Craquelure

Craquelure (French: craquelé, Italian: crettatura) is the fine pattern of dense "cracking" formed on the surface of materials, either as part of the process of ageing or of their original formation or production. The term is most often used to refer to tempera or oil paintings, where it is a sign of age that is also sometimes induced in forgeries, and ceramics, where it is often deliberate, and usually called "crackle". It can also develop in old ivory carvings, and painted miniatures on an ivory backing are prone to craquelure.

Normally, craquelure is formed by the aging of paints. It can be used to determine the age of paintings and to detect art forgery, because craquelure is a hard-to-forge signature of authenticity.

Authentic paint craquelure occurs because paint dries and becomes less flexible as it ages and shrinks. In the case of paintings on canvas, the canvas slackens as it ages as it cannot endure the long-term stress of stretching. Paint at the center of a painting is the least cracked, whereas paint at the edges is the most cracked, or stressed. The precise pattern of craquelure depends on where, when, and under what conditions the picture was painted, and subsequently kept. Cracks caused by stretching or slackening the canvas are quite different from cracks due to other factors, such as drying and ageing of the paint. The paint cracks when the stress upon it is greater than the breaking stress point of the paint layer and the paint will crack approximately at right-angles to the direction of the stress, relieving that stress. The stress at the corners is more than double that of the center.

There are distinct so-called French, Italian, Flemish, English and Dutch "styles" of craquelure, relating to differences in the typical techniques used (and the typical period when each country produced most of its art); however these names just refer to typical patterns, and an Italian painting might show a "French" style of craquelure. The distinctness of these styles has been largely confirmed by studies. The English style arises from the use of bitumen in paint that was pioneered by Sir Joshua Reynolds, and can be especially disastrous, leading to large blisters in the surface. Radiating circular patterns of cracks are a result of impact. One study used images to get subjects to classify paintings according to the following "rules", with some success:

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Wikipedia