This is a direct-positive process, creating a highly detailed image on a sheet of copper plated with a thin coat of silver without the use of a negative. The silver-plated copper plate was polished until the surface looked like a mirror, then it was sensitized in a closed box over iodine until it took on a yellow-rose appearance. The plate, held in a lightproof holder, was then transferred to the camera. After exposure to light, the plate was developed over hot mercury until an image appeared. To fix the image, the plate was immersed in a solution of sodium thiosulfate or salt and then toned with gold chloride.
If the image is exposed to air, the silver in the plate can tarnish and impair the visibility of the image.
George Eastman House Video Series: Daguerreotype