Solvent dammar

A varnish is often appHed on top of the paint layers. A varnish serves two purposes as a protective coating and also for an optical effect that enriches the colors of the painting. A traditional varnish consists of a natural plant resin dissolved or fused in a Hquid for appHcation to the surface (see Resins, natural). There are two types of varnish resins hard ones, the most important of which is copal, and soft ones, notably dammar and mastic. The hard resins are fossil, and to convert these to a fluid state, they are fused in oil at high temperature. The soft resins dissolve in organic solvents, eg, turpentine. The natural resin varnishes discolor over time and also become less soluble, making removal in case of failure more difficult (see Paint and FINNISH removers). Thus the use of more stable synthetic resins, such as certain methacrylates and cycHc ketone resins, has become quite common, especially in conservation practice. 

It is a colorless to straw-yellow, oily liquid with a peppermint-like odor. It will darken and form a solid residue an exposure arxl aging. It is miscible with most organic liquids and it is a good solvent for such substances os nitrocellulose, ethylcellulose, law-viscosity cellulose acetate, polyvinyl chloride, vinyl resins, hydrocarbons, raw linseed oil, kauri gum, rosin, ester gum and synthetic rubber. It will only partly dissolve shellac and dewaxed dammar. 

Glycol diocetote is a colorless liquid having a foint odor resembling that of ethyl acetate. It will dissolve a wide range of cellulose esters, camphor, dammar, ester gum, elemi, mastic, rosin and sandarac. When it is mixed with active solvents its range of solubility is increased far a wide variety of cellulose esters and ethers and for natural and synthetic resins. 

When a solution of daimnar dries in high relative humidities, a bloom made up of small crystals of ammonium sulphate can occur on the surface. Dammar films can develop wrinkles during drying. The wrinkling appears to be the result of an initial oxidation of the resin combined with movement of the underlying solution. Mastic, which develops fewer wrinkles than dammar (Thomson, 1957), retains less solvent than dammar (Feller, 1957b, 1958). 

On ageing, dammar polymerizes and some parts even become cross-linked, yellow and insoluble (Boon and van der Doelen, 1999). Dissolving coatings with solvents will therefore leave coloured insoluble material on the surface. For this reason, a mechanical method of removal should also be considered when the application and reversal of a dammar coating are being proposed. 

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