In daguerreotype photography, the first commercially successful photographic process, a positive image is recorded directly onto a silvered copper plate. Although mercury is traditionally used to develop the plate, there’s a way of creating daguerreotypes called the Becquerel method that eschews mercury in favor of non-lethal ingredients. According to Contemporary Daguerreotypes,
A polished silver plate is sensitized with iodine vapor. After the sensitized plate is exposed to light in a camera, the image will develop if the plate is further exposed to bright light through a red or amber filter. He called this the action of “continuation rays.” The curious aspect is you can watch the image form much like a Polaroid. Depending on how the subject of the image, how the plate was prepared and the development time, Becquerel images can be indistinguishable from mercury developed plates.
Did you catch that? The mysterious process uses sunlight to magically develop the images. In the video above, photographer Jerry Spagnoli shows how the Becquerel method is done, from start (polishing a piece of metal) to finish (a great looking photo).