Introduction

Research on conservation materials at the National Gallery of Art has focused on coating materials, such as paints for retouching, coatings for the protection of outdoor bronzes, and varnishes for paintings. In this research, modern synthetic materials are tested for their use in conservation applications, often as replacements for traditional materials, which may be based on unstable natural products. Extensive use is being made of the expertise and materials available from the chemical industry. The materials are tested using accelerated aging methods. Stabilizing additives may be used to prolong the useful life of the materials. An array of analytical methods is being used to monitor changes during aging, and finally conservators judge the products for handling and appearance.

Artists have commonly applied a final transparent coating to the surface of oil and tempera paintings. These varnishes have protective and aesthetic functions, although their aesthetic function is probably the more important one. Varnishes have the ability of changing the appearance of paintings dramatically, affecting such properties as color saturation and gloss. A varnish is, therefore, an essential part of a painting if it originally had such a layer. With the change in artists' materials and techniques arising in the late 19th century came a decline in the use of picture varnishes, although they are still applied by many artists today. Throughout the centuries, many different materials have been used to prepare varnishes.