium, Gambodium, Gambogium, &c., is the produce of several kinds of trees. The natives of the coast of Coromandel call the tree from which it is principally obtained Gokathu, which grows also in Ceylon and Siam. From the wounded leaves and young shoots the gamboge is collected in a liquid state and dried. Our indigenous herb Celandine yields abundantly, in the same manner, a beautiful yellow juice of the same properties as gamboge. Gamboge is of a gum-resinous nature and clear yellow colour. It is bright and transparent, but not of great depth, and in its deepest touches shines too much and verges upon brown. When properly used, it is more durable than generally reputed, both in water and oil; and conduces, when mixed with other colours, to their stability and keeping their place, on account of its gum and resin. It is deepened in some degree by ammoniacal and impure air, and somewhat weakened, but not easily discoloured, by the action of light. Time effects less change on this colour than on other bright vegetal yellows; but white lead and other metalline pigments injure, while terrene and alkaline substances redden it. In water it works remarkably well, and forms an opaque emulsion without grinding or preparation, by means of its natural gum; but is with difficulty employed in oil, &c., in a dry condition. It dries well, however, in its natural state, and lasts in glazing when deprived of its gum. With regard to other colours it is perfectly innocent, and though a strong medicine, is not dangerous or deleterious in use. Gamboge has been employed as a yellow lake, precipitated upon an aluminous base; but a better way of preparing it is to form a paste of the colour in water, and mix it with lemon yellow, with which pigment being diffused it goes readily into oil or varnish. Glazed over other colours in water, its resin acts as a varnish which protects them; and under other colours its gum acts as a preparation which admits varnishing. It is injured by a less degree of heat than most pigments.
In landscape, gamboge affords with indigo or Antwerp blue clear bright greens, and with sepia a very useful sober tint. For sunrise and sunset clouds, a mixture of gamboge and cadmium yellow will be found useful.
32. EXTRACT OF GAMBOGE
Is the colouring matter separated from its greenish gum and impurities by solution in alcohol, filtration and precipitation, by which it acquires a powdery texture, rendering it miscible in oil, &c., and capable of being employed in glazing. At the same time it is improved in colour, and retains its original property of working well in water with gum. Gamboge is likewise soluble in caustic potash, forming a red liquid, from which it is thrown down by acids.
33. INDIAN YELLOW
Is a pigment long employed in India under the name Purree, but has not many years been introduced generally into painting in Europe. It is imported in the form of balls of a fetid odour, and is produced from the urine of the camel. It appears to be a urio-phosphate of lime, and is of a beautiful pure yellow colour and light powdery texture; of greater body and depth than gamboge, but inferior in these respects to gallstone. Indian yellow resists the sun's rays with singular power in water painting; yet in ordinary light and air, or even in a book, the beauty of its colour is not lasting. In oil it is exceedingly fugitive, both alone and in tint. Owing probably to its alkaline nature, it has an injurious effect upon cochineal lakes and carmine when used with them. The colour is not damaged by foul air, and, as lime does not destroy it, the pigment may be employed in fresco according to its powers.
Indian yellow washes and works extremely well, and is adapted for draperies and for compounding landscape greens—where permanency is not required. Blackness in the darkest shadows of the foliage will sometimes result from too great a use of indigo; should this evil exist, no colour is so fitted to regain the proper tone as Indian yellow employed thickly.