Oil Paintings, (To Clean). The art of cleaning oil paintings has been very much neglected, and several valuable pictures have been destroyed in consequence of the persons operating upon them employing the same means for removing all kinds of dirt, as they do for dust and varnish commingled, so that frequently a valuable painting has actually been scoured away.
Most paintings are varnished, and as the nature of the varnish differs, so also must the means by which they are removed. In some cases it is better to allow the varnish to remain untouched, than to interfere with it, as the painting might be damaged in the Litter instance.
Soluble Varnishes, such as sugar, glue, honey, gum arabic, isinglass, white of egg, and dirt generally, may be removed by employing hot water. To know when the painting is varnished or coated with such materials, moisten some part with water, which will become clammy to the touch. To clean the picture, lay it horizontally upon a table or some convenient place, and go over the whole surface with a sponge dipped in boiling water, which should be used freely until the coating begins to soften; then the heat must be lowered gradually as the varnish is removed. If, however, the coating is not easily removed, gentle friction with stale bread crumbs, a damp linen cloth, or the end of the fore-linger, will generally effect it, or assist in doing so. White of an egg may be removed (if not coagulated by heat), by using an excess of albumen (white of egg), and cold water; but if coagulated, by employing a weak solution of a caustic alkali as potash.
Coated dirt is removed by washing with warm water, then covering with spirit of wine, renewed for ten minutes, and washing off with water, but without rubbing. The process is to be repeated until the whole is removed.
Spots should be washed with warm water dried with soft linen rags, and covered with olive oil warmed; after the oil has remained on the spots for twenty minutes, gentle friction with the finger should be used, the foul oil wiped off and fresh laid on, until the spot disappears. Should this fail, spirits of wine, essence of lemons or oil of turpentine may be carefully applied, observing that only such parts as are dirty must be covered with them; they are to be cleaned off first with water and then with olive oil. Sometimes even these means fail, and then strong soup-suds, applied directly to the spots, and retained there until they soften or disappear, will prove effectual. The spots must then be washed with water.
In employing these means, as indeed throughout the whole process of cleaning, the greatest care should be taken in removing any coating upon the surface of a painting, and it is therefore better to employ mild measures first, then if they fail, to use stronger ; or in the event of these not succeeding, to very carefully apply the strongest. For our own part, we prefer leaving the painting in a half-cleaned state, as it sometimes happens that, with the most scrupulous care, under experienced persons, some of the fine touches or delicate tints of a painting are damaged by the process. When we state this, we mean only such pictures as are covered with insoluble varnishes - varnishes of gum-resins, or old oil varnishes, which cannot well be removed without injury to the painting.
Varnishes of long standing are very difficult to remove, as they generally consist of linseed oil combined with gum-resins, and if not easily taken off by the means given below, it is better to leave them as they were.
To remove these varnishes, use spirits of wine in the manner recommended for coated dirt; or, oil of turpentine, which requires greater care than the spirits of wine; or, warm olive oil: but if the varnish is very hard, the painting should be washed by means of a sponge, with a warm solution of pearl-ash (an ounce to a pint of water), until the coating is removed, when the surface must be washed well with fresh water frequently.