Don’t fear the craquelure

Cracks in the paint of a great work of art may not be quite the problem they seem. In fact, if it’s a painting on wood, they may be doing it good.

Research has found that craquelure patterns – networks of fine cracks on the surface materials – can make historical paintings less vulnerable to environmental variations.

And that, says a team from the Polish Academy of Sciences, Université de Strasbourg, France, and Yale University, US, may explain why old and heavily cracked paintings can remain stable even in less than ideal conditions.

Painted wood is very vulnerable to fluctuations in relative humidity and temperature, which is why museums studiously try to keep things at 45-55% humidity and 21-22 degrees Celsius.

Most susceptible in a wood painting is the ground layer, or gesso, which is a mixture of animal glue and white pigment applied between the wooden support and the paint.

The gesso is strained when the wood expands or contracts.

To get a better idea of exactly what is happening, the researchers created two wooden panels, which they joined with gessoes prepared according to traditional recipes.

Both were stored at 25 degrees but at humidities of 30, 50, 75 and 90% for two weeks before being subjected to a splitting test.

Using computer tomography to scan historic samples of panel painting, the researchers determined the size of existing flaws in the gesso at which new cracks initiate. Those measurements were then used in a computer model of a panel painting to simulate further crack formation.

Factoring in the elasticity of the materials and moisture expansion of wood, they found that the stress on the gesso decreased as the number of cracks increased over time.

“Stress on the gesso occurs in the areas between cracks,” says Lukasz Bratasz, corresponding author of a paper in the journal Heritage Science.

“The larger these areas are, the more easily cracks will form. As cracks multiply and the spaces between them become smaller, stress decreases up to a point where, finally, no new cracks will form.”

Bratasz says the current standards for a “safe range” of variations of relative humidity was determined based on laboratory testing of when cracks start to form in new, undamaged material, which “does not reflect the physical reality of paintings as they age and complex craquelure patterns form”.

“Our research more accurately reflects that physical reality, accounting for changes in the susceptibility to environmental stresses as paintings age.”

The authors do caution that their conclusions are valid for paintings with opened cracks. If cracks are filled in during conservation treatment or varnishing, the vulnerability of a painting to the environment may increase.