A Different Photographic Process: Cuprotypes and Their Chemistry

Abstract

Cyanotypes are almost the last vestige of photographic chemistry in high school and college chemistry courses, with good reason as the process is simple and reliably produces striking results. For instructors looking for something fresh and different, this article describes a recently invented photographic process that in addition to producing beautiful images is complex enough to have interesting chemistry. The “new cuprotype process” needs three reagents (ferric ammonium citrate, copper sulfate and sodium thiosulfate) to produce a yellow-brown image. Toning with iron(III) ferricyanide changes the color to red-brown due to formation of copper(II) ferrocyanide, an analog of the blue iron(III) ferrocyanide produced by the cyanotype process. This article describes a two-week experiment tested by undergraduate nonscience majors, each part taking up 2 h of laboratory time; a UV lightbox is necessary equipment for both. In the first part, students make a cuprotype. In the second, students working in groups disentangle the chemistry by looking at the effect of light on four subsystems brushed on squares of paper; they then test whether thiosulfate dissolves the products of the reaction. Groups discuss their results with the goal of identifying subsystems that do and do not contribute to the final image and are interviewed orally about their conclusions and the experimental evidence on which they were based. All groups in the two tested iterations of the experiment met this challenge successfully. This second part also uncovers simplifications of the cuprotype process that can be pursued as extensions or independent projects. Laboratory reports reinforce basic chemistry skills such as balancing reactions, determining ion charges, identifying oxidation and reduction, and accounting for dilution.