whereby they in some degree correct the tendency of drying and expressed oils to discolorment. Of the essential oils, the most volatile and nearest in this respect to alcohol is oil of sassafras, but that most used in painting is the rectified oil, improperly called spirits of turpentine, preferable only on account of its being thinner and more free from resin. By the action of oxygen upon it, water is either generated or set free, and the oil becomes thickened, but is again rendered liquid by a boiling heat upon water, in which the oxygen and resin are separated from it. When colored by heat or otherwise, oil of turpentine may be bleached by agitating some lime powder in it, which will carry down the color. The great use of this oil, under the name of turps, is to thin oil paints, and, in the larger use thereof, to flatten white and other colors, and to remove superfluous color in graining. It, however, weakens paint in preventing its bearing out, and when used entirely alone, it will not fix the paint.
The name of turpentine is applied to a liquid, or soft solid product of certain coniferous trees, and of the Pistachia terebinthus.
There are several varieties, as follows: American or white turpentine, Bordeaux turpentine, Venice turpentine, Strasburg turpentine, Canadian turpentine, or Canadian balsam, Ohio turpentine and Frankincense.
In nearly all cases the processes of collecting are similar. A hollow is cut in the tree yielding turpentine, a few inches from the ground, and the bark removed for the space of about 18 inches above it. The turpentine trickles down into vessels placed to receive it. The incisions are made about the close of March, and the turpentine continues to run throughout the vegetative season, especially during the summer months. In general character these turpentines have much in common; they are oleo-resins, varying slightly in color, consistency and smell; they enter into the composition of many varnishes.
