A team of scientists led by Western University in London, Ontario, Canada has revealed how art curators can use light to recover images preserved on otherwise damaged daguerreotype silver plates. The research, published in Scientific Reports – Nature, includes two images from the National Gallery of Canada’s photography research unit that were no longer visible because of tarnish and other damage.

“It’s somewhat haunting because they are anonymous and yet it is striking at the same time,” says Madalena Kozachuk, lead author and a PhD student in Western’s Department of Chemistry. “The image is totally unexpected because you don’t see it on the plate at all. It’s hidden behind time…but then we see it and we can see such fine details—the eyes, the folds of the clothing, the detailed embroidered patterns of the tablecloth.”

Kozachuk and her interdisciplinary team have used synchrotron technology for the past three years to learn more about chemical changes that damage daguerreotypes . The group conducted much of its research at the Canadian Light Source (CLS) and the Cornell High Energy Synchrotron Source, publishing a series of results in scientific journals in 2017 and earlier this year. In those articles, the team members identified the chemical composition of the tarnish and how it changed from one point to another on a daguerreotype.

Daguerreotype images were created in 1839 using a highly polished silver-coated copper plate that was sensitive to light when exposed to an iodine vapor. Subjects had to pose without moving for two to three minutes for the image to imprint on the plate, which was then developed as a photograph using a heated mercury vapor.

“We compared degradation that looked like corrosion versus a cloudiness from the residue from products used during the rinsing of the photographs during production versus degradation from the cover glass,” says Ian Coulthard, a senior scientist at the CLS and one of Kozachuk’s co-supervisors. “When you look at these degraded photographs, you don’t see one type of degradation.”

Kozachuk used rapid-scanning micro-X-ray fluorescence imaging to analyze the plates, which are about 7.5 cm wide, and identified where mercury was distributed. With an X-ray beam as small as 10×10 microns (a human scalp hair averages 75 microns across) and at an energy most sensitive to mercury absorption, the scan of each daguerreotype took about eight hours.

“Mercury is the major element that contributes to the imagery captured in these photographs,” says Tsun-Kong Sham, Western’s Canada research chair in materials and synchrotron radiation and co-author of the research. “Even though the surface is tarnished, those image particles remain intact. By looking at the mercury, we can retrieve the image in great detail.”

The team says this research will not only contribute to improving how daguerreotype images are recovered in cases where cleaning is possible but also provide a way to see what’s below the tarnish if cleaning is not possible.

“There are a lot of interesting questions that at this stage of our knowledge can only be answered by a sophisticated scientific approach,” says John P. McElhone, who provided the daguerrotypes after becoming intrigued by the research before retiring as the chief of the Conservation and Technical Research branch at the Canadian Photography Institute of the National Gallery of Canada. “A conservator’s first step is to have a full and complete understanding of what the material is and how it is assembled on a microscopic and even nanoscale level. We want to find out how the chemicals are arranged on the surface and that understanding gives us access to theories about how degradation happens and how that degradation can possibly or possibly not be reversed.”

As the first commercialized photographic process, the daguerreotype is thought to be the first “true” visual representation of history, supplanting the “poetic license” of paintings with a precise reflection of what was photographed. Experts believe that perhaps millions of daguerreotypes were created over 20 years in the 19th century before the process was replaced. The Canadian Photography Institute collection numbers more than 2,700, not including the daguerreotypes in the institute’s research collection.