Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/249

Rh 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