Ironically, the core purpose of portrait photography—inscribing identity in an “irrefutable assertion of existence,” as theorist Roland Barthes noted in Camera Lucida—is often rendered defunct by decades of damage to the physical image.

Such was the case with two 19th-century daguerreotypes housed at the National Gallery of Canada (NGC). Obscured by tarnish and miscellaneous defacements, the plates offered no trace of the images they had once held, which is how they came to be slated for inclusion in a new study of daguerreotype degradation.

When PhD student Madalena Kozachuk of Western University in London, Canada, tested the plates using a process known as rapid-scanning, synchrotron-based micro-X-ray fluorescence, however, she found herself face-to-face with two anonymous figures, a man and a woman whose images had been previously lost to time.

According to a press release, Kozachuk and a team of researchers from Western are the first to use light to peer past daguerreotype damage. Their findings are detailed in a June Scientific Reports article.

Science News’ Katherine Bourzac reports that the researchers used a particle accelerator known as a synchrotron to scan the plates with high-energy X-ray beams and unearth their chemical makeup. Traces of mercury enabled the team to map the contours of the original snapshots and produce digital copies of them. The process of scanning each 8-by-7-centimeter plate was lengthy, requiring about eight hours per square centimeter.

“The image is totally unexpected because you don’t see it on the plate at all. It’s hidden behind time,” Kozachuk said in the statement. “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 table cloth.”

Daguerreotype photography dates back to the 1830s, when French artist and chemist Louis Daguerre invented the groundbreaking, albeit unwieldy, process. Using silver-coated copper plates treated with iodine vapor to increase their sensitivity to light, early practitioners were able to craft images that directly reflected reality.

As subjects sat unmoving for several minutes, their images were exposed to the plates, which were then developed using heated mercury vapor and a gold chloride solution. The final product, Bourzac explains, relied on the formation of silver-mercury-gold particles at points where light had struck the plate during the portrait sitting. At the conclusion of the process, the image was inscribed directly onto the plate, creating a singular representation distinct from later snapshots produced using photographic negatives.

Kozachuk began her project with little hope, or even thought, of recovering the daguerreotypes. According to the Globe and Mail’s Ivan Semeniuk, she initially mapped the plates’ distribution of copper, silver, gold and iron at the Canadian Light Source facility in Saskatchewan. The laboratory did not possess a beam with sufficient energy to track mercury on the plates, so Kozachuk turned to the synchrotron at Cornell University. Here, the two plates revealed their contents with startling clarity.

“When the image became apparent, it was jaw-dropping,” Kozachuk tells Bourzac.

The researchers’ findings offer a powerful tool for the study of daguerreotype photography. Now, scientists and art conservationists will able to recover lost images when cleaning is impossible.

“From a historical perspective, having these images now viewable ... opens a whole new area of discovery,” Kozachuk said in a recent interview with the London Free Press’ Jennifer Bieman. “You can recover portions of history that either were unknown or were thought to be lost.”