Edouard de Saint-Ours clearly described the fascinating work he and his colleagues have done to identify the source of the colors in one of the earliest color photographic processes. In 1848 Edmond Becquerel successfully produced a color photographic image, but himself was unable to identify the cause of the colors. The discovery of several of his early plates in the archives at the National Museum of Natural History in Paris sparked Edouard and his colleagues’ interest in Becquerel’s process and the source of his colors.
Edouard began by explaining the two known ways in which color can be produced in photography: through the use of colorants, or through the production of interference colors. It was assumed that Becquerel had been relying on one of these two types of color, and the research team focused on methods of analysis that would identify either of these two methods of producing color.
Becquerel’s photochromatic images were made by dispersing sunlight through a prism for several hours, exposing the plate in camera to form a direct positive. The images were not fixed, and will fade if exposed to light. In order to understand the physical and chemical composition of the Becquerel plate, Edouard and his colleagues replicated the technique themselves. To make a photochromatic image a silver plate was polished and cleaned, and sensitized by immersion in copper chloride, or by hydrolysis in a bath of hydrochloric acid. The latter is referred to as an electrochemically sensitized plate. Once sensitized, the plate takes on a red-brown hue. In the replication of the process the plates were exposed to a Xenon lamp with colored filters, and the colors produced on the plate corresponded to the color of the light.
Once they had replicated the technique, they set about studying their sample plates in order to identify the cause of the colors they had produced. SEM analysis and cross-sectional analysis showed that there were no surface or structural differences between the different colors. Although this suggested against interferential colors, it did not rule out the possibility entirely.
SEM-EDX offered the researchers more information about the chemical composition of the different colors, but also indicated no difference between the green and red colors on the sample plate. Both were almost entirely comprised of silver chloride. However, Edouard mentioned the very interesting possibility that very small variations in the proportion of silver could cause different sizes of silver nanoparticles to form on the plates. In this scenario, a different size of nanoparticle would form from each color of light, and the color of the silver nanoparticles would vary depending on their size.
From this hypothesis, the researchers performed spectroscopic analysis of the colored surfaces, a technique which can detect the chemical state of an element. However, this analysis showed only oxidized silver on all colors, with no indication of difference between colors, or the presence of metallic silver. Again, this suggests against the presence of silver nanoparticles, but does not definitively rule out that possibility.
Although the project has not returned any definitive results, the research is ongoing. In the meantime, the work has cast light on the complexity of Becquerel’s early process, and the intriguing questions still presented by early color photography.