Time can transform almost any work of art. Most paintings are extremely fragile and can easily become damaged. Many suffer deterioration both through their natural aging process, which is generally regarded as acceptable and from physical or environmental conditions and fluctuations.
All the natural materials used in the creation of a painting exhibit some degree of decay over time. Paintings undergo the greatest change in the first year of their life as the solvent or liquid part of the medium evaporates and the paint begins to dry. Then the chemical process involved in the "drying" or hardening of the paint begins. When some paint dries, a pattern of cracks (craquelure) generally forms. This is often distinctive, giving clues to the composition and consequently may help to date the painting. These cracks are either aging cracks, which form in most paintings in response to the stresses within the painting: or drying cracks which form as the paint dries. Surface cracks may also be an indication that the artist may have used incompatible materials or defective techniques. Generally aging cracks are thin, sharp and angular in comparison with drying cracks which are wide and rounded. If one looks very closely at the point at which the paint goes into the crack the difference is obvious as the aging crack has a broken sharp edge and drying cracks have a smooth rounded edge.
Oil paintings can become less transparent as chemical changes occur over time. Natural varnishes tend to darken and discolor with age, necessitating their removal and replacement. The removal of a varnish layer requires great skill and knowledge and should only be undertaken by a trained paintings conservator. The replacement of a varnish is not necessarily a simple matter either. Conservators must decide whether to replace a natural varnish with another natural varnish, knowing that the natural varnish will yellow and will have to be replaced, or with a synthetic varnish, which may not yellow as rapidly but also may not duplicate the aesthetic effect of the natural varnish. The skill of varnish application has a great deal to do with the final appearance of the painting. Conservators usually attempt to match the varnish to the type and sheen that the original artist may have used.
Other effects of aging include the change in tone or fading of some pigments, for example some greens may turn brown, blues can go grey, and reds can fade. These effects are reduced if the painting has not been exposed to light but, unfortunately, paintings are not usually viewed or displayed in the dark.
Many old paintings are commonly covered with a layer of surface dirt and grime, deposited over time from exposure to smoke and other forms of environmental contamination. Dust and dirt can settle into the paint layers on unvarnished paintings making it nearly impossible to remove without damaging the paint. A white haze can develop in paintings stored in damp conditions where moisture has penetrated the paint layers. This effect is known as bloom.
Basically paintings are composed of incompatible elements, each having differing reactions to changes in relative humidity, temperature and light. Small changes are absorbed by the materials which are reasonably elastic. This elasticity, however, diminishes with age and eventually the painting cannot absorb the stresses caused by these fluctuations.
Paintings all run the risk of being scratched, knocked, dented, torn or punctured. Paintings on canvas generally show the effect of any impact or bump. Even a slight tap from the reverse side can stretch the canvas enough to cause a webbed shaped network of cracks to form in the paint and ground layers over time. A scrape along the back of the canvas will often produce a centipede or fishbone sort of crack.
The supports, too, are vulnerable to change and deteriorate. Paintings on canvas suffer from weakening fibers which lose strength through oxidation and which eventually become too weak or too brittle to support the paint layers. The tacks which hold the canvas to the stretcher can also oxidize (rust) and then further contribute to the weakening of the canvas. The wood of the stretcher is acidic and this produces more loss in strength in the canvas, especially at the angle where the canvas bends around the stretcher. Repeated fluctuations in relative humidity cause the canvas to slacken and then tighten and finally to go permanently slack. As the slack canvas sits in direct contact with the stretcher sharp cracks can form along the lines of the stretcher edge. The movement in the canvas will eventually cause brittle and stiff paint to lift and flake from the support.
Paintings can be protected to some extent from the effects of normal aging by good preventative conservation measures and from physical damage by good hanging, handling and storage procedures.
· Do Not hang paintings in direct sunlight.
· Never store paintings in an attic or basement
· Do Not hang paintings over a working fireplace or HVAC duct.
· Do Not attempt to clean paintings yourself with household cleaners or solvents.
· If your paintings are cracked and discolored, have them inspected by an experienced Art Conservator