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.
Varnishes are the most vulnerable components of paintings, as the large surface-to-volume ratio of these thin layers of organic material maximizes their exposure to the deteriorating effects of the environment. Traditional picture varnishes, which are based on natural materials, deteriorate over time, causing yellowing, loss of transparency, and, eventually, cracking. Because these degraded varnishes obscure the images beneath them, they are removed periodically in conservation treatments and replaced by new ones. Artists and conservators alike have long sought more stable varnish materials. Many of the modern varnish materials that have been tried out, however, proved to have problems of their own.
Careful consideration of the drying process of these solvent-based coatings has led to the insight that natural resin varnishes, which are of low molecular weight and hence produce solutions of low viscosity, level to a greater extent over the microscopically rough paint surfaces than polymeric varnishes. Because of this, natural resin varnishes produce higher gloss and more saturated colors. The appearance obtained with traditional varnishes can be mimicked, therefore, using synthetic resins of low molecular weight. Such novel resins were studied extensively in varnish research conducted at the National Gallery of Art and are now being used by conservators and artists. Detailed results of this research may be found in a series of publications (PDF 308k).