Dynamic Mechanical Analysis (DMA) and its role in paint tempera dosimetry

In the ERA project (Environmental Research for Art Conservation) test tempera paint samples were prepared and some were exposed to accelerated ageing (light , heat, pollutants) and others were exposed in frames in major art galleries, a chapel housing a collection of artworks, and a historic castle where environmental conditions were recorded. The sites were selected on the basis of the wide range of environmental conditions. Analysis post exposure involved an interdisciplinary approach using mass spectrometry , thermal and spectroscopic analysis. The data obtained from DMA will be presented here. Tempera paint based on whole egg was prepared according to traditional recipes and then spread onto Melinex substrate. A selection of pigments was used including smalt, Sienna, azurite, lead white, and Naples yellow. Strips of the differently pigmented materials were cut to about 15mms and mounted in a frame for site exposure. For DMA analysis samples were then cut from these strips. Measurements were made in bending mode at 1Hz from -100°C to 150°C. The best response came from smalt tempera samples and this can be explained in terms of good mechanical integrity of the coating which is determined by factors such as pigment reinforcement and adherence to the substrate [1]. In chemical terms, a stronger response indicates the formation of a polymer network. Cobalt ions interact with egg yolk lecithins to form network structures composed of cross-linked aggregates which will increase the mechanical integrity of the film. The high pigment volume concentration in the smalt tempera sample is another factor that affected its mechanical properties. All the pigmented tempera samples used in this study contained, on an average, approximately 90 wt.% pigment as determined by thermogravimetry (TGA). Specific indicators for change by DMA of the paint tempera films were identified as changes in the glass transition temperature (observed as tan d peaks) of the paint tempera samples. Analysis of ageing effects was conducted by measuring either a shift in peak position to higher temperatures or calculation of peak ratios, based on 2 peaks, or the ratio of intensities at selected temperature regions (e.g. -20°C) and peak maximum. Data will be presented for smalt, azurite and Naples yellow samples and will show that differences were observed between sites with varying environmental conditions.