¶ The majority of pictures nowadays are painted on canvas, though it must be admitted that, especially in respect of mechanical and chemical durability, it is a very unsatisfactory material. Its light weight, its cheapness, and the possibility of rolling up pictures painted upon it, offer some advantages, no doubt, although the last characteristic is of questionable value. On the whole, the drawbacks to the use of canvas preponderate over its merits. In this connexion special reference should be made to the very marked alteration in dimension caused by damp (see p. 36). This persistent working or movement of the ground can be checked in the case of large collections gathered in galleries duly equipped with the latest technical appliances for the careful regulation of humidity and temperature. But the case is different in private rooms where the movement in question is practically unavoidable, and inevitably leads to cracking in the course of time. This change is still further promoted by the action of the oxygen in the atmosphere, which attacks the painting both back and front, and, by producing a gradual alteration in the binding material, brings about a corresponding deterioration in the adhesion and cohesion of the pigments.

Moreover, from the same cause, the linen or hemp fibre itself becomes after a time so brittle that it is scarcely capable of affording adequate support to the painted layer. When it reaches such a state a picture must be 'relined' - i.e., stretched on a fresh canvas. The painting itself may thus prove more durable than the material base which was designed to secure its durability.

¶ The stability of pictures painted on canvas is enhanced if the back be protected by a metallic coating, so as to protect it in a measure, on that side at least, from the inroads of oxygen. The simplest way to secure this result is by coating the back with tinfoil, using to fix the metal in position a strong solution of shellac* in spirits of wine. In order to guard against the penetration of oxygen through the accidental holes occurring in the tinfoil, a second sheet of this metal may be added when the shellac solution has become dry. This treatment may be applied generally to pictures painted on canvas, and virtually doubles their span of life.

¶ But protection from mechanical injury is not ensured by a thin coating of tinfoil. If that further protection be desired the back of the painting may be lined with sheet metal. According to the size of the picture, sheet copper or brass, or even sheet iron coated with tin or zinc, may be employed. Paintings executed on canvas may be fastened to the sheet of metal by the shellac* cement before named or with thick amber varnish. Thus the back becomes protected both chemically and mechanically by the same contrivance, and a high degree of durability is thus assured for the picture.

¶ When it is a question of producing new pictures the painting may be executed directly on metal. Such a procedure applied to sheet copper was frequent with Dutch miniaturists: the flawless condition of their works justifies this method from the present point of view. Yet, as this use of copper involves some risk that its green and blue oxidation-products may give rise to discoloration, a more appropriate painting-ground is offered by sheet aluminium, which yields only colourless compounds. Moreover, when aluminium is exposed to the atmosphere there is formed on the surface a transparent and imperceptible film of oxide which retains oil-colours very firmly. Even on unprepared surfaces of aluminium it is possible to paint very easily, for the metal possesses a peculiar 'tooth' in relation to the paint so that one can readily lay on successive coats of paint, stroke by stroke. Some years ago I covered a piece of sheet aluminium with a coating of oil-paint and exposed it in the laboratory to all the accidents of the place. The paint remains sound to this day and shows no tendency to crack or peel.

A sketch executed on strong sheet aluminium stands in a greenhouse, where it receives all available sunshine and is exposed to great variations of temperature: after exposure during six months of spring and summer it showed no signs of change for the worse. There is therefore good reason for contending that in sheet aluminium we possess an ideal painting - ground, especially for work in oils.

* Perhaps marouflage is preferable (see p. 26).

¶ Another material which seems to lend itself particularly to decorative and monumental painting is linoleum. This consists of a very strong fabric coated with a thick layer of oxidized linseed oil mixed with cork-raspings and other materials. It thus bears some resemblance to canvas which has been primed for the reception of oil colours, but differs in its greater solidity and in the elastic substances which it contains. The fact that the body of linoleum consists mainly of the same substance that forms the binding material in ordinary oil-painting sufficiently guarantees permanent union between picture and ground. From another point of view the massive nature of the material almost completely obviates the risk of mechanical injury, and affords at the same time complete protection from the attacks of air and damp at the back. If the brown colour be not an objection, the surface can be used just as it is as a painting-ground; in this case the various rough and smooth sorts of linoleum provide an agreeable choice of surfaces from the artistic standpoint. But it will be found better to lay on a thin coat of white oil-paint, especially in the case of pictures intended to present a bright general tone.

Or a white pigment may be incorporated with the mass of the linoleum itself and so the brilliancy of the applied colours may be enhanced. As linoleum is manufactured several yards wide, it is possible to paint very large pictures on a single piece, so that this material seems more suitable for monumental or large decorative works. It should not be fixed directly to the wall, but attached to a metal framework covered with galvanized iron wire netting and erected at a small distance from the wall; the danger of injury from damp or fracture is thus avoided.

With reference to the two materials, linoleum and sheet aluminium, recommended in the preceding paragraphs for use as painting-grounds, a few further observations may be advisable. The variety of linoleum in which the canvas-backing is omitted - solid linoleum - is not suitable for the purpose under discussion, for there is some risk when large pieces are used of its sagging, and even splitting, after the lapse of some time. And the experience of coach-builders is not altogether in favour of aluminium for the reception of a coating of oil-paint. In some instances, at all events, there are signs of the disintegration of the metal and consequent cracking of the superimposed layers of pigment. Possibly these drawbacks may not occur where the painted surfaces are not exposed to the external atmosphere. And it must be remembered that the sheet aluminium of commerce varies somewhat in its composition and properties.

'Willesden canvas' appears to resist the attack of moisture and of animal organisms: the copper-compounds which it contains do not interfere with its use as a paintingground, but the absence of soluble salts (chiefly sulphates) from it should be ascertained. This test is easily made by soaking a piece of the canvas weighing 50 grains in distilled water overnight, pouring off the clear liquid next morning, heating it to boiling, and adding a few drops of barium chloride solution and of dilute nitric acid. If a distinct precipitate be formed the canvas cannot be employed safely, but a mere cloudiness may be disregarded. It is just as well to test the watery extract for free acid by means of litmus paper, because an attempt has been made to remove the green colour of Willesden canvas by a bath of dilute sulphuric acid, the presence of which is, on all accounts, to be avoided. 'Willesden paper' possesses properties similar to those of Willesden canvas, and may sometimes be found serviceable for work in oil-colours.