The great danger in the cleaning of paintings lies in the fact that if a painter has used varnish as a medium mixed with his tube colors, nearly all solvents will attack such a painting, and the greatest trouble will result, because the paint itself will come off the surface. So, after having established—let us say, for instance—that the painting is a Linseed Oil painting, the following methods are perfectly safe:
Mixtures of equal parts of No. 1 Denatured Alcohol, Turpentine and Water, thoroughly shaken up, may now be applied with a very stiff bristle brush. This will remove the varnish without disturbing the film underneath.
Another good method to use is to take a soft tooth brush, dip it in Turpentine and gently rub the surface; and, if the varnish is of the single solvent type, like Mastic or Damar, it will come off perfectly clean without dissolving the Linoxin (dry Linseed Oil film).
The latest method is the use of Isopropyl alcohol, known commercially as Petrohol, which will dissolve most of the varnishes without dissolving the dry Linseed Oil film. Isopropyl alcohol can be diluted with Turpentine, Benzine or Kerosene, and when so diluted, can be copiously used.
The other method to be recommended is the use of a material called Cumene, or Cumol, which has the great advantage of dissolving varnishes without dissolving the Linoxin, provided, of course, it is used with a little common sense. It must be understood that the surface of the painting is a very delicate film, and in the restoration of a painting all that is necessary is to dissolve the superficial layer of old yellowed varnish and adherent dirt, and nothing else.
The author has made a series of modifications of Cumene, such as one third Cumene, one third Kerosene and one third Denatured Alcohol, which has the advantage of slowing down the action to such an extent that in case there is the slightest indication of decomposition of the painting itself, the solvent can be wiped off with a dry cloth or a cloth soaked in Kerosene, and the dissolving action will be stopped immediately.
The foregoing description is just the introduction to the various methods employed, and is not intended for novices or those unskilled in the art. At all events, great care must be exercised.
The author superintended the cleansing and restoration of fifteen paintings, some of them very large in size, which had not been cleaned or restored in many years, and some of these were in shockingly bad condition.
The Isopropyl Alcohol and Turpentine methods were employed without the slightest defect, and the varnish in every case came off perfectly clean, and when the pictures were revarnished, they were restored to their pristine condition. If it is desirable to give the picture a patine of age after it has been renovated, it is not a very difficult thing to do, if a slight tinge of a permanent Brown or Yellow Lake is added to the varnish; but, under no circumstances, must any bituminous or asphaltic compound be used for this purpose.