*2.1. The Daguerreotype Plate and Cleaning Solutions*

The daguerreotype plate under investigation, "Little Girl, Pretty Purse", was provided by the Canadian Conservation Institute, National Gallery of Canada (Ottawa, Canada). The reagents used for the chemical cleaning portion included reagent grade ammonium hydroxide (NH4OH, Caledon Laboratory Chemicals) and reagent grade sodium thiosulfate Na2S2O3, 99% assay (Sigma Aldrich). Solutions containing specific reagent concentrations were prepared, including a sodium thiosulfate solution of 3% mass/volume (0.190 M) and 1% and 0.5% mass/volume of ammonium hydroxide (0.294 M and 0.147 M, respectively). Additionally, 0.01 M and 0.1 M reagent grade potassium chloride (KCl BioShop), and 0.01 M reagent grade potassium sulfate (K2SO4, 99% assay, Caledon Laboratory Chemicals) solutions were created for the electrochemical cleaning process. All of the dilution schemes and rinsing processes were done with Type 1 water (18.2 MΩ × cm resistivity).
