The Development and Application of Instrumental Methods for the Identification of Materials and Processes Used in the Manufacture of Orotone, Hand-Colored Orotone, and Silvertone Photographs

Abstract

Orotone photographs are positive photographic images on glass with a gold-colored metallic coating or backing. Although the materials and process used in the production of orotones have been previously documented, there is limited published scientific research on the subject. This study focused on expanding those published works by analyzing a greater number of photographs, including eleven orotones, four hand-colored orotones, two silvertones, and one uncoated orotone. The photographs, dating from the early to mid-20th century, were from the University of Washington (UW) Libraries' collection, except for one orotone that was from the Portland Art Museum. X-ray Fluorescence (XRF) spectroscopy, Fourier Transform-Infrared (FT-IR) spectroscopy, Raman spectroscopy, Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS), and Pyrolysis/Gas Chromatography/Mass Spectrometry (Py/GC/MS) were carried out to identify the materials used in the production of these photographic types. XRF analysis of the orotones revealed that copper and zinc alloys were used in the backing of all the orotones; silver was the imaging material of most of the photographs; there was sulfur and mercury toning of two images; some glass supports contained lead and arsenic; and other detected elements (e.g., potassium, iron, and bromine) remained from the photographic process. Py/GC/MS confirmed that one of the orotones had a gelatin emulsion and a previously unknown backing solution. The backing layer consisted of acrylate polymers and metal flake, which, given the presence of that unusual orotone polymer, may have contributed to the observed delamination of the emulsion and backing layers in that orotone. Analysis of the hand-colored orotones by XRF determined that vermilion and a possible mixture of cadmium yellow and a blue pigment were used as colorants. FT-IR and Raman analysis confirmed that Prussian blue was also used as a pigment. Lastly, XRF spectroscopy and SEM/EDS confirmed that an aluminum metal flake was used in the backing of a silvertone. Analysis of these photographs by a range of instrumental techniques helped increase the body of knowledge on orotones, hand-colored orotones, and silvertones, and this was possible from the collaboration between the Lasseter Clare Lab and the University of Washington Libraries.