Monitoring the photographic process, degradation and restoration of 21st century Daguerreotypes by wavelength-dispersive X-ray fluorescence spectrometry

Abstract

This study applied wavelength-dispersive X-ray fluorescence spectrometry (WDXRF), with its high spectral resolution, to monitor the photographic process, tarnishing and restoration of pristine Daguerreotypes made in the traditional 19th century fashion. The elemental changes upon restoration via thiourea treatment, were evaluated and compared to a method based on bulk electrolysis. The WDXRF system was found to successfully resolve all lines of interest including in the mercury-bromine-gold region. Sulfur, one of the main components of tarnish, was removed efficiently from the surface of the Daguerreotype when restored by electro-cleaning. Restoration by thiourea immersion removed sulfur from the surface of the Daguerreotype to a point at which the residual sulfur was equivalent in content to the residue left after gilding (i.e. from thiosulfate). Thiourea treatment was found to remove a significant amount of gold from the surface of the Daguerreotype, which may explain the sometimes observed dullness imparted to the Daguerreotype after restoration with thiourea. Also, Daguerrian measles were found to accumulate on thiourea treated plates versus those cleaned by electro-cleaning which ultimately negatively imparts further artefacts to the image after restoration.