3. Heating with metallic lead.
4. Heating to a high temperature with manganic oxide.
5. Heating with manganese borate.
6. Heating with manganese oxalate.
7. By the joint action of air and heat upon the oil and manganous oxide, or a solution of manganese dioxide or manganous oxide in the oil.
In the processes 1, 2, 3, there can be no doubt that a lead of linoleic acid is produced, and that this facilitates further oxidation in air, by forming salts with some of the acid products of such oxidation, while the oxidation of the linoleic acid continues. Heating with red lead favours oxidation, by the compound itself conveying oxygen to the oil. In the case of metallic lead, it must be noted that the metal is dissolved. Under certain circumstances, metals become dryers to oils; thus sheets of metallic lead are capable of acting as dryers to linseed oil.
Linseed oil is pre-eminent in its capacity for absorbing oxygen. This action of metallic lead as a dryer is due to the metal becoming oxidised at the expense of the glycerine of the oil, and so passing into solution by combining with the linoleic acid, or with acetic or formic acid, caused by the oxidation of the glycerine. It is the destruction of the glycerine with concurrent oxidation of the fatty acid which causes the drying or hardening of the oil.
When a drying oil which has been treated with metallic lead, or with litharge, is shaken up with a solution of zinc sulphate, all the lead is precipitated from the oil, and zinc passes into solution therein. By manganese sulphate or copper sulphate, the lead is removed by manganese or copper. Oil charged with lead dries in 24 hours when spread out in a thin layer on glass; it will dry completely in 5 or 6 hours if charged with manganese, in 30 or 36 hours with copper, zinc, or cobalt; and it requires more than 48 hours with nickel, iron, chromium, &c.
Although solidification of a drying oil charged with manganese takes place in from 5 to 6 hours when spread in thin films, the solidification of thicker films requires a longer time. A temperature of 122° to 140° F. accelerates the oxidation of the drying oils, partly because the oil becomes more fluid, and partly because the oxygen is more active at a higher temperature. Hence, oil which has been mixed with an equal volume of turpentine, or a light hydrocarbon, such as benzene, dries more rapidly than oil without such admixture.
When a boiled oil, prepared with manganese, is dissolved in an equal volume of benzene, and shaken up with air in a bottle, rapid absorption of oxygen occurs, especially about 120° F. If fresh air is repeatedly provided, the oxidation is sufficient to cause the liquid to become thick, and, on distilling off the sapient, a perfectly dry and elastic solid remains.