Investigation and Optimization of Electrochemical Treatment for Daguerreotypes

Abstract

Daguerreotypes, nineteenth century photographs made entirely of metal, tarnish easily when exposed to atmosphere or other corrosive environments. The removal of such corrosion products has always been a controversial issue due to the irreversibility and potential damage inflicted by the treatment process. When performed using the proper technique and tools, electrochemical cleaning has been shown to remediate daguerreotype corrosion without causing damage to the object. This investigation explores in detail the effects of electrochemically treating daguerreotypes to better comprehend the physical, chemical, and aesthetic changes which occur during tarnish remediation. Such analysis includes understanding the extent to which tarnish is removed from the object, whether the microstructure and surface roughness are altered, and whether deposits are formed on the surface as the result of cleaning. Before characterizing the effects of treatment, a reliable and consistent electrochemical cleaning method was optimized on modern, custom-made daguerreotype coupons. These coupons were tarnished in controlled settings to create silver sulfide or silver oxide, two commonly found daguerreotype corrosion products. They were then treated by the optimized electrochemical cleaning process, which involved immersing the object in a 0.1 M NaNO3 electrolyte and connecting it to a platinum counter electrode, an Hg/Hg2SO4 reference electrode, and a potentiostat. Tests were executed to determine the most effective voltage for removing each corrosion product. Full characterization of the modern coupons was performed prior to corrosion, after corrosion, and after electrochemical remediation to provide full understanding of the treatment effects. Characterization techniques include scanning electron microscopy/energy dispersive x-ray spectroscopy, Raman spectroscopy, atomic force microscopy, x-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The optimized process was also tested on three nineteenth century daguerreotypes, and the changes fully characterized and compared to those of modern samples. Both silver sulfide and silver oxide were noticeably reduced as the result of electrochemical cleaning, while the treatment process did not alter the physical and chemical features of the daguerreotype image particles or silver base layer. The results provide a much better understanding of the electrochemical cleaning process on a chemical and microscopic level, allowing conservators to make informed, scientifically-supported treatment decisions for their clients and institutions.