Varnishes are solutions of natural or synthetic resins in organic solvents that dry when thinly spread onto a surface. Depending on the solution composition, the varnishes exhibit varying qualities of gloss, protective ability, flexibility, and durability.
A good varnish is critical to the restoration and preservation of oil paintings and there are numerous formulas that can be used for these purposes. The varnish layer plays a dual role: it affects the final look of the painting and also functions as a protective coating for the paint surface. Varnishes intensify the appearance of pigments on the painting surface via light refraction; this is referred to as "saturation."
While varnishes are traditionally clear, they can be toned or altered by adding pigments and other materials. Toned varnishes may be used to balance a painting whose appearance has transformed due to age.