Mechanism of craquelure pattern formation on panel paintings

Abstract

The drying shrinkage accumulation from exposure of freshly prepared gesso layers to relative humidity (RH) cycles was determined to elucidate the mechanism of craquelure pattern formation on panel paintings. The progresive drying shrinkage of the gesso is observed only under the cycles going to high RH levels which bring about transitions from brittle to ductile state of the material. The first incidence of fracture on the gesso layers occurred after a limited number of cycles ranging between a few and 100 for a range of layer thickness between 0.5 and 1 mm. The craquelure patterns stabilised also after a limited number of cycles (30 for the 1-mm thick layer). Upon increase in the gesso layer thickness, the strength of the layer is reduced and the spacing of shrinkage fractures increases. The study demonstrated that craquelure patterns, mimicking historical ones, can be realistically produced in laboratory conditions. Such studies would provide useful information for preparing specimens simulating historic panel paintings and would inform the current efforts on automatic, computer-aided classifications of crack formations on paintings.

Fracture saturation in paintings makes them less vulnerable to environmental variations in museums

“…Stiffness of gesso practically did not depend on the PVC ratio within experimental uncertainty. Elasticity modulus varied significantly with RH and was 10.2, 7, 3.8 and 0.4 GPa at 30, 50, 75 and 90% RH, respectively, which agrees with the tendencies established in earlier studies [5,13]. For RH lower than 90%, uncertainty was around 1 GPa and for 90% was 0.4 GPa.…”

“…The Poisson's ratio of the gesso layer υ g , determined experimentally, was 0.2 [5]. Gesso moisture expansion coefficient was assumed to be 0, which seemed a fair assumption as moisture expansion coefficient of gesso is very small [5,13]. The material properties of the wooden support corresponded to lime wood (Tilia sp.)…”

“…The terms 'drying' and 'aging' cracks were used to describe these two groups of cracks, respectively [12]. Investigations on development of cracks in freshly prepared gesso layers demonstrated that historically drying cracks developed fully and stabilized within several years after the painting was executed and exposed in a building with unavoidably uncontrolled climate [13]. Drying leads to isotropic 'mud crack patterns' as the principal stresses in directions parallel to the free surface are similar [14].…”

Stress Assessment in Artistic Materials Using a Micro-Scratching Technique

“…The treatment consisted of placing samples at high humidity (90% RH) and subsequently transferring them to low humidity. In high humidity environment, wood is expanding and gesso is experiencing a dramatic loss of stiffness, due to the transition of the rabbit-skin glue from the brittle to the ductile (gel like) state taking place at approximately 75% RH [3]. Therefore, expanding wood easily deforms (stretches) the gesso layer.…”

“…Gesso is a brittle, granular material composed of chalk grains connected by animal glue. The total stress generated in the ground layer is the result of external loading and drying shrinkage during material preparation as well as the history of RH variations the material experienced, especially excursions into very moist conditions [3]. Total stress is the main factor defining the performance of brittle materials.…”