Restore Old Paintings!
Cleaning includes surface dirt removal, varnish removal, and overpaint removal. Cleaning is an attempt to restore the painting to its intended appearance. Solvent mixtures and sometimes mechanical means are used to clean paintings.
Surface dirt is anything that accumulates on the surface of the painting such as: air pollution, cigarette smoke, grease particles, pollen, insect deposits, mold growth, dust etc... Over time these accumulated deposits darken and interfere with the viewing of the painting. Surface dirt that has accumulated directly on the paint layers of an unvarnished painting can be difficult to impossible to remove. It is important to try and protect paintings from these accumulations by hanging and storing paintings in a climate controlled environment. A proper varnish will further protect the painting from surface dirt. Not all paintings were meant to be varnished however. As a consequence, it is even more critical to maintain clean environmental conditions.
Varnish is a coating intentionally applied to protect the painting and to saturate the colors for viewing. Over time varnish can yellow, become brittle, become dull, flake etc.. These conditions arguably disturb the viewing of the painting. Removal or reduction of the old varnish and reapplication of a fresh varnish can greatly improve appearance.
Overpaint is paint that has been applied to cover damage in a previous restoration campaign. It usually is oil paint and it usually has been applied over damaged and undamaged areas. Oil paint is not a good paint to use for restoration purposes because it is not easily reversible. Removal can present all kinds of problems.