Cracks actually protect historical paintings against environmental fluctuation

Extensive cracking actually makes historic wood paneled paintings less vulnerable to environmental variability, not more.

According to a new study, wood paneled paintings with significant cracking patterns are surprisingly resilient, even in substandard storage conditions.

Wood paneled paintings feature three layers. The so-called ground layer, or gesso, a mixture of animal glue and white pigment, is found in between the wood and paint. When changes in humidity and temperature cause the wood to expand or contract, the gesso can form cracks.

Scientists have closely studied this process in order to develop better standards for storage conditions, so as to prevent cracking.

"The current environmental standards for the display of painted wood allow for only moderate variations of relative humidity," study co-author Lukasz Bratasz, researcher at Yale University's Institute for the Preservation of Cultural Heritage, said in a news release. "The safe range was determined based on laboratory testing of when cracks start to form in new, undamaged material."

"However, this does not reflect the physical reality of paintings as they age and complex craquelure patterns form," Bratasz. "Our research more accurately reflects that physical reality, accounting for changes in the susceptibility to environmental stresses as paintings age."

For their research, scientists stored paintings at 25 degrees Celsius and several different levels of humidity for two weeks before subjecting the wood panels and their gessos to stress tests.

Researchers used computer tomography to image the structures within each gesso and measure the size of flaws from which cracks are most likely to form and proliferate. Based on their measurements, the team of scientists developed a computer model to simulate further crack formation.

Their models showed stress on the gesso diminishes over time as the paintings develop more and more cracks.

"Stress on the gesso occurs in the areas between cracks. The larger these areas are, the more easily cracks will form," Bratasz said. "As cracks multiply and the spaces between them become smaller, stress decreases up to a point where, finally, no new cracks will form."

According to Bratasz and his colleagues, their research -- described this week in the journal Heritage Science -- is only applicable to paintings that have open cracks, not those with cracks that have been filled-in as part of a restoration effort.

"Our findings offer a potential explanation as to why historical panel paintings with developed craquelure patterns remain stable, even if the environmental conditions they are stored in are far from ideal," Bratasz said. "We hope that this knowledge may contribute to development and acceptance of more moderate-cost climate control strategies in historic buildings and museums, especially ones that may have limited potential for tighter climate control."

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