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 off by 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.

Since it has been tried to substitute zinc oxide for white-lead in painting, researches have been made to replace litharge as a drier by a substance free from the inconveniences which caused the abandonment of white-lead. If sulphuretted hydrogen impairs the whiteness of painting done with white-lead, it is not logical to employ a lead drier with zinc paints, because the latter substances will lose their advantage of not becoming dark. Several metallic oxides and salts, especially zinc sulphate, manganese oxide, and umber, have the property of combining with oils, which they render drying. To these may be added the protoxides of the metals of the third class, i.e. iron, cobalt, and tin. But these oxides are very unstable and difficult of preparation; hence it became desirable to discover some means by which they might be combined with bodies which would enable them to be prepared cheaply, and at the same time leave unimpaired their desiccating powers. Moreover, it is acknowledged that driers in the dry state are preferable in many respects to drying oils.

Following are some of the recently-introduced driers: -

(1) Cobalt And Manganese Benzoates

Benzoic acid is dissolved in boiling water, the liquid being continually stirred, and neutralized with cobalt carbonate until effervescence ceases. Excess of carbonate is removed by filtration, and the liquor is evaporated to dryness. The salt thus prepared is an amorphous, hard, brownish material, which may be powdered like rosin, and kept in the pulverulent state in any climate, simply folded in paper. Painting executed with a paint composed of 3 parts of this drier with 1000 of oil and 1200 of zinc-white, dries in 18 to 20 hours. Manganese benzoate is prepared in the same way, substituting manganese carbonate for that of cobalt. Applied under similar circumstances, it dries a little more rapidly, and a little less is required. Urobenzoic (hippuric) acid is equally efficacious.

(2) Cobalt And Manganese Borates

These salts also, in the same proportions, are found to be of equal efficacy. The latter is extremely active, and requires to be used in much smaller proportions.

(3) Resinates

If an alkaline resinate of potash or soda be dissolved in hot water, and this solution be precipitated by a solution of a proportionate quantity of a cobalt or manganese chloride or sulphate, an amorphous resinate is formed, which, after being collected on cloth filters, washed, and dried, forms an excellent drier.

(4) Zumatic (Transparent) Drier

Take zinc carbonate, 90 lb.; manganese borate, 10 lb.; linseed-oil, 90 lb. Grind thoroughly, and keep in bladders or tin tubes. The latter are preferable.

(5) Zumatic (Opaque) Drier

Manganese borate, as a drier, is so energetic that it is proper to reduce its action in the following way: - Take zinc-white, 25 lb.; manganese borate, 1 lb. Mix thoroughly, first by hand, then in a revolving drum; 1 lb. of this mixed with 20 lb. paint ensures rapid drying.

(6) Manganese Oxide

Purified linseed-oil is boiled for 6 or 8 hours, and to every 100 lb. boiled oil are added 5 lb. of powdered manganese peroxide, which may be kept suspended in a bag, like litharge. The liquid is boiled and stirred for 5 or 6 hours more, and then cooled and filtered. This drying oil is employed in the proportion of 5 to 10 per cent, of the zinc-white.

(7) Guynemer's

Take pure manganese sulphate, 1 part; manganese acetate, 1 part; calcined zinc sulphate, 1 part; white zinc oxide, 97 parts. Grind the sulphates and acetate to impalpable powder, sift through a metallic sieve. Dust 3 parts of this powder over 97 of zinc oxide, spread out over a slab or board, thoroughly mix, and grind. The resulting white powder, mixed in the proportion of } or 1 per cent, with zinc-white, will enormously increase the drying property of this body, which will become dry in 10 or 12 hours.