The project built upon expertise developed in the pilot project synchrotron x-ray microtomography of paint samples: The characterisation of the porosity of ground layers in canvas paintings.
In paintings, many reactions that lead to change in colour, transparency and mechanical strength are linked to or driven by moisture. Examples include the efflorescence of salts at the painting surface and formation and mobilisation of metal soaps with often serious consequences to the integrity and appearance of the painting. Furthermore, the interaction between moisture and paintings on canvas can cause differential swelling and shrinkage of the individual materials/layers and lead to internal stresses.
Paintings are multi-layered systems of complex composition with a typical build-up of textile support treated with a sizing layer, primed with a ground layer over which the paint is applied. Not only we aim to monitor water activity and uptake in such a complex system, we also aim to do so in a time and space resolved manner.
Art technological context
A link was made with the recently completed technological study of the early oeuvre of the Swiss painter Cuno Amiet (1868-1963) (Kunsttechnologische Forschungen zur Malerei von Cuno Amiet 1883–1914). The samples prepared and tested were based on materials used in late 19th century and early 20th century paintings, which had been identified in the oeuvre of Amiet.
Hydrogen has a high neutron scattering cross section which makes neutron radiography a highly suitable technique for moisture interaction studies. For the study of the spatial distribution and kinetics of such interaction, a moisture exposure chamber was purpose built for neutron radiography experiments. New collaborations with EMPA (Dübendorf) and the Technical University of Eindhoven allowed to obtain relevant material parameters and to use magnetic resonance (NMR) as an alternative technique to measure water uptake in the paint layers.
Results and publications
The first results on a simplified build-up system (canvas, size and ground) showed that we can measure the moisture uptake quantitatively in a time-resolved experiment and with spatial resolution of 13.5 micrometers, sufficient to distinguish the contribution of the different layers of a typical paint multiplex. The flax fibres of the canvas and the glue sizing appeared to have a much stronger water uptake than the chalk glue ground. We also found that the way the glue size was applied had an influence on the distribution of the glue, and therefore also on the location where more moisture was absorbed. The findings were published in Journal of Applied Physics A.
The observations could be clarified by the outcome of extensive material characterisation, undertaken in the labs of EMPA. The moisture capacity and permeability of the different layers within a painting was measured in cup tests and by Dynamic Vapour Sorption. The results were submitted to the Journal of Cultural Heritage, and have been published online.
During the final phase of the project the gathered results of three experimental campaigns at the neutron beamline ICON and of two sessions of NMR measurements (June 2015 at University College London and November 2015 in Eindhoven) were re-analysed in order to draw conclusions about the behaviour of different types of painting build-ups. These include samples with and without oil paint layer, with and without lining, and with various types of glue sizing. The results were published in Studies in Conservation.
Water uptake distribution through the sample as a function of time.
Evolution of shape change of an oil painting sample during moisture uptake (until 240 minutes) and during the subsequent drying phase. The painting canvas expands more in the direction of the warp than of the weft. Data obtained using neutron radiography at the ICON beamline, PSI, Villigen.
Evolution of moisture distribution during absorption and drying in an oil painting sample. Because the linen canvas is the component with the strongest absorption, the structure of the canvas is visible in the moisture content distribution. During the first four hours the initially dry sample is subjected to a high relative humidity of 90% and absorbs moisture; then the relative humidity is decreased to 15% and the sample dries rapidly. Data obtained using neutron radiography at the ICON beamline, PSI, Villigen.
The project was supported by
Werner Abegg-Fonds, Bern