Time and experience have shown us that a varnish layer protects oil paintings from environmental dirt and dust.
Varnish increases the sense of depth in oil paintings.
In antiquity, varnish was used to protect water-soluble media and fugitive colors, and to intensify colors. Natural resins are secretions from plants, mainly trees. Nut oils and poppy oil were commonly available throughout the Mediterranean region. Oil varnishes are common to boat building civilizations. These varnishes were prepared by dissolving resins in hot oil.