varnish

varnish.
Solution of natural or synthetic resin dissolved in liquid, used as a protective coating on the surface of a painting or sometimes as a paint medium. Ideally varnish should be colourless and transparent, but most have darkened with age. Natural resin varnishes are usually either oil based or spirit based. Oil varnishes, which have been used since the Middle Ages, are made by dissolving resins (such as mastic or sandarac) in hot oil (usually linseed). These were the main types of picture varnishes until well into the 20th century, when they were largely replaced by synthetic equivalents. Spirit-based varnishes are made by dissolving resin in a rapidly evaporating liquid such as turpentine. Other organic varnishes have used gum arabic (notably for manuscript illuminations) and egg white (particularly for temporary varnishes, removed a year or so later and replaced with a permanent varnish).