make a green; red and yellow, an orange; and white earth and cochineal, a lake, &c.
IV. The last and most important division of colours, is into true and false: the former retain their pristine tinge, without fading, under every possible variety of circumstances; the latter either lose their colour entirely, or change into some other shade.
Colours are chiefly affected by being exposed to the sun during the summer, and to the cold air in winter. White lead, however, forms an exception; as, when ground with oil, it retains its whiteness, if it be exposed to the weather, but degenerates into a brownish or yellowish cast, in a confined situation. Nevertheless, when it is immersed in water, it is totally divested of its colour, whether it be exposed to the effects of the air or not. In the making of colours, the chief object is, that they may not fade, from the influence of the weather; though it must be regretted that the most beautiful are, in general, the least permanent. It may, however, for the most part be assumed, that the more simple any colour is, the less liable it will be to change by exposure to the air.
Having thus briefly stated the general theory of colours, we shall also give some account of the different pigments, which are most commonly employed by colour-makers.
1. Black, consists of several sorts, such as lamp-black, ivory-black, blue-black, and Indian ink. The first of these is the finest of what are called soot-blacks, and is more used than any other. Its preparation depends on the manufacture of common resin. The impure juice collected from incisions made in pine, and fir-trees, is boiled down with a small quantity of water, and strained, while hot, through a bag; the dregs and pieces of bark remaining in the strainer, are burnt in a low oven, whence the smoke is conveyed through a long passage into a square chamber, at the top of which is an opening, with a large sack affixed, made of thin woollen stuff: the soot, or lamp-black, concretes partly in the chamber, whence it is swept out once in two or three days, and partly in the sack, which is occasionally agitated, in order to take down the soot, and to clear the interstices between the threads, so as to admit a free current of air. This method of preparing lamp-black, was originally invented in Sweden, but has also been introduced into this country; and is now carried on to a considerable extent in the turpentine-houses, from the refuse of resinous matters.
Ivory-black is prepared from ivory, or bones, burnt in a close vessel; and, when finely ground, affords a deeper and more beautiful colour than lamp-black; but it is, in general, so much adulterated with charcoal, and so grossly levigated, as to be unfit for use. An opaque deep black, for water-colours, may be prepared, by grinding ivory-black with gum water, or with the aqueous liquid that settles from the whites of eggs, which have stood some time to subside.
German Black, see Frankfort Blacking, vol. i. p. 277.
Blue-black is said to be prepared from the burnt stalks and tendrils of vines. This is, however, seldom done by colour-makers, who gene-
