“All the pictures of the early masters of the Italian school, and those of Claude and William Vandervelde, which are painted on chalk and absorbent grounds, are in the greatest danger if washed with water. It penetrates through the small crevices which may exist in the paint, and often totally destroys the picture. If the painting be upon canvas, like those of the two latter-named masters, it breaks into a thousand small lines or cracks; and if upon panel, like the pictures of Raffaelle, Andrea del Sarto, or Fra Bartolomeo, it breaks up the paint by scaling it off in small points of the size of a pin’s head. If the picture, again, is of the Spanish school, and is painted upon the red absorbent grounds and upon a rough canvas, water not only breaks the unity of its surface, but from the canvas being of a coarser texture than the pictures of Claude or William Vandervelde, it often penetrates in a greater proportion, and frequently scales off pieces as large as a sixpence, especially in the dark shadows, or where the ground has not been sufficiently protected by a thick impasto (heavy coat or ground) of colour. At all times and to all pictures water is more or less dangerous, unless used with the greatest caution, and then it should only be applied by means of a piece of thick buckskin leather well wrung out, and left just wet enough to slip lightly over the surface of the picture. In the case of some masters, as with those we have specialised above, the free use of water may be regarded as next door to absolute destruction; and the warmer and drier the weather the more active and ruinous the operation. Instances have occurred in which an Andrea del Sarto, a Claude and a William Vandervelde, were destroyed in a few minutes by the injudicious use of simple water.”
I have given this quotation in full, because water is generally the first thing freely resorted to clean pictures by the ignorant. Thus I have heard of very valuable pictures being actually given to common servants or the washerwoman to scour clean, which was effected with soap and hot water and sand, to the speedy ruin of the work. Nor is it any great wonder that this should be done, when we find in Goupil’s work that, while he admits that cold water “infiltrates itself partially to the fissures of a painting and does great harm,” he declares that “hot water acts differently,” giving the impression that it may be very freely used, and declaring that “clean cold water harmlessly dissolves grease and dirt resulting from dust deposited by the air.” This is true, but he does not seem, like Mr. Mogford, to have fully understood the other side of the question. (Manuel Général et Complet de la Peinture à l’Huile, par F. Goupil.)
For first cleaning away impurities from a surface Mogford recommends ox-gall to be applied with a soft brush. This may be obtained in shilling or six-penny bottles from Winsor & Newton, or any other dealers in artists’ materials. “It is,” he adds, “an excellent detergent, which may be freely applied without fear. It must, however, be well washed” (i.e., wiped) “off with pure water, or it will leave a clamminess on the surface that may prevent the varnish, afterwards applied, from drying.” But a distinction must be carefully borne in mind between washing with water and letting it soak into a picture and simply wiping off the surface with a damp chamois or buckskin or soft old linen handkerchief. In fact, this latter is the first thing to be done before slightly cleaning the surface with the diluted ox-gall. It is very necessary that the skilled cleaner shall understand exactly the nature of varnishes, so as to know on what he is to work. Thus, according to the picture, he may employ “liquor potassæ, oil of tartar, spirits of wine, pure alcohol, liquor ammoniæ fortis, naphtha, ether, soda, and oil of spike or lavender. The very nomenclature of these powerful agents will at once show the great risk of their being injudiciously or carelessly employed.”
Great care should be taken not to allow an excessive or unequal quantity of cleaning fluid to gather in one place. Therefore all pictures should be laid flat while being restored, as streams, for instance of ammonia, would cut very irregularly into a surface. With pictures of any value, the process of cleaning is always very delicate, requiring much practice and very perfect knowledge of all the principles of the art.
Where the varnishes are tender and thin, such as mastic, Mogford advises the use of spirits of wine; but to be sure that no harm can be done by it, it is desirable that “the spirit, which is usually sold at 58° of strength, should be diluted by a fourth part of water, or by the same proportion of rectified spirits of turpentine, or it may be used with an addition of a sixth part of linseed oil, added to the diluted or pure spirit.” In every instance the mixture is to be “well shaken before taken,” or applied. Care should be taken to prevent oil from softening the paint, which it is apt to do. As a rule it is best to begin with the lightest or brightest portions of a picture—as, for instance, the face of a portrait—as these parts are always the hardest. Beginning by wiping the surface with white cotton wool and turpentine, observe if any varnish comes off on it, and as soon as it is seen change the part of the rubber used, else you will go on simply taking up “dirt” from one place and rubbing it into another. This is elsewhere explained as regards cleaning cloth or absorbing ink, that we must continually subtract from and not add again to the ground.
“Turpentine is a counteracting medium, which instantly arrests the action of the solvent spirit.” When all the varnish has thus been removed, the whole may be wiped over with spirits of turpentine, and then when dry revarnished, if nothing more be required.
Rubbing with the fingers, or powders, or any kind of dry cleaning must be avoided, or else practised with great care, since it produces an effect known as woolliness, which will begin to show very decidedly after some time. But when a picture has had no varnish it can only be cleaned mechanically, as by using tripoli, pumice-stone, or whiting. This method requires great skill. Sometimes a very fine-edged scraper or knife is used to thin the varnish before using turpentine.
“Solvents,” adds Mogford, “are only necessary to remove varnish.” Unvarnished pictures are best cleaned by carefully wiping with buff or chamois leather, damp, not wet, aided by a little powdered whiting.
Varnish, when not on a picture, may, however, be removed by rubbing it with the fingers, or palm, or leather, aided by powdered resin, or rosin. For certain purposes, as to make a panel of a piano thoroughly seasoned for heat, and, as it were, enamel it, a coat of varnish is applied, and when dry is rubbed down smooth with pumice-powder or resin, and this process is repeated many times.