61. The oil most generally used by the house painter is obtained by the compression of flaxseed. This oil varies greatly in quality, as well with reference to the seed from which it is extracted, as in respect of age and consequent clearness. When a large quantity is, for six months or more, kept stored, an accumulation of refuse forms at the bottom and cannot be used, save in mixing coarse paint for outside work. Linseed oil is, from its cheapness, the only oil in common use for house painting, and may, under proper management, answer for every kind of work. The marked defects in linseed oil are its brown color and its tardiness in drying. These defects may, however, be greatly diminished, if not entirely removed, by keeping the oil for a length of time before use. It then becomes a good vehicle for color without any mixture. It is generally, nevertheless, used with a proper drier, and it never by itself becomes sufficiently pure to use with white or other light tints, without imparting a brown color. The drying of raw linseed oil may be accelerated by adding about 1 pound of white lead to every gallon of oil, and allowing it to settle for at least a week. Not only is the color of the oil thus improved, but the lead may be afterwards used for ordinary work.

Several methods have been contrived for bleaching and purifying linseed oil, but there is no known process for entirely preventing discoloration after drying.

One method for the purification of linseed oil is to place the oil in a bottle or jar, and drop into it some powdered whiting. Then stir or shake up the mixture, allowing it afterwards to stand for a time in a warm oven. The whiting will very soon carry down all color and impurity, and form a precipitate at the bottom. The refined oil at the top may then be drawn off. In the rare instances wherein the least yellowness in the oil might prove injurious, nut or poppy oil may, with advantage, be used, but linseed oil, for general purposes, is to be preferred.

62. Boiled linseed oil, usually designated boiled oil, is prepared by heating raw oil with certain driers. The drying qualities of the raw oil are greatly improved by the mere process of boiling, but when such substances as those below mentioned are added, this improvement is greatly enhanced. Dark drying oil may be made of these ingredients: 1 gallon of linseed oil; 1 pound of red lead; 1 pound of umber; 1 pound of litharge. Note, however, that, after boiling, the oil becomes much thicker, and, being too fat and likely to clog, cannot be used for purposes of grinding.

In the process here in question the linseed oil is heated to about 200° F. When it looks brown and the scum is all burned off, the other substances are added; the whole is then brought up to 400° F., and, for two or three hours, kept at that temperature. The oil is then drawn off, and the albuminous matter allowed to deposit, after which it is ready for use. The umber is added simply to give the oil a dark color.

Boiled oil to be used with zinc paint must be free from oxides of lead. About 5 per cent. by weight of powdered peroxide of manganese is boiled in the oil, for five or six hours, the mixture allowed to cool, and then filtered.

Pale drying oil may consist of 1 gallon of linseed oil, mixed with about 7 pounds of litharge or acetate of lead, raised to a moderate temperature.

Drying oil for common work may be made by boiling Impounds red lead in a gallon of raw linseed oil, and allowing the mixture to settle.

Linseed oil is, sometimes, boiled with litharge to cause it to dry quickly, but, when thus treated, is unfit for the better classes of work. The quality of linseed oil may be determined by looking through a vial filled with it and turned towards the light. If poor in quality, the oil tends towards opacity, appears turbid or milky, while its taste is strong and rancid. Good fresh oil should be, in appearance, clear and pale; in taste, sweet, and emit little or no odor.