How Varnishing Artwork Affects Its Appearance

Varnishing artwork can correct issues and enhance its appearance. But what varnish should you use? An ideal varnish should be nonpolar and stable and should not oxidize or yellow. Unfortunately, the opposite is true: commonly used varnish materials, natural resins such as dammar, are unstable, oxidize very quickly, and yellow considerably. Thus, they need to be replaced regularly.

The market for synthetic polymers in the past century produced a broad range of resins that seemed very promising for use as varnishes in the field of conservation: the polymers are very stable, not prone to yellowing, and their composition is much more defined and less variable as in natural resins.

Initial experience with high molecular weight (HMW) polymers (e.g., polyvinyl acetate, polyester, and acrylic polymers) was disappointing. The varnishes were found to be less stable than expected. Crosslinking resulted in aged varnishes that were difficult to dissolve or were insoluble in common solvents. Such varnishes are often very difficult to remove without severely damaging the painting.

The sheen of these polymeric varnishes was often found to be inferior to those of natural resins. They do not produce the same gloss and color saturation as traditional, natural resin varnishes.

Later in the century, new materials, low molecular weight (LMW) resins, were developed, showing great promise. Many of the problems associated with high molecular weight polymers can be avoided by using low molecular weight synthetic resins, such as ketone resins (MS2A), aldehyde resins (Laropal A81), or hydrocarbon resins (Regalrez 1094). Due to their lower molecular weights, these varnishes usually form smooth, glossy surfaces and remain soluble after aging. They have many good characteristics, and some are used in commercial artist’s varnishes, such as Regalrez 1094 in Gamblin Artist’s Oil Colors Gamvar.

Although synthetic resin varnishes do not yellow considerably, the optical properties after aging are sometimes inferior to that of natural resins. Blanching seems to be a characteristic aging phenomenon of synthetic resin varnishes.