Wood-ashes, very finely sifted, are spread on the face of the picture, and delicately, or carefully and lightly, rubbed with a soft sponge. This must be carefully washed away as soon as the surface is cleaned.
Other detergents failing, borax dissolved in water may be employed. This works slowly but surely; but, as M. Goupil remarks, this lessive, like wood ashes, must not be left long on the colours, but be promptly wiped away with a sponge. Lime-water will serve as well as the solution of borax.
Soaps of different qualities are also used for cleaning, according to the state of the picture. It may be here again remarked that no exact rule can be given regarding an art specially founded on skill and experience. The beginner should first try his hand on a few common old pictures.
Soap made into a foam or lather with water will generally clean a surface, however dark it may be from smoke. Let the foam settle completely, and then wipe it clean with a damp sponge.
Essential oils, especially turpentine, or those of spikenard, lavender, and rosemary—of either two parts of spirits of wine to one of turpentine, &c.—are commonly used to clean pictures.
Pictures not varnished require great care and skill in cleaning. For these yeast with water, or flour mixed with lime-water, is employed; also spirits of wine or vinegar. Ammonia is also used. Goupil mentions that one of the most dangerous mediums for this purpose is the old one of urine, and that it should never be used.
When the canvas of a picture is very old and rotten, it may be replaced by a process requiring the utmost nicety. If only certain portions are injured, it will suffice to glue pieces of fine canvas on the back.
To completely transfer the painting, gum over its surface two coats of soft paper. Lay it on the face, and carefully remove the old canvas ground. This is effected by wetting every thread till soft, and then picking it away. A piece of pumice-stone and tweezers are also used. When all fibres are removed, carefully glue a canvas and apply it, pressing it well on the back of the paint. Before it is quite dry, press the picture with a warm flat-iron, not too hot. Then remove the paper carefully with a damp sponge and by tearing.
To transfer a picture on wood, the back is sawn into many small triangles or squares, which are carefully chiselled away one by one. Then with files and scrapers approach the paint till only a thin film of wood remains. The last remnant is wetted with a sponge, and picked or scraped away. First, use paper on the face and restore as before.
There is a great enemy to pictures in mould or mildew, which has quasi-equivalents in must, dry-rot, mucor, or robigo. It is divided by Goupil into apparent softening and actual softening or mildew. The former is mildew or mere superficial mould; i.e., a light vegetation which gathers on the surface from germs in the air. It can easily be wiped away, and is caused by dampness. Sometimes, when long rooted, it destroys the varnish, which must be replaced. There is also a mould which is properly decay, or a radical destruction of fabric, for which there is, in fact, no cure, save in renewing the canvas and retouching the picture.