Cracks ‘make historical paintings less vulnerable to environmental variations’

Cracks in historical paintings make them less vulnerable to changes in the environment, a new study has found.

Wood panel artworks with developed craquelure patterns – networks of fine cracks in the paint- are significantly less vulnerable to environmental variations than previously thought.

The findings suggest a potential explanation as to why heavily cracked historical paintings remain stable in environments far from ideal museum conditions, scientists say.

Painted wood is among the most precious and frequently exhibited category of heritage objects and among the most vulnerable to relative humidity and temperature fluctuations.

Researchers at the Polish Academy of Sciences, Universite de Strasbourg, France, and Yale University in the US, investigated changes in susceptibility to fracture for the most brittle component of a wood painting.

This is the ground layer, or gesso – a mixture of animal glue and white pigment that is applied between the wooden support and the paint.

The gesso is strained when wood expands due to increasing humidity, or contracts due to drying out, which can lead to cracking.

Knowledge of this as well as development of technical capabilities to control the environment precisely have led to stringent climate control specifications for museums, both in temperature (21C or 22C) and humidity (45-55%).

Corresponding author Lukasz Bratasz said: “The current environmental standards for the display of painted wood allow for only moderate variations of relative humidity.

“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.

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

To investigate the development of cracks in gesso subject to different environmental conditions, the researchers designed specimens of two wooden panels, which they joined with gessoes prepared according to traditional recipes.

The specimens were stored at 25C and relative humidity of 30, 50, 75 and 90% for two weeks, and then tested to measure resistance to cracking.

Researchers scanned historic samples of panel painting, and determined the size of existing flaws in the gesso at which new cracks initiate.

The measurements were then used in a computer model of a panel painting to simulate further crack formation.

Published in Heritage Science, the study found that the stress on the gesso decreased as the number of cracks increased over time.

Mr Bratasz said: “Stress on the gesso occurs in the areas between cracks. 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.”

The authors 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.

Mr Bratasz concluded: “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.”

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