Raman Spectroscopy of Hematite in the Paint Layer of a Model Fresco before and after Heating

The Raman spectra of hematite in the paint layer of a model fresco sample were compared before and after heating to 740°C. A significant obstacle to the analysis of the Raman spectra of hematite is the significant difference in the relative line intensities in the spectra measured at different points on the sample. It is shown that such differences are due to the orientation factor – a change in the Raman line intensities with changes in the relative orientation of hematite microcrystals and the wave vector of the exciting radiation. To minimize the orientation effect, it is proposed to average a series of spectra measured at different points on the sample. Comparison of the averaged spectra before and after heating allows us to identify a number of changes.  An increase in the intensity of the calcite lines may be associated with thinning of the paint layer, and the appearance of amorphous carbon lines is possibly due to the charring of organic components in the plaster. Heating also leads to an increase in the intensity of the line at 660 cm^–1, indicating an increase in the defectiveness of hematite microcrystals. It is hypothesized that the decrease in the intensity of the line at 221 cm^–1 and the increase in the intensities of the lines at 406 and 605 cm^–1 may be due to the orienting effect of heating on the pigment microparticles. The obtained data can be used to interpret the condition of historical frescoes in churches exposed to fire.

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