Art Encyclopedia - Varnish
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Varnish is an oil-soluble or spirit-soluble resin that will harden into a transparent and colourless film. Varnish can be combined with other painting media, or it can be used to give protective, transparent and glossy coats to a finished work.
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 in hot oil. 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 and egg white.