Setting up a methodology for restoring degraded daguerreotypes using UV lasers and atmospheric non-thermal plasma cleaning techniques

Abstract

Since the early years of the development of daguerreotypes – the first commercial form of photography made public in 1839 – cleaning procedures have been necessary due to their tendency to tarnish. Cleaning daguerreotypes is challenging, and conventional methods often result in undesirable effects or prove impractical, especially for hand-colored plates. In recent decades, advanced techniques, such as laser and plasma cleaning, have been optimized for tarnish removal. However, their effectiveness in removing other substances, such as by-products of previous cleaning treatments, has not been evaluated. To address this issue, we investigated the efficacy of laser and atmospheric non-thermal plasma cleaning on two 19th-century daguerreotypes exhibiting two different degradation conditions: one with typical tarnish, and the other with a complex layer composed of cyanides, calcium carbonate, and an organic compound, likely produced during a previous cleaning attempt. This work reports systematic tests using visible (VIS) and ultraviolet (UV) lasers emitting at four different wavelengths: Nd:YAG (532 nm, 355 nm), KrF excimer (248 nm), and ArF excimer (193 nm). All the lasers operated in the nanosecond regime, except for the KrF excimer laser, which was also tested with femtosecond (500 fs) pulse duration. This is the first report of results using the 248 nm wavelength in both ns and fs regimes for daguerreotype restoration. For comparison, we also applied atmospheric non-thermal plasma cleaning to both types of degradation. The experiments allowed us to assess the advantages and limitations of both techniques, showing that both are suitable depending on the specific surface condition. Additionally, we evaluated a combined cleaning strategy that involved laser and wet cleaning with two different substances – distilled water and an EDTA solution – to optimize the cleaning outcomes. To make an informed selection of the parameters and evaluate treatment efficacy, we characterized the chemical composition and morphology of the daguerreotypes and their degradation products before and after cleaning using optical microscopy, μ-Raman spectroscopy, and SEM-EDS. Besides the identification of the degradation products, the analytical results provided a critical review of previous literature on laser cleaning of daguerreotypes and supported the determination of the best experimental conditions for the safe removal of degradation layers from the surface of daguerreotypes.