Craquelure

Craquelure in the Mona Lisa

In art, craquelure (French: craquelé, Italian: crettatura) is the fine pattern of dense "cracking" formed on the surface of paintings in particular due to the aging of paints. It is often used to determine the age of paintings and also to detect forged art, as craquelure is a hard-to-forge signature of authenticity.

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. The precise pattern of craquelure depends upon where, when, and under what conditions the picture was painted. Cracks caused by stretching or slackening the canvas are quite different from cracks due to other factors such as drying and aging 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 in the direction of the stress, relieving that stress. In the middle of the picture the cracks tend to run parallel to the short sides. They spread from the middle towards the stressed locked edges, while the cracks starting at the short sides curl round. The stress at the corners is more than double that of the center. There appear to be distinct French, Italian and Dutch "styles" of craquelure.

The craquelure is almost impossible to accurately reproduce artificially in a particular pattern, although there are some methods such as baking or finishing of a painting wherein this is attempted. These methods, however, can get a crack at most uniform in appearance, while genuine craquelure has cracks with irregular patterns. The precise pattern depends upon chemical characteristics of pigments used--from the finest light colors to the less perceptible the dark, painting style of the painter, whether wood or canvas was used as a background, and furnish a record of the environmental conditions the painting has experienced during its lifetime--such as temperature and humidity, and also can reveal details about the painting's history of handling, transportation, and restoration. The craquelure can reveal environmental conditions in which a painting was stored and also details about its history, methods of transport and previous restorations.

Craquelure gives objects such as ceramics and furniture that "antique" look. The effect is achieved by a chemical reaction that results in more or less regular looking craquelure. The regularity is given by the thickness of the product spread. They can be mixed with bitumen of Judea or oil paints. Flatting and cracking can be replaced by shellac and gum arabic.

In recent years the modern decor industry has used the technique of craquelure to create various objects and materials such as glass, ceramics, iron. This was possible by a special marketing kit that reacts with the colors used in decorative acrylic colors. These kits, according to the percentage of reagent and time of use, make the craquelure more or less accentuated by offering more possibilities of realization based on the final result to be obtained. To highlight the cracks, use glitter powder usually available in copper, bronze and gold. Mixing different brands on the market of products ready to make the craquelure result in various sizes and patterns of cracks. Software programs are available for creating craquelure in digital photos.