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 2-5 per cent. by weight of the oxides or dryers is gradually stirred into the oil after it has been slowly raised to a temperature of about 300° F. 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, either by hand or steam. The material of which the pans are made is either 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 them 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 means of an earthenware pipe. Since zinc oxide has been introduced as a substitute for white lead in painting, researches have been made to replace litharge as a dryer, because it is not logical to discard the lead pigment and then use a lead dryer with a zinc pigment. 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 dryers in the dry state are preferable in many respects to drying oils. Following are some of the recently introduced dryers:--