Early Varnish Materials
The earliest varnishes were oil varnishes, consisting of a drying oil and a natural resin. From the 16th century onward, oil varnishes were gradually replaced by so-called spirit varnishes. The latter consist of a solution of a natural resin, such as gum mastic, in a volatile solvent, traditionally oil of turpentine.
Cleaning of Paintings
The removal of varnishes (referred to as "cleaning" in the art conservation field) is accomplished using organic solvents or other cleaning agents. Aged picture varnishes require relatively polar solvent mixtures to be removed. Although the paint layers are often much less soluble than the varnish layers, solvent action may cause swelling and leaching. Some paint layers cannot be treated safely with solvents.
Modern Varnish Materials
Dammar resin was introduced in the 19th century and is generally considered to be more stable than other natural resins. Nevertheless, it deteriorates rapidly and demonstrates many of the same defects as, for example, gum mastic. In the 20th century, with the advent of the chemical industry, synthetic polymers were introduced. Polymeric coatings are generally much more stable than natural resins, and several have been applied to paintings as varnishes almost as soon as they became available. Some of these new coatings proved to have problems of their own. For example, cross-linking reactions may cause insolubility to occur in some acrylic coatings upon aging, possibly jeopardizing future removal; poly(vinyl acetate) coatings, which were applied to paintings as early as the 1930s, have relatively low glass transition temperatures, which may result in dust and dirt becoming embedded into the coating. The main problem associated with modern synthetic coatings, however, is that they create a different appearance in general than that produced by traditional varnishes.
Varnish Research at the National Gallery of Art
In research at the National Gallery of Art several avenues were pursued: accelerated aging studies were carried out to investigate the possibility of inhibiting oxidative degradation of natural resin and other varnishes using stabilizing additives, and a search for new, stable, low-molecular-weight resins was conducted. It was found that dammar resin can be stabilized effectively using a hindered amine light stabilizer, but only in an environment free of ultraviolet radiation below 400 nm. In addition, novel low-molecular-weight resins, such as hydrogenated hydrocarbon resins and urea-aldehyde resins, were tested and found to be suitable replacements for traditional varnish materials.