The action of these substances is not thoroughly understood. Chevreuil has shown that the drying of linseed oil is caused by the absorption of oxygen, and there can be no doubt that for the most part driers act as carriers of oxygen to the oil, a very small quantity producing considerable effects.
They are sometimes used to improve the drying qualities of the oil with which the paint is mixed, as explained at page 410, or they may themselves be ground up with a small quantity of oil and added to the paint just before it is used.
Litharge, or oxide of lead, is the drier most commonly used, and 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, being produced by heating lead to an extent insufficient to fuse the oxide.
Sugar of Lead (acetate of lead) ground in oil, and Copperas and White vitriol (sulphate of zinc), are also used as driers, especially for light tints.
2 Cresy's Encyclopaedia.
Oxide of Manganese is quicker in its effects, but is of a very dark colour, and seldom used except for deep tints.
Japanners' Gold Size and Verdigris (acetate of copper) are also used for dark colours. Care must be taken not to apply too much of the size, or it will make the paint brittle.
Red Lead (oxide of lead) is often used as a drier when its colour will not interfere with the tint required. It is not so rapid in its action as litharge or massicot.
Sulphate of Manganese is the best drier for zinc white, about 6 or 8 ounces only being used for 1 cwt. 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.1
Sulphate of Zinc is also a good drier for zinc paint.
Patent Driers contain oxidising agents, such as litharge or acetate of lead, ground and mixed in oil, and therefore in a convenient form for immediate use.
Terebine consists of a powerful drier dissolved in spirits of turpentine; it is used as a substitute for patent and other driers, and is used in the proportion of 1 oz. to 1 lb. of paint. Alone it will dry in about half an hour.
Xerotine Siccative is a species of terebine, but differs from it in that when mixed with oil the mixture does not become cloudy. The siccative becomes dangerously explosive when stored.
"The following points should be observed in using driers : -
"Ist. Not to use them unnecessarily with pigments which dry well in oil colour."
"2d. Not to employ them in excess, which would only retard the drying" and tend to destroy the paint.
"3d. Not to add them to the colour until about to be used."
"4th. Not to use more than one drier to the same colour." 2