Why Do Old Paintings Crack? The Layers Move at Different Rates

A painting is a stack of materials that behave differently, typically a fabric or wooden support, a preparatory ground, one or more paint layers and a varnish. Each responds to temperature and humidity by expanding and contracting, and each does so by a different amount and at a different speed. Oil paint also continues drying and stiffening for decades after it appears dry, shrinking slightly as it does. Over time the accumulated differential movement exceeds what the layers can accommodate, and they crack. The cracking releases stress, which is why the pattern stabilises rather than continuing indefinitely, and why a painting kept in stable conditions cracks far less than one moved between climates.

Conservators distinguish cracks by cause and the difference matters:

• •Ageing cracks, formed slowly by drying and by movement of the support
• •Mechanical cracks, formed by a blow, a fall or pressure from behind
• •Ageing cracks form networks across the whole surface
• •Mechanical cracks radiate from a point, frequently in a spiral pattern
• •Drying cracks in the paint itself, caused by a fast-drying layer over a slow one
• •Cracks following the weave of the canvas or the grain of a panel

The network that forms depends on the support, the ground, the paint and the history, and those vary systematically by place and period, which makes the pattern informative. Paintings on wooden panels crack along the grain in a strongly directional pattern. Paintings on canvas produce more rectangular networks reflecting the weave. Italian, Flemish and Dutch practice used different grounds and produced characteristically different networks, which specialists learn to recognise. The pattern also interacts with the image, since cracks propagate differently through thick and thin paint, so a network that ignores the picture entirely is suspicious. Analysis of these patterns has been formalised with image processing and is used as supporting evidence in attribution.

The cracks are damage and are also part of the object's history, which makes treatment a matter of judgement rather than repair. Stabilising comes first, since paint lifting at the edges of a crack will eventually flake away, and a consolidant is introduced under the lifted edges and the paint laid back down with gentle heat and pressure. Filling and retouching are applied only where a loss disturbs the reading of the image, and modern practice keeps retouching reversible and confined to the loss rather than extending over sound paint. Filling every crack is regarded as falsification, since the network is evidence of age. Preventive work matters more, and stabilising the temperature and humidity around a painting does more for it than any treatment.

Faking the pattern convincingly is extremely difficult, and failures to do so have exposed a number of forgeries. Artificial cracking is produced by heating, by rolling the canvas, by adding materials that shrink, or by painting cracks in directly, and each leaves detectable signs. Genuine cracks go through all the layers down to the ground, while induced ones frequently stop in the paint. Genuine cracks have edges that lift slightly and accumulate dirt over decades, which forgers imitate by rubbing ink or dust into fresh cracks, detectable under magnification. Painted cracks sit on the surface rather than in it. And the overall network in a forgery tends to be too uniform, since real ageing produces variation that follows the picture.

A painting is a stack of materials expanding and contracting at different rates while oil paint continues stiffening for decades, and the accumulated stress cracks the layers. Networks differ by support, ground and regional practice, which makes the pattern informative about origin. Forgers induce cracking by heating, rolling or painting it in, and genuine cracks run through every layer and collect dirt in a way fresh ones cannot imitate.