Dosimetry of paintings: determination of the degree of chemical change in museum exposed test paintings (lead white tempera) by thermal analysis and infrared spectroscopy

Thermoanalytical (DSC and TGA) data on the basic lead carbonate pigment alone have been previously published [J. Chem. Soc., Dalton Trans. (1996) 3639]. This paper draws attention to the modifying effect of basic lead carbonate on the thermal stability of the paint medium, and on the degradation of the paint medium under controlled light ageing conditions. The various light dosages received by the samples were found to produce differences in the shape of the leading edge of the DSC curves. Quantification of the data shows that there is a systematic trend, i.e. the difference in the leading edge of the DSC curve, with respect to the control sample, increases with the duration of light exposure of the samples. In the natural ageing of the samples exposed in various galleries for a period of 9 months, the dominant change is a shift in the overall peak. Changes in artificially and naturally aged samples were also identified by Fourier transform infrared spectroscopy. The broadening of the carbonyl peak due to the formation of lipid oxidation products could be quantified to determine the degree of chemical change.