Photographic plates consist of an image layered on a glass support. Glass plates emerged as a common support medium for photographic negatives in the mid-nineteenth century. In general, black and white photographic negatives are made up of fine silver particles, (or color dyes for color negatives), which are embedded in a thin layer called a binder. Combined with the image substance, these two elements make-up what is called the emulsion. This emulsion layer sits upon what is called the support, which can be paper, metal, film, or as in the case of photographic plates, glass. Depending upon the time period, there can be variants to the binder and thus, the chemistry of the image. This is extremely important with the conservation and restoration of photographic plates because it helps the conservator prevent further chemical reactions. In the case of the Wet Plate Collodion, the image is run under a wash bath to stop the development of the image after exposure. An important part of the photographic process, "fixing," is then used to wash the silver particles that are not part of the image, which then produces a stable negative image. The fix bath will ensure that the remaining silver halide crystals are no longer sensitive to additional light exposure, removing all excess. This negative image can then be used over many years to produce paper positives. Glass plates as a form of support were popular in use between 1851 and the 1920s.
