Fig. 3

Scanning electron microscopy-X-ray microanalysis performed in a portion of the cross section of the altered blue sample (area analysed is delimited in the square in Fig. 2d): a backscattered electron image of the the cross-section of the sample (Fig. 2d), b X-ray spectra acquired in a white grain of lead white located in the lower paint layer (arrow a) and c X-ray spectra acquired in a blue area located in the upper paint layer (arrow b)