If a thin layer of olive oil protected from dust is exposed to the air, it will remain fluid for years and retain its characteristic oily consistency. The only change it undergoes is that it becomes somewhat more viscid and rancid, and acquires a darker color, but it never dries up.
Linseed oil treated in the same manner solidifies in the course of a few weeks to a hard, tough and elastic mass, resembling, as regards its physical qualities, resin or rubber.
By compounding a drying oil with a small quantity of litharge, pyrolusite, manganous borate, etc., and heating the admixture to the boiling-point, it acquires the property of drying in a few hours when exposed to the air in a thin layer. Oil so treated has been changed to a varnish.
By bringing a drying oil in contact with a body possessing strong basic properties a peculiar process takes place; the sebacic acids contained in the oil combine with the basic bodies to solid combinations which are insoluble in water, and, on exposure to the air, change gradually into masses as hard as stone. Such combinations, as regards their chemical composition, resemble ordinary soap, and for this reason are called insoluble soaps to distinguish them from ordinary soap which is soluble in water.
Burned lime, calcined magnesia, whiting, ferric oxide, litharge, and minium possess the capacity for forming insoluble soaps on coming in contact with drying oils and, still more quickly, with varnishes prepared from them.
The hardness of these soaps in time increases considerably by the oil not saponified drying in. The oil cements are principally used for tightening water and gas pipes, as they resist the action of water, steam and gas.
The only drawback connected with these cements is that they must reach a certain age before becoming entirely hard, and that, on account of the high price of drying oil or varnish which is absolutely required for their preparation, they are rather expensive. The ordinary glazier’s putty and the red lead and linseed-oil cement used in constructing water and gas conduits belong to this group.
Resinous cements. By resins are understood a number of constituents of plants which exude in thick viscous masses through incisions made in the trees, and on exposure to air are gradually converted into less transparent, brittle masses. When heated they melt more or less readily, forming a thick, ropy liquid, and brought in contact with an ignited body they burn with a bright flame and much sooty smoke.
By making incisions in the bark of any of the whole genus of Pinus belonging to the Coniferæ family, a viscous mass of a strong odor, called turpentine, is obtained. It consists of a solution of common rosin in the essential oil of turpentine, and when distilled yields from 75 to 90 per cent. of colophony or rosin, which remains in the retort, and from 25 to 10 per cent. of the essential oil, commonly called spirits of turpentine. Pure rosin is a brittle, tasteless, and almost inodorous mass of a light yellow color and a smooth, shining fracture.
The various resins found in commerce, such as shellac, mastic, elemi, copal, etc., are formed in a similar manner.