Laser desorption mass spectroscopy has been used to characterize both modern and ~ 150-year-old daguerreotypes. Such investigations are a necessary prelude to attempts to clean them of tarnish and other contaminants by laser ablation of the surface layers. Both positive- and negative-ion time-of-flight spectra were obtained following YAG laser ablation/desorption at 1064, 532, and 355 nm. Major peaks obtained from several daguerreotypes reveal expected elements from the substrate (Ag, Cu) as well as the developing (Hg) and gilding (Au) processes. Silver clusters (Agn) may reflect surface desorption of molecules or, alternatively, aggregates formed in the ejection process. Silver sulfide molecules observed from old daguerreotypes are the signature of the tarnishing process.
Danel L. Hogan, Valerie V. Golovlev, Michael J. Gresalfi, John A. Chaney, Charles S. Feigerle, John C. Miller, Grant Romer, and Paul Messier, "Laser Ablation Mass Spectroscopy of Nineteenth Century Daguerreotypes," Appl. Spectrosc. 53, 1161-1168 (1999)