Paint (FR., Peinture; GER., Anstrichfarbe).—" Paint " consists essentially of two parts, (1) the vehicle or medium, and (2) the pigment (see Pigments). In the case of oil-paints, a third substance, becomes necessary, to facilitate the drying or solidification of the vehicle ; this is termed a " drier. A perfect vehicle should mix readily with the pigment, forming a mass of about the consistency. of treacle. It should itself be colourless, and have no chemical action upon the pigments with which it is mixed. When spread out in a thin layer upon LI, non-porous substance, it should solidify, and form a Elm not liable to subsequent disintegration or decay, and sufficiently elastic to resist a slight concussion.
Unfortunately, we possess no vehicle which complies with all these conditions ; those which most nearly approach them are the drying oils (see Oils and Fatty Substances, p. 1467). The use of oil in painting is said to have been invented in the 14th century, and, in a short time, it reached a considerable degree of perfection. We have only to compare a vau Eyck with a painting by a modern master, Turner, for instance, to see that even the best of recent painters have not succeeded in giving to their works that durability which the originators of the method attained. All organic substances are liable to a more or less rapid oxidation, especially if exposed to light and heat. Oil is no exception to this rule ; but it seems that, in its pure state, it is much more durable than when mixed with other substances. Although ground-nut- and poppy-oils (see pp. 1391, 1409) are sometimes employed by artists wbere freedom from colour is essential, yet linseed-oil (see p. 1393) is the vehicle of by far the larger proportion of paint used both for artistic and general purposes.
Oil-paint appears to have been unknown to the ancients, who used various vehicles, chiefly of animal origin. One of these, which was in high repute at Rome, was the white of eggs beaten with twigs of the fig-tree. No doubt the indiarubber contained in the milky juice exuding from the twigs contributed to the elasticity of the film resulting from the drying of this vehicle. Pliny was aware of the fact that when glue is dissolved in vinegar and allowed to dry, it is less soluble than in its original state. Many suggestions have been made in modern times for vehicles in which glue or size plays an important part. In order to render it insoluble, various chemicals have been added to its solution, such as tannin, alum, and a chromic salt. None of these vehicles, however useful for special purposes, has become sufficiently well known to warrant description here.
Linseed-oil, to be suitable for painting, must dry well. Tho test described in the article on Floordoth (see p. 1002) will indicate whether this be the case or not. Another reliable test is to cover a piece of glass with a film of the raw oil, and to expose-it to a temperature of about 38° (100° F.). The time which the film requires to solidify is a measure of the quality of the oil. If the oil has been extracted from unripe or impure seed, the surface of the test-glass will remain " tacky " or sticky for some time, and the same will happen if the oil under examination has been adulterated with either an animal or vegetable non-drying oil.
Until recently, linseed-oil was frequently adulterated with cotton-seed-oil (see p. 1385), extracted from the waste seeds of the cotton-plant. Where the admixture was considerable, it could easily be detected by the sharp, acrid taste of the cotton-seed-oil. Now, however, means have been found for removing this disagreeable taste, and the consequence has been that cotton-seed-oil is so largely used for adulterating olive-oil, or as a substitute for it, that its price has risen above that of linseed oil. Another adulterant which is rather difficult to detect is rosin. Oil containing this substance is thick, and darker in colour than pure oil. When the proportion of rosin is considerable, its presence may be ascertained by heating a film of the oil upon a metallic plate, when the charac teristic smell of burning rosin will be perceptible. When the percentage of rosin is too small for detection in this planner, a film of the oil should be spread upon glass and allowed to dry. When quite hard, the film should be scraped off, and treated with cold turpentine, which will dissolve any iosin which may be present, without materially affecting the oxidized oil. The presence of rosin may also be detected by the following simple chemical test. The oil is boiled for a few minutes with a small quantity of alcohol (sp. gr. 0.9), and is allowed to staud until the alcohol becomes clear. The supernatant liquid is then poured off, and treated with an alcoholic solution of acetate of lead. If the oil be pure, there will be but a very slight turbidity, while the presence of rosin causes a dense flocculeot precipitate. Should linseed-oil he adulterated with a non-drying oil, it will remain sticky for months, when spread out in a thin film upon glass or any other non absorbent substance.