Varnish
From Principles of Applied Arts
Varnish is a finish applied to wood or other surfaces in order to provide a clear, hard, durable, protective finish. Varnish is usually a combination of a drying oil, a resin, and a thinner or solvent. Varnish finishes are often glossy, although not always. As opposed to paint, which contains pigment and is opaque, varnish has little or no color and is transparent.
There are many types of both natural and synthetic drying oils used in varnishes, and they including linseed oil, tung oil, and walnut oil.
The resins hystorically used in varnishes have included amber, copal, and rosin. But today, alkyd and polyurethane resins are most commonly used.
Traditionally, turpentine was used as the thinner or solvent, but it has been replaced by white spirits or paint thinner.
Violin varnish is most often prepared by combining walnut oil and linseed oil with amber, copal, rosin or other resins. The oil is prepared by cooking or exposing it to air and sunlight. The resin is "run" by cooking it and reducing its weight and concentrating its color. The thickened oil and prepared resin are then cooked together and thinned with turpentine into a brushable solution.
After being applied, varnishes harden either by evaporation of the solvent, or by a chemical reaction. Oil varnish drying time depends on the ratios of oil to resin and turpentine, and may be sped up by exposure to UV light.
Most natural varnishes consist of a gum or resin dissolved in a solvent. Depending on the solvent used, the three main types of natural varnishes are: spirit varnish (which uses alcohol as a solvent), turpentine varnish, and oil varnish. Spar varnish (also called marine varnish) is high quality waterproof and sunlight resistant varnish named for its use on the masts of ships (called spars).
The word lacquer refers to quick-drying, solvent-based varnishes or paints. Lacquer is also a very durable, slow-hardening, varnish obtained from the sap of a varnish tree.
Shellac uses alcohol as a solvent. Although not a particularly durable varnish, shellac is sometimes used as a primer or sealer for paint or varnish, or as a finish by itself.
Synthetic varnishes include urethane or polyurethane varnishes, varnishes that use water as a solvent, and epoxy varnishes.
