The maximum drying power is obtained by the addition of certain metallic oxides, which not only part with some of their own oxygen to the oil, but also act as carriers between the atmospheric oxygen and the heated liquid. This heating of the oil with oxides is known as boiling, although the liquid is not volatilized without decomposition, as is the case with water. At about 500° F. (260° C.), bubbles begin to rise in the oil, producing acrid white fumes on coming into contact with the air. The gas thus given off consists chiefly of vapour of acrolein mingled with carbonic oxide. There is no advantage in heating the oil higher than 350° F. (176° C.); the drying properties of the oil are not increased by heating beyond this point, while its colour is considerably darkened. For the finer qualities of boiled oils, it is essential that the raw oil should have been stored for some time, so that it may be free from mucilage. This mucilage is the chief source of the dark colour of some boiled oils; when heated, it forms a brown substance, which is soluble in the oil itself, and extremely difficult to remove. The oxides usually added to the oil during boiling are litharge or red-lead, the former being preferred on account of its lower price.

About 2 to 5 per cent. by weight of the oxides or driers is gradually stirred into the oil after it has been slowly raised to about 300° F. (149° C). The stirring should be continued until the litharge is dissolved, or it would cake on the bottom of the pan, and cause the oil to burn. Litharge may even be reduced to a cake of metallic lead when the fire is brisk. Some pans are furnished with stirrers and gearing by which the latter can be worked by hand or steam. The material •of which the pans are made is wrought- or cast-iron. Copper pans are sometimes used with the object of improving the colour of the oil. Little is known respecting the chemical reactions which take place during the boiling of oil. Even when the air is excluded during the process, the drying properties are greatly increased, and, if boiled long enough, the oil is converted into a solid substance. The loss of weight which ensues is dependent upon the temperature and the time during which the operation continues. It is less when the air is freely admitted than if the pan is covered with a hood. The vapours given offby the oil are of an extremely irritating character, and should be destroyed by passing through a furnace.

As their mixture with air in certain proportions is explosive, this furnace should be situated at some distance, and the gases be conducted into it by an earthenware pipe.