OILS, DRIERS, VARNISHES, ETC.
We shall now briefly occupy ourselves with those fluids which are indispensable to the worker for compounding with pigments mixed in paint.
Oils are usually divided into two classes, and are termed fixed oils and volatile oils. Fixed oils are further distinguished, by their nature and source, into fat oils and drying oils. Fat oils are those which contain an excess of oleic acid, or stearine, as the animal and fish oils, and these are consequently non-drying oils. Drying oils are those which harden into a solid form—as, for instance, linseed, poppy, and nut oils.
Good and reliable fluids are as necessary in the mixing of paint as are good pigments. Lin seed oil occupies a position amongst oils similar to that taken by genuine white lead amongst the solids; and just as white lead forms the basis in nearly all light color paints, so linseed is the principal solvent in the preparation of varnishes and other vehicles.
Drying oils, and linseed oil particularly, owe the characteristic of drying to their excess of resinous properties; and therefore, when used under the influence of oxygen, they dry or harden into a film of horny substance.
Linseed oil, then, is the most useful of all oils for the painter, but it turns to a yellow-brown color rapidly, and darkens by age. To help to prevent this, turpentine is mixed with the color, and it is best to use the oil sparingly when mix ing paint, for the reason that in drying it darkens the lead. Color and smell are the tests for good oil (some painters even taste it). A good way is to compare the odor with that of crushed linseed meal. By mentioning that animal oils are turned brown by chlorine, and that vegetable oils re main colorless, a test is revealed for any oils whose nature is unascertained.
Linseed oil should always be transparent, free from any rancid smell or taste, and of a light yellow or amber color.
Boiled Oil—that is, boiled linseed oil—is a very serviceable preparation. It is the ultimate product of the raw linseed oil boiled with litharge, or some similar substance. By this
process the oxidizing or drying qualities of the litharge are communicated to the oil, which fur thermore gains body and brilliancy. The boiling of linseed oil causes it to become much darker, and hence it is seldom used for light colors, and but rarely for interior painting. For preserva tive work, boiled oil is almost indispensable, and especially with dark pigments; its color is then no disadvantage, while its extra body and hard ening qualities are a decided gain. In the proc ess of manufacturing boiled oil, raw linseed oil is brought up to the boiling point, a little man ganese being steeped in it during the boiling; it thus becomes the boiled oil of commerce. Badly boiled oil has the fault of ropiness. It should be nearly as limpid as raw oil, and should, if spread on a piece of glass, either dry or rapidly have a skin over it.
Like linseed oil, turpentine is largely used in the manufacture of varnishes and other paint ers' vehicles.
Oil of Turpentine, commonly called turps, and sometimes, but incorrectly, termed spirit of turpentine, has a colorless appearance and strong pungent odor, as well as an inflammable nature. Turpentine contains a slight proportion of resin and other matter which will not evap orate by exposure or heat: which fact demon strates the fallacy of calling it a spirit; but its volatile nature makes it invaluable to the painter for thinning the drying oils and for making flat ting paint. Since oil of turpentine contains but a small proportion of the resinous properties common to the expressed oils, it follows that its binding quality is very poor; and paint com pounded with turps alone can be rubbed away by friction.
There are several methods of testing turpen tine, and the points upon which it is possible to tell its purity are: (1) The specific gravity; (2) the boiling-point; (3) the action upon polar ized light; (4) the absence of fluorescence; (5) the residue on evaporation. Turpentine is some times adulterated by the addition of paraffin oil.