Old paintings and sculpture can become covered with dirt, grease and accumulations of smoke or mold. The paint, stone, marble or wood can be chipped, cracked or broken. Past repairs may have been sloppy or may even have damaged the work. The canvas may be saggy, in poor condition or it may be torn. Patching may have been done with incompatible materials.
Solvents are used to dissolve the dirt and grime from the painting or sculpture. Old layers of original varnish and paint not original to a painting must be stripped away layer by layer. Weathered sculpture must be cleaned of dirt and the effects of pollution.
Paint that closely or exactly matches the original paint must be applied to spots where the paint is missing. Sculpture is sometimes patched with like materials, but often that is not attempted. Torn canvases are patched by filling them in with various materials, sanding and repainting them. Wrinkled canvases are straightened. Saggy canvases are re-stretched. A backing is placed on the painting to protect it, and the image is revarnished.
Art restoration can be controversial. Art historians and owners wonder if restoration changes the work into something that is not in keeping with the intentions of the artist. Others see a greater benefit in repair and conservation and are willing to take the risks.
Infrared reflectography and ultraviolet (UV) imaging are used in art restoration today. They help restorers to see beneath the layers of paint and varnish to show the original structure of the painting and reveal subsequent changes to it.