Extraction and Purification

The percentage of oil in linseed varies between 28 and 45: by cold-pressure 20 percent is the average yield; by hot-pressure, 27 per cent.; by extraction with carbon disulphide, 33 percent The linseed oil in common use by artists is hot-pressed oil, and is very rarely, if ever, obtained from absolutely pure seed. The seed should be kept three months before it is pressed. The expressed oil should be exposed to light in covered glass vessels or tanks, and kept at a temperature of 212º F. for some time. It thus loses colour and becomes clear, a slimy deposit containing mucilage, albuminoid matter, and traces of a cyanogenetic glucoside, being formed. When thus bleached and clarified, the oil should be preserved in corked bottles filled quite full; the longer it is kept, the better it becomes for painting, provided the access of air is prevented. The specific gravity of good linseed oil varies very little. At 60° F. (15.6° C.) it is .935; a bottle which will hold 1,000 grains of water at this temperature will therefore hold but 935 grains of linseed oil. It expands considerably with heat, its specific gravity at 50º C. being .913 only. One part of linseed oil requires 36 parts of cold absolute alcohol for solution, but only 4 parts of boiling alcohol.

It may be purified by solution in boiling alcohol or in petroleum ether. Other methods of purification are generally employed. Amongst these may be named the following: Filtration through felt or carded cotton and charcoal, and then through pyrolusite; contact for some weeks with 3 percent of a mixture of equal parts of kaolin and aluminium hydrate, both these compounds having been previously dried at about 50° C.; agitation with a solution of common salt, followed by washing with water, and drying by a heat of 220° F.; treatment with one four-hundredth part of oil of vitriol, addition of hot water, washing, and drying. Various other processes and reagents have been employed for purifying and bleaching linseed oil. Aqueous solutions of sulphurous acid, green vitriol, potassium permanganate, potassium bichromate, and peroxide of hydrogen may be included in this list. The addition of 1 percent of oil of turpentine to the oil, and then passing a mixture of air and steam through it, has also been tried. Whatever process be adopted, no acid, saline matter, or moisture must be left in the oil.

The general and usual result of all the very different kinds of treatment to which linseed oil is subjected, in the above-named and in many other processes, seems to be the more or less complete removal of impurities. The effect on the properties of the purified oil is chiefly seen in its greatly increased rate of absorbing oxygen and consequent hardening.