Contemporary paintings, naive, or folk art may not have a ground layer or varnish coating. Paintings that do not have all of the traditional layers may be more fragile and susceptible to change or damage. The paint can be applied on a wide variety of supports. Although the most common are canvas and wood, other supports include paper, cardboard, pressed board, artist's board, copper, ivory, glass, plaster, and stone. Paintings on canvas are usually stretched over an auxiliary wood support.
Paintings change over time.
Some inevitable results of aging, such as increased transparency of oil paint or the appearance of certain types of cracks, do not threaten the stability of a painting and may not always be considered damage. One of the most common signs of age is a darkened or yellowed surface caused by accumulated grime or discolored varnish. When a varnish becomes so discolored that it obscures the artist's intended colors and the balance of lights and darks, it usually can be removed by a conservator, but some evidence of aging is to be expected and should be accepted. However, when structural damages occur in a painting such as tears, flaking paint, cracks with lifting edges, or mold, consult a conservator to decide on a future course of treatment for your painting. It is important to maintain a proper environment for your paintings.