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.