68. Driers are substances added to paint in order to cause the oil to thicken and solidify more rapidly. The drying of linseed oil is caused by the absorption of oxygen, and there is little doubt that driers usually act simply as carriers of oxygen to the oil, a very small quantity yielding quite extensive results. The best driers, therefore, are those containing a large proportion of oxygen, such as litharge, acetate of lead, red lead, sulphate of zinc, verdigris, etc., which, added to the oils, improve their drying qualities by causing the more rapid absorption of oxygen. These driers are ground up in oil and mixed in small quantities with the suspended pigment. Some colors will not dry without driers, but remain tacky, and thereby exposed to much injury, through the accumulation of dust and the degeneration of the tints.

69. The following is a list of driers, each of which is suited to some particular quality or color of paint:

Red lead makes a good cheap drier, but can, of course, be used only in situations or in paints where its color is unobjectionable.

Sugar of lead (acetate of lead), ground in oil, is the best but most expensive of all driers, and is, like copperas (sulphate of iron) and white vitriol (sulphate of zinc), used as a drier, especially for light tints.

Litharge, or oxide of lead, the drier most commonly used, is produced in the oxidation of lead containing silver. It can be procured, on a small scale, by scraping off the dross which forms on molten lead, exposed to a current of air.

Massicot is a superior kind of litharge, produced by heating lead to a degree insufficient to fuse the oxide.

Oxide of manganese, though not as rapid as litharge or massicot, is quick in effect, but, being of a very dark shade, is seldom used except for the deep colors.

Sulphate of manganese is the best drier for zinc white, about 6 or 8 ounces only being used for 1 hundredweight of ground zinc paint. The manganese should be mixed with a small quantity of the paint first, and then added to the bulk. If great care be not taken in mixing the drier, the work will be spotted.

Japanner's gold size (acetate of copper) is for some uses an excellent drier, but care must be taken not to use too much or it will make the paint brittle and cause it to crack.

Patent driers containing oxidizing agents, such as acetate of lead, litharge, etc., are ground and mixed in oil, ready for immediate use. Some of the inferior descriptions, depending for their drying qualities upon lime, should be avoided.

Terebene is a powerful drier, used as a substitute for patent driers, and mixed in the proportion of 1 ounce of terebene to 1 pound of paint, will dry in about half an hour.

Xerotine siccative is a species of terebene, but differs from it, in that when mixed with oil, the mixture does not become cloudy. In the use of any of the above driers, the following should be observed: First, not to use them unnecessarily with pigments which dry well in oil color; second, not to employ them in excess, which not only retards the drying, but injures, if it does not destroy, the paint; third, not to add them to the color until ready for use; fourth, not to use more than one kind of drier to the same lot of paint.