How a particle accelerator helped recover tarnished 19th century images

Researchers used an accelerator called a synchrotron to produce strong, but nondamaging beams of X-rays to scan the damaged photographs, called daguerreotypes, and map their chemical composition. This allowed chemist Madalena Kozachuk of Western University in London, Canada, and colleagues to trace mercury deposits in the plates and create digital copies of the hidden images, the team reports June 22 in Scientific Reports. One image revealed a woman; the other, a man who had been completely obscured by tarnish.
An early form of photography, daguerreotypes were popular from the 1840s through the 1860s. Photographers crafted the images by making a silver-coated copper plate and treating it with iodine vapor to generate a light-sensitive surface. Subjects sat still for the several minutes required to expose the plate and create an image. Then photographers treated the plate with heated mercury vapor and a gold solution to develop the image, forming tiny silver-mercury-gold particles where light struck the plate during the exposure process. These particles make up the image, reflecting white light. Lighter parts of an image, such as the woman’s hands and collar, have a higher density of these particles.
The researchers used mercury to map the contours of the original images, because that metal remains fixed in place under years of cloudy tarnish. The scans revealed where the original particles were, letting researchers reconstruct the image.
Scanning the roughly 8-by-7-centimeter daguerreotypes, provided by the National Gallery of Canada, was time-consuming, taking about eight hours per square centimeter.