Of the expressed or drying oils appropriate to painting, the Linseed is by far the strongest, and that which dries best, most tenaciously, and firmest under proper management; which properties it owes to its being at once resinous, glutinous, and oleaginous. Having more of the quality of a resin than a fat oil, it never totally loses its transparency while liquid, in the manner of fat oils by cold, but preserves it during the most intense frost in the manner of a resin; and, like the resins also, it becomes ultimately fixed, hard, and solid, by combining with the oxygen of the atmosphere: but it lies under the great disadvantage of acquiring, after drying, and by exclusion from light and pure air, a semi-opaque and yellow-brown colour, which darkens by age. To obviate this as much as possible, when painting with the oil alone, it is best to work the colour as stiff as may be, so as to use as small a proportion of the vehicle as may suffice; for it is a fact proved by direct and repeated experiments, that little oil diffused through much colour is subject to little change upon the canvass, and that a thin coating of linseed oil is similarly preserved by light and the action of the atmosphere, as is the case in glazing or oiling out, when coat-upon-coat is not applied in these ways, or a redundance of oil has not been consumed in the under-painting; yet the practice of oiling out is to be deprecated when sponging with water may suffice; the latter, however, is insufficient to remove a film of oil which rises to the surface of every layer of colour, and interposes an obstacle to the uniting of that which follows and disposes it to flake off, which is best remedied, when the work is hard enough to bear it, by oiling out with thin turpentine instead of linseed oil.

Linseed oil varies in quality according to the goodness of the seed from which it is expressed; the best is yellow, transparent, comparatively sweet-scented, and has a flavour somewhat resembling that of the cucumber: great consequence has been attributed to the cold-drawing of this oil, but it is of little or no importance in painting whether moderate heat he employed or not in expressing it. Several methods have been contrived for bleaching and purifying this oil, so as to render it perfectly colourless and limpid; but these give it mere beauty to the eye in a liquid state, without communicating any permanent advantage, since there is not any known process for preventing the discolourment we have spoken of as sequent to its drying: and it is, perhaps, better upon the whole that this and every vehicle should possess that colour at the time of using to which it subsequently tends, that the artist may depend upon the continuance of his tints, and use his vehicle accordingly, than that he should he betrayed, by a meretricious and evanescent beauty in his vehicle, to use it too freely. If, indeed, the oil were tinged with a fugitive transparent brown colour previously to being employed in painting, the original disposition of the oil to acquire colour by age would be compensated by the disappearance of the brown tinge given to it, so as to preserve the original freshness of the painting. Indeed, linseed oil that has been long boiled upon litharge in a water-bath, to preserve it from burning, acquires such a fugitive colour; and is, when diluted with oil of turpentine, less disposed to run than pure linseed oil, and affords one of the most eligible vehicles of the oil painter.

The most valuable qualities of linseed oil, as a vehicle, consist in its great strength and flexibility; some have preferred it when fattened by age, or exposure to sun and air; others, when new and fresh, or that which is cold-drawn; but that is the best which will temper most colour in painting; bees'-wax: and we have seen heads painted by another eminent artist, Mr. Joseph, of Cambridge, of the most fascinating transparency and texture, with oil copal varnish alone. Copal, in every mode of dissolving, swells or augments in bulk more than any other resin, like glue in water, and contracts proportionably in drying; and it is this which disposes it to crack, in which respect it is inferior to mastic, when unsustained by a drying oil.

As a general sketch of the progress of vehicles is rather a light to shew the way, than a hand to guide in practice, we will subjoin such observations of a more particular and tangible nature as experience has supplied concerning the materials of vehicles and varnishes; and, first, of oils.