Gelatin-silver process
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The '''gelatin-silver process''' is the [[photography|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 fiber-based 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 led 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'' in [[sodium thiosulfate|sodium thiosulphate]] or [[ammonium thiosulphate]], 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 comeback recently amongst art photographers.
==References==
[http://www.alternativephotography.com/process_dryplate.html Silver gelatin dry plate process]
[[category:photographic processes]]
[[pt:Processo da prata coloidal]]
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