INVESTIGATION OF A SURFACE TARNISH FOUND ON 19TH-CENTURY DAGUERREOTYPES
LEE ANN DAFFNER, DAN KUSHEL, & JOHN M. MESSINGER
6 SCANNING ELECTRON MICROSCOPE
Due to the delicate nature of the daguerreotype surface and the extremely thin tiers of tarnish and oxidation, it was necessary to use methods of analysis that are both nondestructive and suitable for thin-layer analysis. The first choice was scanning electron microscopy (SEM). The surface topography of daguerreotype plates has been well documented with SEM (Swan et al. 1979; Barger and White 1991), and this method is available to many conservators. Small plates fit inside the vacuum chamber of the scanning electron microscope, with no harm to the artifact.
Daguerreotype plates (figs. 3, 4) were examined with a Hitachi Instruments, Inc. S-800 field emission scanning electron microscope equipped with a Princeton Gamma Tech IMIX microanalysis system for energy dispersive x-ray analysis. The following x-ray spectra results were obtained in one area unassociated with the fluorescence and two areas associated with fluorescence: (1) Spectra from areas of the plates that did not fluoresce under UVC irradiation showed one substantial silver peak. Smaller peaks of gold and mercury were also present. (2) Spectra obtained from an accretion present at the center of a fluorescent ring showed one substantial peak of copper, with smaller peaks of silver, chlorine, and iron. (3) Spectra from the fluorescing ring showed one substantial peak of silver, accompanied by smaller peaks of copper. Trace peaks of iron and mercury were also indicated.
These results suggested that the unknown fluorescing material is primarily composed of silver and copper. With the exception of the trace iron elements, these inorganic elements comply with the typical daguerreotype plate structure.
Swan et al. (1979) singled out similar spots during their examination of daguerreotypes using SEM. They described “thin orange and iridescent films” and rings of orange films containing copper and traces of iron, surrounded by a clear ring, then a second wider ring of orange tarnish that also contains copper. The rings and films Swan et al. analyzed using SEM have spectra similar to the fluorescing areas investigated in this study.