The value of an ocher does not depend upon the amount of oxide of iron present, but upon the purity of the tint. Good ocher is invariably a mixture of ocher and chrome yellow.
The French ochers, which are the finest, are regarded as standards of quality.
Purple-Brown is itself an oxide of iron. Iron is treated with lime and acid to produce it. It is very permanent in itself, but weakens tints with which it is mixed, not because it chemically destroys the other colors, but because it retains its own strength, while the others fade. As its name implies, it is the lowest in the scale of brightness, being, in fact, a dense brown. It is most in demand for external painting, when it forms the basis of most so-called chocolate hues. Although not being useful for admixture with white—muddiness resulting—it presents in mass a comfortable-looking appearance, used alone, in oil or water processes.
Red Lead is a preparation of burnt massicol (an oxide of lead), of a bright scarlet color. Al though, if used by itself in oil or varnish, it will retain its brightness for some time, it is useless for mixing tints, and is destructive to other colors when mixed with them. It is altogether useless, too, with water, which turns it black. It is, however, well adapted for priming, or first coat, either alone or mixed with white lead, and is a great preservative to wood and iron. Being a good drier it is used for hardening white lead, when mixed as a putty or as paint for preserva tive purposes. Red lead is an important pigment, and as a paint is the best preservative that can be used. It should never be mixed with whiting or similar bodies, but oxide of iron may be used in conjunction with it to advantage.
Red Ochers comprise the most valuable, and also the cheapest, red pigments in general use, the best of these, for house-painting purposes, being Venetian and Indian reds and purple brown.
All these reds having, if properly mixed and used, sufficient covering power to hide, in one coat, almost any other colored surface, they are therefore very useful and economical for pre servative and plain painting. Although existing in a native condition, these commercial reds are usually manufactured pigments—namely burnt others, They are sold in powder form; but Ven etian red and purple-brown are generally to be had ready ground in linseed oil. Vermilion and
Indian red, from the reason of their more heavy nature, would soon become solidly caked, and, therefore, are ground in oil or turpentine only when so ordered. Of course, this does not apply to colors in collapsible tubes. Venetian red, In dian red, Turkey red, rose-pink, and red oxide owe their color almost entirely to oxide of iron.
Indian Red, originally coming from Bengal, is a rich deep red, of slightly purple tone, with all the good qualities of body and permanence of its preceding pigment; it is a beautiful color in itself, is a splendid stainer, and is very perma nent and satisfactory.
Indian red cannot be classed as pure if it con tains less than 95 per cent of oxide; the paler the color, the greater is its tinting strength, and rosier the tint; the deep-colored give purplish tints.
Turkey Red is essentially a mixture of In dian red with a lake color or with rose-pink. The object to be sought for in Turkey red is its brilliancy, and, next, its opacity or body. It should also be very finely ground.
Venetian Red is a natural red ocher with more iron in it than the other ochers; it consists usually of oxide of iron and sulphate of lime. Its comparative brightness must be examined, and also its fineness of grinding. It is of good body, makes clean tints with white in both oil and water, and is sufficiently cheap for any purpose. It is permanent; but there is an imitation, made from vitriol, which is not so permanent, and which is altogether unsatisfactory.
Vermilion is a mineral sulphide of mercury, preeminent for its brilliancy and purity of red, and is one of the pigments known and used by the ancients. Although it may be found in a na tine state, principally in China and California (requiring grinding, however, before being fit for use), the vermilion of commerce is principal ly an article manufactured from mercury. It is a crystalline pigment. The larger the crystal, the deeper the tint, therefore the paler shades have most body. Sulphide of mercury is found in the earth. It formerly came from China; and Chinese vermilion has the reputation of being the best. The natural sulphide is called, commer cially, cinnabar.