International audienceMany wooden objects from cultural heritage consist in wooden panels, painted on one face. Some of these panels show permanent cupping, micro-cracks of the painted layer, and cracks of the painted support itself. Different physical and mechanical phenomena are at the origin of these damages: wood is a hygroscopic material (its dimensions vary with humidity), it is highly anisotropic, the paint layer on one face has properties of permeability different from those of raw wood of the back face, and a rigid frame possibly restrained the deformation of the panel. Experimentations on our mock-up panels combined with numerical simulations of these panels in real situations of hygrothermal fluctuations will allow us to test specific situations and eventually to make suggestions to conservators and restorers and guide them in their interventions. Hygrothermal treatments are often used to improve wood durability thanks a reduction of its hygroscopicity. They have been considered as means to reproduce the physical properties of ancient wood. We intend to model the mechanisms involved in mechanical and chemical effects of wooden painted panels exposed to climatic variations. To develop such conservation tool, we need to work on mock-up, which replicate panel painting. So we will investigate the process of hygroscopic ageing and compression set generation of the back of painted panels to explain their permanent cupping and replicate their ageing state. For this purpose, digital image correlation is used to evaluate the strain field of the section of a wood piece submitted to variations of relative humidity

It is essential for the preservation of cultural heritage that the effects of climate change are investigated. With this in mind, the daily temperature and relative humidity (RH) cycles within the Brown Gallery at Knole House, Kent, have been reconstructed for the period 1605 -2015 enabling the study of low-cycle environmental fatigue on a set of 17 th century panel paintings. By establishing a relationship between the temperature in the Brown Gallery and the Hadley Centre Central England Temperature (HadCET) dataset over a sixteen year period (2000 -2015), it is possible to use the full HadCET dataset to obtain the daily minimum and maximum temperatures in the Brown Gallery for the period 1878 -2015. Using a Fourier series to fit the periodic data it is then possible to extrapolate back to 1605. Furthermore, correction factors derived using the HadCET average daily temperature in the period 1772 -1877 and average monthly temperature in the period 1659 -1771 are applied to the temperature data to increase the model accuracy. The daily minimum and maximum RH for the period 1605 -2015 are obtained using the Brown Gallery maximum and minimum temperatures respectively, and assuming that the daily dew point temperature at Knole is calculated by subtracting a monthly-dependent constant from the daily minimum temperature at Knole, thus enabling the calculation of the daily actual water vapour pressure of air. Changes in RH are a result of the daily temperature cycle changing the saturation vapour pressure of air in the gallery. This data is valuable as it enables a study of the effects of low-cycle fatigue on the 17 th century panel paintings housed in the Brown Gallery at Knole House, Kent due to these temperature and relative humidity cycles. Furthermore, the method presented offers a technique that can be utilised to replicate the internal environment for any unheated monument building so that the effects of past and future temperature and humidity cycles on cultural heritage can be examined.

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This article describes the experimental devices and the processes used to study the hygromechanical behaviour of a historic painted wooden panel (La Sainte Trinité couronnant la vierge, 1516, anonymous, Fabre Museum of Montpellier). A climate showcase was designed for in-museum use, with two glasses allowing visitors as well as scientists to observe both sides simultaneously. The evaluation of the hygroscopic behaviour was done by measuring relative humidity (RH), temperature and variations of panel weight. Variations of panel shape, strain and curvature were observed by two methods: locally with three pairs of displacement transducers fixed on the rear face, and over the entire panel surface by a non-contact optical technique, stereo mark tracking, used simultaneously on both faces. The correlation between local and whole-field measurements was very good. Continuous monitoring (several data per hour over three years) was required to observe the behaviour of the panel during imposed climate variations. The first test, detailed in this paper, was performed before frame removal and panel restoration. It consisted in a stabilization of the panel at 52% RH during 2 months and an increase to 63% RH during 3 months. The small amount of total mass variations, from 31.980 kg to 31.910 kg at 52% RH and 32.088 kg at 63% RH, could not be explained without taking into account the phenomenon of sorption hysteresis. The whole field relief measurement exhibited a concave shape with a maximum amplitude of 16 mm. It showed that shape deformation, ie out-of-plane displacement, was convex bending with a maximum deflection of 1.5 mm and a maximum strain of 0.1 %. Comparable local data had been obtained by the optical and DKs methods. DKs results were ten times more accurate than optical ones. The horizontal strain exhibited a global shrinkage during initial stabilization and a global swelling during RH increase. Heterogeneity of the strain field can be related to the cracks observed on the back face, and to the glued crossbeams. Assuming a linear relationship between swelling and moisture content, we expected, for a 0.6% increase of moisture content, a radial strain of 0.10% and a tangential strain of 0.22%. These results are higher than the ones measured, which can be explained by the cross bars glued on the boards and the cracks. This comparison will be continued for the next stages (crossbars removal, panel restoration).