E. René de la Rie
National Gallery of Art
Washington, DC

Abstract-Résumé

NEW VARNISHES FOR OLD MASTERS
Until the mid-to-late nineteenth century it was common practice to apply final transparent coatings to the surface of oil and tempera paintings.  These picture varnishes can have a dramatic influence on the appearance of a painting and can affect color saturation and gloss.  A varnish is, therefore, an essential part of a painting if it is intended to be varnished.  Since the advent of Impressionism, many unvarnished paintings have been produced and because of the potential change in appearance, such paintings should remain unvarnished.  Varnishes are however still used by many artists today.

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 the triterpenoid resins mastic and dammar, in a solvent, which traditionally was oil of turpentine.  Varnishes are the most vulnerable parts of paintings as they are on the surface and 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 deteriorate rapidly, causing defects such as cracking, loss of transparency and yellowing.  Because these degraded varnishes obscure the images beneath them they are removed regularly during restoration procedures and replaced by new ones.  The removal (referred to as "cleaning" in the field) is accomplished using organic solvents or other cleaning agents, which may involve risk for the paint surface.  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 any solvent.

Modern solvent-based polymeric coatings are generally much more stable than natural resin varnishes and have been applied to paintings ever since these materials became available.  Some of the new coatings proved to possess problems of their own.  For example, cross-linking reactions may cause insolubility in some poly(alkyl methacrylates), possibly jeopardizing future removal; poly(vinyl acetate) coatings have relatively low glass transition temperatures, which may result in embedded dust and dirt.  The main problem associated with modern synthetic coatings, however, is that they produce an appearance different from that achieved using traditional varnishes.  After a period of experimentation with these coatings, many painting conservators went back therefore to using natural resin varnishes.

Previous research efforts have focused for the most part on the degradative and mechanical properties of varnishes, perhaps due to the misconception that the function of a varnish is primarily a protective one.  Although the varnish layer may protect the underlying paint surface to some extent, its primary function is aesthetic.  Much confusion has existed in the field concerning optical differences between varnishes.  Painting conservators may describe synthetic clear coatings as varnishes that do not "wet" or "saturate" the paint surface.  In general, a feeling that these coatings do not penetrate the porous and absorbent paint surfaces, which may be several centuries old, appears to accompany this opinion.  No evidence, however, can be found of lack of contact between coatings and the paint surface.  Occasionally it is heard that the observed differences occur because natural resin varnishes are yellow and synthetic coatings are not.  Since it can be demonstrated, however, that thin films of natural resins, when unaged, have a flat transmission spectrum throughout the visible part of the electromagnetic spectrum, the difference in appearance must be due to other factors.
 
Careful consideration of the drying process of solvent-based varnishes led to the insight that natural resin varnishes, which are of low molecular weight and produce solutions of low viscosity, level to a greater extent over microscopically rough paint surfaces than polymeric varnishes, which produce solutions of high viscosity and which can reproduce the underlying roughness.  Varnishes that form smoother films have higher gloss, and produce less surface scattering and more saturated colors.

At the National Gallery of Art, several avenues to a solution of the problem 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 varnish could be stabilized effectively using a hindered amine light stabilizer, but only in an environment free of ultraviolet radiation below 400 nm.  It was also found that the appearance produced by a natural resin varnish can be mimicked using synthetic resins of low molecular weight.  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.

As a result of this research, many conservators now use stabilized dammar, or a stabilized synthetic low molecular weight resin to varnish their pictures.