All the fixed oils have an attraction, more or less powerful, for oxygen, and by exposure to the air they either become hard and resinous, or they only thicken slightly and become sour and rancid. Those which exhibit the first property in a marked degree - as the oils of linseed, poppy, rape and walnut, are called drying oils, and are used as vehicles for colors in painting; the others are termed glutinous or non-drying oils.

The resinifying or drying qualities of the oils are greatly increased by boiling them, either alone or along with litharge, sugar-of-lead or white vitriol, when the product forms boiled oil, or drying oil of commerce. The efficacy of the process depends on the elimination of substances which impede the oxidation of the oil.

The following methods of preparing drying oils are culled from various sources; the quantities of each formula are given as in the originals, but these can, of course, be used in relative proportions when the preparation is to be carried on on a smaller scale.

Linseed oil, 1 gallon; powdered litharge 3/4 pound. Simmer, with frequent stirring, until a pellicle begins to form; remove the scum, and when it has become cold and has settled decant the clear portion. Dark colored, used by house painters.

Three hours boiling, with litharge one-tenth in weight of the oil, renders the oil more perfectly drying than when the boiling is continued for a much longer time, when the oil acquires a darker color and so becomes injured in transparency the longer it is boiled. Merely heating linseed oil to 170° Fahrenheit, along with a small quantity of peroxide of manganese, as completely renders it siccative as any amount of boiling, and without any deterioration to its color or transparency. It appears probable that litharge acts more by its mere presence in inducing the oxidation of the oil than by actually giving up oxygen to it, and those engaged in boiling oils have remarked that the old litharge, with which linseed oil has been already boiled, acts more energetically in producing the siccative property in it than new litharge.

Pale Linseed or Nut-Oil 1 pint, litharge, or dry sulphate of lead in fine powder 2 ounces; mix, let it stand, frequently stirring it for ten days, then set the bottle in the sun, or in a warm place to settle and decant the clear portion.

Sugar of lead 1 pound, dissolved in 1/2 gallon of rain water; 1 pound litharge in fine powder is then added, and the mixture is gently simmered until only a whitish sediment remains; 1 pound of levigated litharge is next diffused through 2 1/2 gallons of linseed oil, and the mixture is gradually added to the lead solution previously diluted with an equal bulk of water; the whole is now stirred together for some hours with heat and is lastly left to clear itself by exposure in a warm place. The lead solution which subsides from the oil may be used again for the same purpose by dissolving in it another pound of litharge as before.

Into linseed oil, 236 gallons, pour oil of vitriol 6 or 7 pounds, and stir the two together for three hours, then add a mixture of fuller's earth 6 pounds, and hot lime 14 pounds, and again stir for three hours. Next, put the whole into a copper, with an equal quantity of water, and boil for about three hours; lastly, withdraw the fire, and, when the whole is cold, draw off the water, run the oil into any suitable vessel, and let it stand for a few weeks before using.