As part of the research for the exhibition, Robert Cornelius: Portraits from the Dawn of Photography (20 Ocotober 1983-22 January 1984, National Portrait Gallery, Smithsonian Institution, USA), approximately half of the extant Cornelius Daguerreotypes, as well as Daguerreotypes by Joseph Saxton and Walter Rogers Johnson, were submitted for scientific analysis. This collection, containing plates dating from 1839 to 1843 and including the oldest photograph made in the United States, is the largest coherent body of the earliest American photographic objects. While the thrust of the analyses was to understand the working methods of Robert Cornelius and to date the plates, this study also presented an opportunity to examine long-term aging and the results of past preservation treatments on Daguerreotypes of known provenance. The varieties of corrosion products affecting Daguerreotypes were followed through the research. In addition, the related effects of deterioration of cover glasses and mats were observed. Further, the characteristic deleterious consequences of both cyanide and thiourea cleaners, as well as the corrosion products associated with their use, were noted. Recommendations for stabilization of Daguerreotypes pending the publication of reliable, non-destructive cleaning methods are given. See also AATA 20-2225.