Varnish

For this reason a picture must either be protected by a transparent material, such as glass, or be sealed off from the atmosphere by a coating of transparent varnish, so that when its surface becomes soiled, it can be cleaned with mild solvents without endangering the upper paint films.

Other reasons exist for the application of varnish. A coating of varnish will protect the painting against the wear of handling and any light scratches which might otherwise mar its surface. Varnish also creates that uniform degree of either matteness or gloss across the entire picture area, which makes it possible to see a picture well from many angles. Finally, the varnish restores the original depth and intensity of color that may have become dull and gray as the paint dried.

When a varnish ages, it may become darker, lose its transparency, or become brittle and crack. In such cases it must be removed and replaced by a fresh coat of protective varnish. Usually the restorer employs a relatively mild solvent on a wad of soft cotton to remove the old varnish, while leaving intact the final glazes and layers of paint. Thus one of the requirements of a picture varnish is that it should be easily removable by means of solvents which are not so strong that they quickly attack films of dried oil paint. Of course, the varnish should remain transparent as long as possible and should retain enough flexibility to follow any movements of the paint films, supports, and grounds used in the picture.

The most common materials found in protective picture varnishes are simple solutions of dammar resin, mastic resin, or methacrylate resin in turpentine or petroleum solvents. These all satisfy, to roughly the same degree, the requirements listed before: (1) they remain transparent for a long time; (2) they may be easily removed; (3) they can be applied easily in a thin film; (4) they dry rapidly, lessening the possibility that dust may be caught and embedded during the drying period; (5) they do not use thinners which easily dissolve dried films of oil paint.

Varnishes made with hard resins [such as copal] cooked in hot oils are not suitable for picture varnish, because when they darken, it is very difficult to remove them from the picture. Picture varnishes may be bought ready to use at any reliable art supply store or may be made in good quality easily and inexpensively by the artist as described in the next section. [pp. 83-84]