Laser-Induced Breakdown Spectroscopy for the Analysis of 150-Year-Old Daguerreotypes

Abstract

Laser-induced breakdown spectroscopy, or LIBS, has been used to characterize the elemental composition of the surface and interior of nineteenth-century, metal-based photographs called daguerreotypes. A surface plasma was produced by the infrared beam of a nanosecond Nd:YAG laser and the emitted light was dispersed in a monochromator to yield the spectroscopic identification of elements. Major components of the daguerreotype surface included silver, copper, and gold. The gold was only present on the surface, as evidenced by its absence after the first one or two laser shots. Minor components such as silicon, aluminum, calcium, and magnesium reflect contaminant particles arising primarily from the cover glass. Several “colored” or “tinted” daguerreotypes were analyzed and the pigments identified. Depth profiling, accomplished by monitoring the spectra as a function of multiple laser shots, allowed the thickness of the silver layer to be measured as the ablation laser “drilled” into the surface. The LIBS technique is shown to be useful in understanding the composition of daguerreotypes and for identifying the specific pigments used by the photographer. These data also provide a scientific basis for attempts to optimize laser cleaning of tarnished daguerreotypes and may also provide clues to the provenance of particular examples.