Oil boiled with 10 per cent, of litharge for three hours, is a much better dryer than when heated without this oxide.
Oil boiled alone for five hours is an inferior drying oil to one heated for only three hours.
Oil previously boiled alone for five hours, and boiled alone again for three hours, is scarcely altered in drying power, but it becomes a better drying oil if it is boiled the second time with litharge. It is inferior to a drying oil which has been boiled only three hours with litharge, without being submitted to a previous boiling.
Oil boiled alone for five hours, boiled for a further period of three hours with manganese dioxide which has already been used for one operation, is very nearly as strong a dryer as that which has been boiled with litharge under the same conditions; but it is superior to an oil which has been boiled with manganese dioxide for eight hours. This no doubt arises from the longer boiling with manganese having caused a larger quantity of manganese to dissolve, and that the quantity dissolved is in excess of that which yields the best result.
Finally, oil boiled for five hours, and then boiled alone once more for eight hours, becomes viscous, and the first coat requires a considerable time to dry. We thus see that the oxides of lead and of manganese in certain proportions concur with heat in increasing the drying power of linseed oil. This drying of oils is a process of slow oxidation.
The following points of Chevreul’s appeared to be difficult of satisfactory explanation, and suggested to Prof. Hartley an examination de novo of the facts, as well as an investigation of the chemistry of the subject generally:—
1. Linseed oil not boiled acted as a dryer to the same oil boiled with manganese dioxide.
2. Linseed oil, boiled with either litharge or manganese dried more rapidly when mixed with turpentine.
3. Oil, mixed with white lead, zinc white, antimony white, and arseniate of tin, acts differently, thus:—The white lead dries most rapidly, the zinc white next, but antimony white and arseniate of tin are incapable of acting as dryers, in fact, they retard the drying process.
4. Oil boiled alone for five hours, and boiled for a further period of three hours with manganese dioxide becomes a superior drying oil to one which has been boiled with manganese dioxide for eight hours.