ADVANCES Science reveals hidden daguerrotype images

10 July 2018

SIG: Imaging Science

Conventional photograph (left) and XRF reconstructed image (right ) of a corroded daguerrotype plate. Image by Madalena Kozachuk/Western University (CC-BY)

Large numbers of daguerrotype images have been lost over time due to tarnishing and corrosion. However, exciting new research published in Nature (click here for the original paper) offers a non-destructive method for finding out what is hidden beneath years of damage.

X-rays have been used for decades to find hidden images in paintings and similar objects. However, daguerrotypes are made using silver-coated copper plates that block normal X-rays. A team at the University of Western Ontario in Canada have made use of X-ray fluorescence (XRF) to extract the images. You will be familiar with the idea of fluorescence - you shine ultraviolet 'black' light onto something and it glows with a visible colour. If you use X-rays of the right energy, it makes metals fluoresce in different X-ray 'colours' that can be picked up by a special detector. Each metal gives off a specific wavelength of X-rays.

In the original daguerrotype process, the image is made by developing the exposed plate using mercury vapour. This forms an amalgam (a bit like an old tooth filling) with silver particles in the coating on the plate that form the image. So, find the mercury and you've found the image.

The team scanned daguerrotype plates placed in a research X-ray facility called CHESS. Using a 20-micron square sensor, the scanned image was built up pixel by pixel in a process that takes over eight hours. A sophisticated algorithm then isolates the signal strength due to mercury in each pixel, enabling images to be revealed that haven't been seen in decades. Best of all, the process is non-destructive so no damage is done to the (possibly priceless) original.

Many museums and historical collections around the world must be considering the amazing process, so who knows what treasures might be discovered.