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The Vehicle or Medium
As already stated, the vehicle or medium of a paint may be a drying oil, a varnish, or a mixture of both.Varnishes will be considered elsewhere and we will here deal only with the principal oils used in paints.
Linseed Oil.-Although in recent years new forms of drying oils have been developed or discovered for use in paints, linseed oil still remains the most widely used paint vehicle, not only because of its own valuable properties but because it is obtainable in abundant quantities more or less wherever flax is grown.
The decorator who makes up his own finishes rarely, if ever, uses any other kind of oil.
Up to the end of the last century, North Russia was the most important source of supply and Baltic oil was regarded in the painting trade as the best of its kind.
Nowadays, the Argentine is probably the chief exporting country, though Southern India, the USA, and Canada also produce and export considerable quantities.
The principal method of obtaining the oil is by grinding or pressing the flax seed, though a good deal is now extracted by means of solvents.
Before crushing, other seeds, such as rape or hemp, which are often found among the flax, are removed by machinery. The oil, when it first emerges from the press, contains a certain proportion of impurities, in the form of water, natural salts, and albuminous matter, and these, too, must be removed.
The presence of these impurities gives the oil a cloudy appearance; in storage it clarifies and the mucilage (commonly known as " foots ") sinks to the bottom of the tank.
Formerly, the method employed was to expose the oil for many months in glass-covered shallow tanks; on ageing, in this way, it became progressively lighter in colour.
This process produces an oil which is probably superior in quality to that obtained by any other means, but it is too slow and costly for ordinary purposes, and the bulk of the oil produced is now refined by chemical means.
Traces of gummy matter in the crude oil remain in solution in the oil, but turn insoluble and cause a jelly-like deposit to separate out when the oil is heated-as when boiled oil or varnish is produced. It is more economical to remove these traces by preliminary refining than as " foots " from the finished product. Removal is effected in various ways; the oil may, for instance, be treated with Fuller's earth which absorbs the glutinous matter: alternatively, caustic alkali can be added to neutralise the free fatty acids and form a soap which separates out; or, again, the oil can be treated with sulphuric acid. after treatment, the oil is allowed to settle until it is clear and is then refiltered.
The refined oil is paler in colour and perfectly limpid. Linseed oil dries by absorbing oxygen from the air, and this property of absorbing oxygen is increased by heating the oil for some hours and adding a proportion of driers in the form of lead oxide and manganese oxide.
The latter decompose in the heated oil and form soluble metal compounds of the oil itself, the heating of the oil in the presence of these drying agents imparting a dark colour to the oil.
Whereas raw linseed oil tends to become lighter when exposed to strong sunlight, boiled oil is inclined to become slightly darker.
There are various types of boiled oil, some of which are extra pale and some extra-quick drying.
Blown Oil.-In the preparation of various types of paints, varnishes, and other finishes, linseed oil treated in a number of ways may be employed.
It may, for instance, be thickened or "bodied" by having a current of air blown through it. "Blown" oil, as it is called in this form, is slightly thicker in consistency than ordinary boiled oil and yellower in colour.
Its smell is reminiscent of that of oiled silk. The decorator seldom encounters blown oil, which has a tendency to form rather thick, soft skins.
For the manufacturer, it has useful properties, being rather more elastic and moisture-resisting than boiled oil, as well as paler in colour.
Stand Oil.-In the preparation of certain forms of slow-drying enamels and enamel paints, what is known as " Stand" oil is frequently used.
This form of linseed oil originated in Holland about 1865; it is produced by heating the oil without the addition of any driers, until the required degree- of stoutness is obtained.
Cooking proceeds for several hours at a temperature of just below 6000 F., and steps are taken to ensure that the oil does not become too dark in the process.
This, again, is essentially a manufacturer's oil and is seldom used by the decorator to make up his own paints in this country, though some continental painters occasionally employ it for this purpose.
Stand oil is a viscous fluid, slightly thicker than treacle.
It yields a film which is rather soft and is, therefore, often combined with varnish in order to harden it.
An enamel properly made with first-class stand oil has great durability and elasticity.