A perfect vehicle mixes readily with the pigment, forming a mass of about the consistency of treacle. It is colourless, and has no chemical action upon the pigments with which it is mixed. When spread out in a thin layer upon a non-porous substance, it solidifies, and forms a film not liable to subsequent disintegration or decay, and sufficiently elastic to resist a slight concussion. No vehicle complies with all these conditions; those which most nearly approach them are the drying-oils. The use of oil in painting is said to have been invented in the 14th century, and soon reached considerable perfection. 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 are sometimes employed by artists where freedom from colour is essential, linseed-oil is the vehicle of by far the larger proportion of paint for both 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 white-of-egg 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.
Linseed-oil, to be suitable for painting, must dry well. A test which will indicate whether this be the case or not is to cover a piece of glass with a film of the raw oil, and to expose it to a temperature of about 100° F. (38° C). 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 testglass will remain " tacky " or stinky for some lime, and the same will happen if the oil under examination has been adulterated with an animal or vegetable non-drying oil.
Until recently, linseed-oil was frequently adulterated with cottonseed-oil, 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 cottonseed-oil. Now, however, means have been found for removing this disagreeable taste, and the consequence has been that cottonseed-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 61m of the oil upon a metallic plate, when the characteristic smell of burning rosin will be perceptible. When the percentage of rosin is too small for detection in this manner, 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 rosin 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 stand until the alcohol becomes clear. The supernatant liquid is then poured off, and treated with an alcoholic solution of lead acetate. If the oil be pure, there will be very slight turbidity, while the presence of rosin causes a dense flocculent precipitate. Should linseed-oil be adulterated with a non-drying oil, it will remain sticky for months, when spread out in a thin film upon glass or other non-absorbent substance.
The sp. gr. of linseed oil is in some cases of value in estimating its quality; but;as the variations are slight, it would be difficult to detect them in so thick a liquid by means of an ordinary hydrometer. A simple method of obtaining an approximate result is to procure a sample of pil of known good quality, and to colour it with an aniline dye. A drop of this tinted oil will, when placed in the oil to be tested, indicate, by its sinking or swimming, the relative density of the liquid under examination. Freshly-extracted linseed-oil is unfit for making paint. It contains water and organic impurities, respecting the composition of which little is known, and which are generally termed "mucilago." By storing the oil in tanks for a long time, the water and the greater part of the impurities are precipitated, forming at the bottom of the cistern a pasty mass known as "foots."
To accelerate the purification of the oil, and to remove at least a portion of the colouring matter, various methods are in use. The action of sulphuric acid upon linseed-oil is not so favourable as upon other oils. It is, however, sometimes employed, in the proportion of 2 parts of a mixture of equal volumes of commercial sulphuric acid and water to 100 of oil. The dilute acid is poured gradually into the oil, and the mixture is violently agitated for several hours, then run into tanks, and allowed to settle. A concentrated solution of zinc chloride has been substituted for sulphuric acid in the proportion of about 1 1/2 per cent, of the weight of the oil. When the reaction is complete, steam or warm water is admitted into the liquid to clarify it. Oil treated in this way loses a considerable proportion of the colouring matter which it originally contained. When the oil is to be used for white paint, it is sometimes bleached by exposing it to the action of light. On a large scale, this is done in shallow troughs, lined with lead and covered with glass. The lead itself appears to have some influence upon the bleaching of the oil, for the decoloration is not so rapid if the troughs be lined with zinc.
For small quantities, a shallow tray of white porcelain gives very good results, the white surface increasing the photo-chemical action. It is not quite clear whether the presence of water accelerates the bleaching of oil by this method; some manufacturers consider its presence necessary, others omit it. Various salts are added to the water, the one most in use being copperas. However the oil may have been prepared, it will, if kept for a long time, deposit a sediment. At first this contains mucilage; but the sediment from old oil consists chiefly of the products of decomposition of the oil itself. Oxygen is not necessary for this decomposition; but it is increased by the action of light. Raw linseed-oil dries more slowly than boiled; but the resulting film is more brilliant and durable. Raw and boiled oils are therefore usually mixed in proportions varying according to the time which can be allowed for the paint to dry, or to the properties required of the film. For ordinary kinds of paint, equal parts of boiled and raw oils are customary.
Linseed oil heated to 350° to 400° F. (176° to 204° 0.) dries much more rapidly than in its raw state.