“Indigenous Spots”: Microscopic Imaging and Spectroscopic Analysis of Galvanic Halos Associated with Metal Inclusions in the Paper of Mid-19th Century Salted Paper Prints by William Henry Fox Talbot

Abstract Metallic inclusions in the paper substrate of historic salted paper prints are responsible for the formation of light-colored or white image regions immediately surrounding the particle, which are especially noticeable in areas of high image density. These low-density regions or “halos” often appear around a dark central spot. Our work shows this formation is connected with the electrochemical reaction of the metallic inclusion with the silver salt solution applied during the preparation of sensitized photographic paper, suggesting that these blemishes found in early prints can be termed “galvanic halos.” Early photographers were aware of the flaws caused by metal impurities and attempted to purchase the best quality paper to mitigate the problem. An ongoing study of prints in William Henry Fox Talbot’s The Pencil of Nature revealed numerous examples of galvanic halos, prompting an initiative to document and characterize the phenomenon, and an investigation into formation mechanisms. Talbot’s seminal publication was the first commercially produced photographically illustrated book, containing 24 hand-printed photographs. This paper documents observations related to the forty galvanic halos identified and analyzed in the course of this study using X-ray fluorescence (XRF) and Raman spectroscopy, microscopic imaging, and the preparation of simulacra using historic methods. Elemental analysis, including the use of scanning micro- X-ray fluorescence spectroscopy (scanning XRF) has shown that copper, brass, and lead particles all cause the formation of halos.