Plate
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| Two packs of Kodak plates. These packs are dated (in pencil) to 1953 and 1956. image by Nicholas Middleton (Image rights) |
Plates are a form of photographic medium, which may broadly be described as the predecessor of film. The term plate refers to the fact that the photographic emulsion is supported on a sheet (or plate), usually of glass.[1] Even when plates were poured by the photographer, the glass had to be in standard plate sizes to fit the plate holders.
Wet plates
The first use of glass plates was as the support for the emulsion in the wet collodion process, which replaced negatives made on paper in earlier processes (the Calotype and variations on it). The collodion emulsion had to be spread on the glass plate, made sensitive, exposed and developed before the collodion dried. This is therefore often called a wet-plate process. The photographer would require either a darkroom adjacent to the studio, or a portable dark-tent if working in the field, to prepare the plates. There were several variations in the collodion material itself, and in the light-sensitive components added to it. These and the glass plates themselves were increasingly made available as factory-prepared products.
Just as the photographer was expected to pour his or her own plates, photographic manuals from the wet-plate period describe methods for the photographer to carry out other supporting work such as cleaning the collodion from failed exposures for the glass plate to be reused, checking the purity of bought chemical supplies, and recovering the valuable silver from used chemicals.
Even early wet plates were much more sensitive than the paper-negative media. They were also much less convenient, however. The need to prepare each plate singly before exposure was a great constraint. At the least, it limited the speed of work. For fieldwork, it was also inconvenient to carry any number of heavy, fragile glass plates. A large amount of equipment was needed to prepare and pour the emulsion, although manufacturers put great effort into making photographic field equipment more convenient; discussing early dry plate alternatives, Smith (1866) describes a dark-tent two feet wide, one tall and one deep, which may also be used as the camera.[2] There was not a sharp changeover from paper media to plates; both were in use for some years, because both had advantages, and also perhaps because individual photographers chose to continue using the process they knew.
Dry Plates
Since the late 19th century, the term plate has been more or less synonymous with dry plate. As early as 1866, Smith describes two methods of preparing dry collodion plates; wet plates, sensitized and then allowed to dry to a controlled extent, with a protective coating to prevent them from drying fully.
These were somewhat slower (i.e. less sensitive) than wet plates, but overcame some of their main disadvantages: none of the plate-pouring apparatus was required in the field; a number of dry plates loaded into holders allowed a series of exposures in rapid succession; and a stock of plates could now be prepared in advance. Dry plates were available factory-made.
Later, the use of albumen (the protein material in egg-white) or gelatine (again, animal protein, obtained from rendering of animal bones or hide) in place of collodion led to more sensitive dry plates.
