Oil, for delicate machinery, should be purified from its stearine, or fatty mat ter, which is effected by gradually boil ing it with eight times its weight of alco hol. When cold, the stearine separates in a precipitate, and the liquid is to be evaporated to a fifth, which is pure elaine, or oil, without any chemical action or odour.
Oils in painting afterward fatten and do not dry, owing either to want of com bination in the pigment, or to the oil not being old enough. Olive oil, for exam ple, will not dry—even several samples vary in this defect. Keeping, and the use of some drying substance, are the best remedies. Oils are adopted because they give an equal surface and a subse quent body.
Drying oils, by boiling, or sometimes by setting on fire, by which they cease to stain paper, are linseed, walnut, hemp, poppy, castor, croton, grapeseed, night shade, tobacco, henbane, sun-flower, and cress.
Drying oils are best prepared by boil ing a gallon of linseed oil with if lb. of red lead, and leaving it to stand till the lead has subsided. Other materials effect the same purpose, as white vitriol, sugar of lead,. gum mastic, &c., where long boiling is inconvenient. Or, take half a gallon of linseed oil, and slowly boil it with 6 oz. of litharge, and li of white vitriol, till no more scum arises. Let it cool and settle, and then pour oft the clear into small vessels, and in ten days it will be fit for use. Or, suspending in boiling oil a bag of litharge and white vitriol for 4 or 5 hours. Or, well stir a lb. of white lead with a gallon of linseed oil, and leave it to settle for 8 or 10 days. Fat oils generally may acquire a drying quality by the following treatment:— Take of nut oil, or linseed oil, 8 lbs.; white lead, slightly calcined ; sugar of lead, also calcined ; white vitriol : of each 1 oz. Litharge, 12 oz., a head of garlic, or a small onion. When these are pul
verized, mix them with the garlic and oil over a fire capable of maintaining the oil in a slight state of ebullition : continue it till the oil ceases to throw up scum, assumes a reddish color, and the garlic, or onion, becomes brown. A pellicle in dicates that the operation is completed. Take the vessel from the fire, and the pellicle, precipitated by rest, will carry with it the unctuous parts. When the oil becomes clear, separate it from the deposit, and put it into wide-mouthed bottles, where it will completely clarify itself.
In all cases, where preparations of lead are employed for freeing oils from greasy principles, the mixture should not be stirred. It is sufficient to leave the mix ture over a gentle fire, capable of pro ducing slight ebullition. The garlic merely indicates the moment when the aqueous part is evaporated.
Drying oil is employed by those who paint pictures, and it enters' into the composition of varnishes. It serves it self also as varnish, either employed alone, or diluted with oil of turpentine.
For house painting, it is advantageous to for the last coating, resinous dry ing oil, as a varnish. It is prepared as follows :—Take 10 lbs. of drying nut oil, if the paint be designed for external sur faces, or 10 lbs. of drying linseed oil, if for internal. Yellow resin 3 lbs.,, com mon turpentine 6 oz. Melt the resin, to which add the turpentine, and lastly, the oil, so as not to coagulate the resin ; leave the varnish at rest, by which means it will often deposit portions of resin and other impurities ; preserve it in wide-mouthed bottles. It must be used fresh : when suffered to grow old, it deposits some of its resin. If this re sinous oil become too thick, dilute it with a little oil of turpentine, or with oil of poppy, if intended for articles shelter ed from the sun.