Nineteenth-century paper-based photographic images, in their simplest forms, consist of metallic nanoparticles embedded in a cellulose matrix, where the properties of these particles determine the aesthetic tonality of the print. As in modern nanochemistry, traditional chemical photographic printing methods require tightly controlled reaction conditions to influence product morphology and access specific optical properties. For example, recent work has shown that the tone of a platinum-palladium print, ranging from sepia to neutral gray, stems from the size and distribution of the image material, which is a direct result of the moisture content of the paper during printing. The analogous silver-based process, the salted paper print, naturally produces prints with a reddish-brown coloration. Photographers quickly learned to manipulate the tones of these photographs with the use of precious metal baths containing gold or platinum salts. A suite of silver prints toned with platinum and/or gold was generated following historic recipes and analyzed by X-ray fluorescence spectroscopy, scanning electron micrography, and colorimetry to investigate the interplay between metal content and perceived tone. Thin sections of the samples were also prepared by focused ion beam milling and imaged using scanning transmission electron micrography to visualize the metal particles <i>in situ</i>. Prints toned in an acidic platinum bath undergo galvanic replacement of silver for platinum, converting nanoscale aggregates of silver to evenly distributed bimetallic particles that impart a neutral gray hue. Those toned in an alkaline platinum bath experience subsequent metal redeposition, which increases the print’s optical density. Detailed analysis of laboratory-created samples and comparison to other plain paper processes and variants, including platinum prints and silver-platinum prints, leads to insights into traditional photographic printmakers’ working methods and can help direct the long-term care of these objects. Additionally, understanding the implications of these synthetic methodologies for current nanotechnology fields, such as paper-based sensors and supported nanocatalysts, is an important consequence of this work.

Nanochemistry of silver and platinum in nineteenth-century photography

254th National Meeting (2017)

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