Art objects are delicate by nature. The aging process alone causes inherent physical and chemical changes to occur; however, others factors such as adverse environmental conditions (too much heat or light; extreme humidity levels); biological infestation (insect or mold spores); and pollution (dirt, soot, smog, and cigarette smoke) can accelerate these changes. Although paintings created in oils are susceptible to these influences, in many cases they can be cleaned or restored to their original condition, either by the owner or, preferably, by a trained and skilled art conservator.

Most oil paintings are works in four layers. There is a canvas held taut on a wooden framework, a ground (a layer of chalk or pigment in glue or drying oil) that stabilizes the paint and provides a smooth, nonabsorbent surface, and the paint layers—pigments in a binding (usually linseed oil) emulsion.

Before the mid-twentieth century, most artists applied a varnish finish—the fourth layer—to the completed oil painting that both protected the thick applications of paint beneath (called the impasto), and brought a glossy clarification to the colors. Unfortunately, many of these paintings hung in rooms that were lit and heated by candles, wood-stoked fireplaces, or coal furnaces and over time were inundated with soot and smoke. This as well as other factors such as dust and dirt accumulation, or an overexposure to light may have caused the varnish to darken, discolor, yellow, or crack.

A common restoration procedure of this type is to remove this original layer of varnish with a cotton swab dipped in a special solvent found in art supply stores. The solvent should first be applied gently to one small, unobtrusive corner of the painting, and if all appears well, continued slowly to the rest of the painting. If unsure in the least bit about performing this painstakingly slow procedure, it should be done professionally. If the oil was painted after the1950s, in all liklihood it does not contain a layer of varnish.

In this case, unless the paint itself is cracked, the dirt, smoke or grime that has accumalated over the years can be cleaned with a soft detergent diluted in water and applied with a soft cloth. The cloth should be damp, not wet; the surface should be gently dabbed, not rubbed, as this could loosen or dislodge the paint.

Caution must always be taken before dusting an oil painting. First, it should be closely examined for lifting and insecure paint (if the paints are not firmly attached to the canvas, the mechanical action of dusting may further damage them). Providing that there are no signs of loose or flaking paint, the build-up of dust can be safely removed using a clean, soft, natural-hair artists’ brush. The painting should be positioned on a clean padded surface and held upright at a forward angle so that the dust falls away from the face of the painting. Brushing should be carried out gently by moving in one direction across and down the canvas, followed by a second brushing in the opposite direction. Never use stiff bristles brushes or feather dusters because they can scratch the surface.

If in doubt about self-cleaning of an oil painting, it should be taken to an art conservator who can carefully clean and repair it by studying each layer of paint and analyzing the composition of the paints and varnishes by using various techniques, including a thorough inspection with infrared and ultraviolet light. Later additions that had been made can even be traced and paintings can be restored in a manner that slows the aging process, while preserving the integrity of the original work of art.

A professional conservation treatment generally begins with the removal of dirt, grime, and other accretions from the varnish layer by means of aqueous solutions. After that, the discolored natural resin varnish is reduced with chemicals that will not harm the original paint layers. Finally, the distracting, discolored retouching and overpainting in oils from previous restorations are reduced or removed.

The next step usually performed is cleaning the reverse of the canvas of dirt, dust, and debris. Necessary repairs are made to the tacking edges, and the stretcher is adjusted ("keyed-out") to place even, proper tension on the canvas.

Attention then turns to cosmetic problems. A non-yellowing, stable, synthetic resin varnish is applied to the painting, and losses in the paint and ground layers are filled and textured to match the surrounding original paint layers. (This process, know as inpainting is done only after the original painting has been revarnished). Inpaint is applied on the losses, areas of abrasion, and to diminish stains and old restorations (if they are not removed). Torn or weak canvases can be strengthened and supported by lining. Small punctures or tears can be repaired with patches or sometimes invisibly mended with special adhesives and reweaving techniques.

The final procedure is usually securing a nonacidic protective backing board to the reverse of the stretcher that keeps out dirt and debris, prevents impact or puncture damage, and enhances the stretcher's structural stability. The painting is now ready to be reframed with new and secure hardware and hanging materials. As the final step, the conservator writes a report detailing all aspects of the treatment, and documents the processess and procedures that were employed.