Gelatin Silver Process

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 a support such as glass, flexible plastic or film, baryta paper, or resin-coated paper. These light-sensitive materials are stable under normal keeping conditions and are able to be exposed and processed even many years after their manufacture. This is in contrast to the collodion wet-plate process dominant from the 1850s–1880s, which had to be exposed and developed immediately after coating.

The gelatin silver process was introduced by Richard Leach Maddox in 1871 with subsequent considerable improvements in sensitivity obtained by Charles Harper Bennet in 1878. Research over 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 silver halides in the presence of free silver atoms. An 'amplification' of the latent image occurs as the silver halides near the free silver atom are 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 thiosulphate or ammonium thiosulphate, and then the negative or print must be washed in clean water. The final image consists of metallic silver embedded in the gelatin coating.

All gelatin silver photographic materials are subject to deterioration. The silver particles that comprise the image are susceptible to oxidation, leading to yellowing and fading of the image. Poor processing can also result in various forms of image degradation, due to residual silver-thiosulfate complexes. Toning increases the stability of the silver image by coating the silver image with a less easily oxidized metal such as gold, or by converting portions of the silver image particles into more stable compounds, such as silver selenide or silver sulfide.

Gelatin silver printing was the dominant photographic process from introduction in the 1880s until the 1960s when it was eclipsed by consumer color photography. The gelatin silver or black-and-white print is thus a primary form of visual documentation in the 20th century. Its widespread use in applications as wide-ranging as fine art, snap shots, and document reproduction led to an extraordinary variety of papers with a wide range of available surface texture and gloss, and paper thickness.