President Abraham Lincoln's 2nd Inaugural Poster

The daguerreotype 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 process required great care. The silver-plated copper plate had first to be cleaned and polished until the surface looked like a mirror. Next, the plate was sensitised 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. Exposure times for the earliest daguerreotypes ranged from three to fifteen minutes, making the process nearly impractical for portraiture. Modifications to the sensitisation process coupled with the improvement of photographic lenses soon reduced the exposure time to less than a minute. Although daguerreotypes are unique images, they could be copied by redaguerreotyping the original. Copies were also produced by lithography or engraving. Portraits based upon daguerreotypes appeared in popular periodicals and in books.