Sprickorna i Konstnärsförbundets styrelse

Sammanfattning: All oil paintings do degrade with time and a clear sign of this ageing is the cracks seen at the surface of the painting. Cracks can , however, also be hidden underneath the very surface and in fact extend all the way down to the very substrate. Knowledge about the crack pattern and origin(s) is useful information for a conservator, but cracks are notoriously difficult to analyse. Here, the cracks in the painting The Council of the Society of Artists by the Swedish artist Richard Bergh have been investigated by performing artificial ageing of several mock-ups of the painting, foremost in order to understand the influence of the pigments and focussing especially on zinc white and cobalt blue. The mock-ups have been analysed by heritage science methods and in particular by non-destructive methods: technical and X-ray photography, and X-ray tomography. For comparison purposes microscopy methods have also been used, by removing small samples from the mock-ups. Apart from the role of the pigments, also the influence of two different type of substrates have been investigated: a linen canvas and a Melinex film. Overall, zinc white affects the elasticity of the paint, which hampers the crack formation, but the cracks that are observed most likely originate at the substrate and grow towards the surface. The type of substrate seems to be negligible as do the thickness of the impasto, but this might be a consequence of the relatively short study. Moreover, the cracks are dominantly found to be perpendicular to the brush direction, but indeed sometimes also alongside the direction. The crack pattern found when studying the mock-up has clear similarities with those in the original painting. The main agreement is found in the border between the green background and the black costumes; the zinc white contributes to cracks less net-like, but in contrast the results cannot unambiguously determine the influence of cobalt blue for the crack pattern. Altogether, the use of a mock-up and (non-destructive) heritage science techniques combined open for future studies not limited to the study of crack formation but one can also envisage studies of pigment degradation.