work should be stocked for at least twelve months after purchase. Very pale and old linseed oil should be used in the thinning, otherwise it will probably soon discolor. It is, however, about the best pigment for preserving wood from the effects of the weather. Zinc white is an oxide of zinc. It does not discolor and is a very pure pigment. It is a substitute for white-lead, but is not so employed in the practice of graining.
Vermilion is used only in the most exceptional cases; it can be had as a fine dry powder, free from grit, and is a very brilliant color in oil. The best quality only is permanent, and that is a sulphuret of mercury. Chinese red, or vermilion, is of a deep crimson tone, but has bad covering power, and, unless well protected, will soon fade under the action of light and impure air.
Indigo possesses great body, and is a good glazing color. It is not very durable and is injured by impure air.
Ivory black is made by placing ivory dust in a covered crucible exposed to a great heat. An inferior color known as bone black is made by treating bones in a similar way. Ivory black, the deepest and purest of the blacks, being somewhat hard, requires very careful grinding, and unless ground very fine is useless. It is best ground in turpentine, and diluted for use with turpentine, gold size and a little varnish. In drying it will become dull, so that it should not be used unless it is afterwards to be varnished. If thinned down too much with turpentine it will not bind, so that when the varnish is applied it will rub off onto the rest of the work and spoil the whole. Ivory black, when purchased unground, resembles drops and is sometimes called drop black, but bone black is prepared in the same way.
The various ochres, Oxford, yellow and Italian, are used only in the composition of grounding paint, and never in graining color. Really, commercial yellow ochre is the only one of this class of pigment there is need to use, since the addition of a little Venetian red will give any warmer
