It has not been thought necessary to deal with the summary, but only too common, method of repair which consists in gluing a patch at the back of a tear in the canvas. Only the very smallest tears can properly be mended by means of patches. If the patch is of any size it gives the original canvas insufficient support, and after a certain time the surface of the picture will be spoilt by undulations. Re-lining is the least that is necessary if the two edges of a tear are to be absolutely prevented from showing. If re-lining is not done properly, the remedy is worse than the disease. Supposing one canvas were simply to be glued hastily on to the other, there would soon be a number of air-bubbles between the two, while in other places there would be spots where the glue was too thick, and these would ferment and cause mildew. In addition, the edges of the tear would gradually detach themselves from the new canvas and would rise up. Hundreds of celebrated pictures have been re-lined, Velazquez's "Venus" (National Gal lery), Rembrandt's "La Religieuse" and so on.
In order to prevent any portion of the original painting from falling or being displaced, two thicknesses of tarlatan and two of paper are first lightly glued to the painted side of the picture. The painting itself being thus protected, the picture is turned over, and the process of removing the supporting structure is begun. If the support in question is a canvas, it can as a general rule be removed fairly easily either by damping it, or by subjecting it to heat so as to dissolve the original glue between the canvas and the isolating coating on which the actual painting was done. If there is no glue it is more difficult to detach the canvas, and it has to be gradually scraped away until the isolating coat is reached. This is the case where the support is a wooden panel ; the only possible means is generally to plane it away. When the reverse of the picture has been laid bare in this way, it only remains to re-line it. This can be done by several methods, more or less simple or complicated as preferred by the operator. Some cover
the reverse of the painting with a coating of white lead in order to strengthen it, and interpose layers of tarlatan between this and the new canvas; others glue the new canvas as directly as possible to the painting by means of caseine. Here again it must be remem bered that the process involves a number of detailed operations which it is not necessary to mention here, and that the utmost care and great skill are required.
Difficult as the operation is, it may unhesitatingly be recom mended in preference to re-lining. However skilfully re-lining may be carried out, it necessarily involves fixing closely together two tissues of different composition and age, which are not equally sensitive to changes of temperature. It follows that after a certain lapse of time the two canvases undergo dissimilar changes and become loosened; the edges of the old tear in the picture cease to adhere to the new canvas, because the place of the tear is a weak point since at that point there is only one thickness of canvas; it thus forms a sort of hinge which comes into play whenever the canvas is expanded or contracted by heat or damp.
After a certain time the mastic which has been used to fill the gap made by the tear is bound to scale off because it is inelastic ; the foundation of the restoration is thus destroyed. The impor tance of this danger has no doubt been exaggerated in the above account in order to make clear the possible defects of re-lining.
No such danger however exists if the process of transference is applied, since there is only a single tissue to be affected by varia tions of temperature ; in addition, the volume of the mastic used to fill the gap is negligible, since its thickness is only that of the layer of paint and not that of the canvas.
The public is still very much impressed by the difficulty of the process of transference, and since the operation is a long one and cannot be carried out except by a really expert worker, picture restorers often hesitate to undertake it. Thanks to transference, we still have masterpieces like Raphael's "La Petite Sainte Famille" and "Saint George." They were treated by Picault in 1751 and this is probably the date of the invention of the process. De-varnishing.—It is logical to deal first with the varnish, since the restorer has to deal with it before actually restoring the painting, and because, owing to the action of various factors, it is rare for a picture to retain the original quality of its varnish. There are cases in which the original varnish can and should be preserved; but as a general rule, if the restoration which a picture requires is at all extensive, it is necessary first to remove the varnish. Of all the problems which arise in connection with pic ture restoring, that of de-varnishing is by far the most important. Defective re-lining can be remedied, and repainting which has been badly done by an ignorant operator can be removed without damage to the original picture, but bad de-varnishing may do irreparable harm.