From The Art and Popular Culture Encyclopedia
The daguerreotype was the first commercially successful photographic process. The image is a direct positive made in the camera on a silver plated copper plate. The raw material for plates was called Sheffield plate, plating by fusion or cold-rolled cladding and was a standard hardware item produced by heating and rolling silver foil in contact with a copper support.
The surface of a daguerreotype is like a mirror, with the image made directly on the silvered surface; it is very fragile and can be rubbed off with a finger, and the finished plate has to be angled so as to reflect some dark surface in order to view the image properly. Depending on the angle viewed, and the color of the surface reflected into it, the image can change from a positive to a negative.
The very first daguerreotypes used Chevalier lenses that were slow, and the light sensitive material was silver iodide made by fuming the the plate with iodine vapour. This meant that the exposure in the camera was too long to conveniently take portraits, and the first subjects taken were street scenes and architectural studies. When Petzval lenses were introduced, with lenses of a larger diameter and the plate was sensitized with iodine and bromine forming light sensitive crystals of silver iodide and silver bromide, the exposures were reduced so that portraits could be taken. Usually, it was arranged so that the sitters leant their elbows on a support, or else head rests that did not show in the picture were used to help the sitters sit motionless, and this led to most daguerreotype portraits having stiff, lifeless poses. There were exceptions with lively expressions full of character by photographers who saw the potential of the new medium, and these are represented in museum collections and are the most sought after by private collectors today.
Dagguerreotypes were mounted in cases under glass with a cover, or else in a frame that could be hung on a wall. Photographic processes that were invented soon after: ambrotypes and tintypes were mounted in similar cases, but were made by the later wet plate process using collodion on glass or on a bitumen coated iron plate. These can be distinguished from daguerreotypes by the image quality. The polished silver surface of a daguerreotype gives a feeling of presence where the image appears to be floating in space.
The process was developed by Louis Daguerre together with Joseph Nicéphore Niépce. Niepce had produced the first photographic image in the camera obscura using asphaltum on a copper plate sensitised with lavender oil that required exposures as long as eight hours.
The image in a daguerreotype is often described as being formed by the amalgam, or alloy, of mercury and silver because mercury vapor from a pool of heated mercury is used to develop the plate; but using the Becquerel process (using a red filter and two-and-a-half stops extra exposure) daguerreotypes can be produced without mercury, and so the actual composition of the crystals that form the image is not clear.
Exposure times were later reduced by sensitising the plate with other silver halides: silver bromide and silver chloride, and by replacing the Chevalier lenses with much larger, faster lenses designed by Joseph Petzval. A reduction in camera size and the size of the image will always result in more light reaching the image plane and consequently reduced exposures, and a small camera that produces small images was made by Voigtländer.
The image is formed on the surface of the silver plate (resembling the surface of a mirror) and is unstable; it can easily be rubbed off and will oxidize in the air, so from the outset daguerreotypes were mounted in sealed cases or frames with a glass cover.
When viewing the daguerreotype, a dark surface is reflected into the mirrored silver surface, and the reproduction of detail in sharp photographs is very good, partly because of the perfectly flat surface.
Although daguerreotypes are unique images, they could be copied by redaguerreotyping the original.