The essential characteristic of these methods is to be found in the use of a binding material which is in itself insoluble in water. The painting-ground employed should be dry, and free from alkali and from soluble salts. If it be primed canvas or panel, it is a good plan to cleanse it with oxgall and water, or with a very weak solution of carbonate of ammonia, before commencing work. A discoloured lead-priming should be restored to its original brightness by laying a sheet of white blotting-paper upon it, and then just saturating this paper with a solution of peroxide of hydrogen. The moist surface is now exposed to a moderate degree of heat - as by holding it in front of a fire - which greatly quickens the activity of the peroxide. When the paper has become dry, it may be removed, and the bleaching of the tarnished ground will be found to have been effected, the brown sulphide of lead having been oxidized into the white sulphate. In order to learn whether a plaster-ground or a wall is sufficiently dry to be safely painted upon in oil or spirit-fresco, the gelatin-test may be employed. A small oblong piece of coloured sheet-gelatin is held firmly and closely against the plaster or wall, by means of a stick applied at the centre.

If hygroscopic equilibrium have been established between the wall and the air, the gelatin will remain flat; if the wall be moister than the air, the sheet will curl outwards, the inner surface becoming highly convex. Slate and several other suitable painting-grounds may be dried and further prepared for work in oil or spirit-fresco by heating them gradually in a water-oven up to the temperature of boiling water, and then rubbing them with a piece of hard paraffin-wax. The slate is again heated in the water-oven, withdrawn, and then at once rubbed with a dry, warm cloth, so as to remove all excess of paraffin wax. Other methods of treating stone, etc., for the reception of oil-colours have been previously given. A very convenient means of neutralizing the residual alkalinity of a lime-plaster ground intended for oil or spirit-fresco painting is afforded by linoleic acid.* This liquid fatty acid is an article of commerce, moderate in price, and easily obtainable. A wide-mouth tin of it is placed in a vessel of boiling water; when the linoleic acid is hot, it is paid on to the surface of the plaster with a wide brush, any excess being removed by wiping the ground with a cloth.

Solid stearic acid may be melted and used in the same way, but its effect is inferior.

The vehicles employed in these methods of painting are not miscible with water - are, in fact, hydrofuge materials repellent of moisture. If an absorbent ground or other porous material be soaked with water, and then covered with oil, as the water evaporates the oil penetrates, and at last completely takes its place. But, on the other hand, the reverse process cannot be carried out, since the water outside will not displace the oil inside. These vehicles are either oils or else solid substances in solution - solids which, though insoluble in water, may be dissolved with more or less ease in one or other of a long series of liquid solvents (Chapters V., VI., XL, and XII.). The changes experienced by these vehicles and their constituents during the painting process may be thus summarized:

* By linoleic acid is here meant the mixture of fatty acids obtainable from raw linseed oil.

(a) The oils used absorb oxygen from the air, increasing in weight thereby to the extent of 10 or 11 per cent. - such increase in weight being accompanied by a considerable increase in bulk. This latter change is clearly shown when a layer of a drying oil, spread upon glass, is allowed to dry; it then becomes rippled or wrinkled from expansion; such expansion, owing to the viscosity of the oil, takes place mainly in a direction perpendicular to that of the surface of the glass.

(b) The above-described absorption of oxygen by the oil employed in painting results in the formation of a substance or mixture of substances called linoxine. Now this product is not only solid instead of liquid, but it is almost insoluble in the usual solvents of oils unlike the oil from which it has been formed. But there are circumstances, not yet accurately defined, in which linoxine itself occasionally suffers a peculiar change, finally becoming brown in colour, tacky in consistence, and soluble even in spirits of wine. This degradation of linoxine is, however, of very rare occurrence in the ordinary practice of oil-painting. A singular circumstance connected with the transformation of 'linolein' into 'linoxine' has been noticed; this change is accompanied by the formation of hydrogen peroxide, a compound which is also produced during the oxidation of the terpenes. The continuous production of the peroxide may be recognised on the surface of an oil-painting long after it has been completed by the blue colour which it develops in starch-paste containing potassium iodide.

(c) The resins present in varnishes and media contract for some time after the major part of their volatile solvent has escaped by evaporation, and thus leave a residue which becomes fissured. In a properly-proportioned medium this contraction should be balanced, or rather more than balanced, by the expansion of the oil present. Hence the desirability of associating a varnish (or a resin dissolved in a volatile solvent) with a drying oil, in this method of painting.

(d) Waxes and solid paraffins, when once deposited from a solution by the escape of the solvent, neither expand nor contract by desiccation or oxidation, but only through changes of temperature.

(e) Most of the liquid solvents simply evaporate, leaving no fixed residue due to their previous presence. But spirit of turpentine and oil of spike generally behave differently. Some kinds of spirit of turpentine differ from the majority in this particular, but the remainder suffer two simultaneous changes. A portion evaporates; another portion absorbs oxygen from the air, becoming converted into a sticky, yellow, and resinous substance, which remains behind. The resin thus formed is a very objectionable constituent in the structure of a picture, and its production should be avoided either by employing a variety of turpentine not subject to easy resinification, or by using a freshly-distilled turpentine which has been secluded from the air, and in which a few lumps of freshly-burnt lime have been placed, to remove water and such resinous matters as may be produced.