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Preservation of Canvas

There are two methods of preserving canvas from decay by the application of solutions to the back after the priming-has been completed on the front. One of these solutions contains corrosive sublimate (mercuric chloride) dissolved in methylated spirit: a 5 percent solution is sufficiently strong. The other solution is made by dissolving tannin in methylated spirit. One or other of these solutions should be applied once to the back of the canvas by means of a broad stiff varnish-brush. When the spirit has evaporated, the coating of white lead ground in starch paste previously recommended in this chapter may be applied. The two solutions we have named act by coagulating some of the size in the canvas: the tannin turns it into leather. Corrosive sublimate prevents the development of mould or mildew, and is a good preservative against the attack of animal organisms.

It may be observed that the employment of size in the first preparation of canvas constitutes an element of weakness. Many attempts have been made to substitute a less hygroscopic and changeable substance. A plain collodion containing a little ceresin (the hard paraffin from ozokerite, or earth-wax) has been used for the purpose in question. It is, however, very difficult to secure the adhesion to the canvas of the film left behind when the collodion dries. I have found that oil-paintings executed upon collodion-ized canvas cannot be rolled up without damage. It would probably be found that the formation of a viscose film (see the account previously given of paper-sizing) would prove a good substitute for ordinary size in the first treatment of canvas.

¶ 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.

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


Last Update: 2011-01-23