Non-destructive study of lead glasses’ alteration by reflectance spectroscopy

Non-destructive detection of the early stages of glass deterioration is important for stopping the degradation and for restoring works of art. With this aim, two sets of reference glasses, with varying amounts of lead, have been damaged using a defined and controlled degradation process. The measurements of the reflectance spectra of the samples, implemented with a portable fiber-optics goniospectrophotometer in a back-scattered configuration, underline very fine levels of deterioration. For glasses containing a low amount of lead, an increase in the roughness of the upper surface with the duration of the deterioration is observed and can be quantified by an upward vertical translation of the spectra. For glasses containing an important amount of lead, an increase followed by a decrease in the roughness is observed during the alteration. Moreover in the latter set, the reflectance spectra present oscillations, due to interferences inside a superficial layer created by the loss of lead during the deterioration and involving an iridescent appearance. The analysis of these oscillations leads to the quantification of the thickness of the altered layer and to its evolution with time. The obtained results have been validated by RBS analysis. The same measurements have been implemented on archeological samples that have been damaged by naturally occurring reactions while being buried in soil. Comparisons of features and oscillations of the reflectance spectra discriminate non-weakly and strongly altered areas of the glasses, no matter the amount of lead in the samples.