Surface characterization of Austrian daguerreotype portraits

Abstract

Six daguerreotypes from Austrian collections dating from 1841 to 1850 were examined using digital light microscopy, scanning electron microscopy and µ-X-ray fluorescence. This enabled the observation of corrosion states, surface features due to different production steps, possible conservation interventions and storage conditions. The morphology and size distribution of Ag-Hg and Au-Hg-Ag amalgam platelets and image nanoparticles were studied. The platelets ranged in diameter from 1 to 2 µm. The image particles featured a crystalline morphology with diameters of 200–400 nm. The Ag coating grain size was 40–160 nm. Long-term exposure of the daguerreotype surface to fingerprint sweat caused the transformation of local Ag surface areas into AgCl crystallites with sizes of the order of µm. interestingly, in two samples, the pupils of the eyes were scratched with a needle, which was intended to increase the three-dimensionality of the eyes. In one case, corrosion caused the Ag layer to detach from the copper substrate, creating a crater on the exposed Cu substrate surrounded by the intact Ag surface. In another case, a greenish corrosion product of basic Cu carbonate with increased volume occurred.