This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
Oil paintings should under no circumstances be varnished over before the colors are surely and unmistakably dry, otherwise the Assuring and early decay of the surface may be anticipated. The contention of some people that oil paintings need the protection of a coat of varnish is based upon the claim that the picture, unvarnished, looks dead and lusterless in parts and glossy in still others, the value and real beauty of the color being thus unequally manifested. It is not to be inferred, however, that a heavy coating of varnish is required. When it is deemed advisable to varnish over an oil painting the varnish should be mastic, with perhaps 3 or 4 drops of refined linseed oil added to insure against cracking. A heavy body of varnish used over paintings must be strictly prohibited, inasmuch as the varnish, as it grows in age, naturally darkens in color, and in so doing carries with it a decided clouding and discoloration of the delicate pigments. A thinly applied coat of mastic varnish affords the required protection from all sorts and conditions of atmospheric impurities, besides fulfilling its mission in other directions.
Oil paintings, aquarelles, etc., may be also coated with a thin layer of Canada balsam, and placed smoothly on a pane of glass likewise coated with Canada balsam, so that both layers of balsam come together. Then the pictures are pressed down from the back, to remove all air bubbles.
When old oil paintings have become dark and cracked, proceed as follows: Pour alcohol in a dish and put the picture over it, face downward. The fumes of the alcohol dissolve the paint of the picture, the fissures close up again, and
the color assumes a freshness which is surprising. Great caution is absolutely necessary, and one must look at the painting very often, otherwise it may happen that the colors will run together or even run off in drops.
See Cleaning Preparations and Methods.