Gelatin-silver process
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The Gelatin-silver process is the photographic process used with currently available black and white films and printing papers. A suspension of silver salts in gelatin is coated onto acetate film or resin coated paper and allowed to dry (hence the term dry plate). These materials remain stable for months and years unlike the 'wet plate' materials that preceded them.
The Gelatin-Silver process was introduced by R. L. Maddox in 1871 with subsequent considerable improvements in sensitivity obtained by Charles Harper Bennet in 1878. Intense research in the last 125 years has lead to current materials that exhibit low grain and high sensitivity to light.
When small crystals (called grains) of silver salts such as silver bromide and silver chloride are exposed to light, a few atoms of free metallic silver are liberated. These free silver atoms form the latent image. This latent image is relatively stable and will persist for some months without degradation provided the film is kept dark and cool. Films are developed using solutions that reduce the free silver atoms. An 'amplification' of the latent image occurs as the silver salts near the free silver atom are also reduced to metallic silver. The strength, temperature and time for which the developer is allowed to act allow the photographer to control the contrast of the final image. The development is then stopped by neutralizing the developer in a second bath.
Once development is complete, the undeveloped silver salts must be removed by fixing, and then the film (or paper) must be washed in clean water. The final image consists of metallic silver embedded in the gelatin coating.
See photographic processes for older methods that have seen something of a come-back recently amongst art photographers.