Silver gelatin photographs have been produced since the 1840s.
Before the "color revolution" in photography, almost all photographs (including snapshots) were gelatin silver prints. These types of photographs exploited the reaction that occurs between silver salts and light to print images from film onto specially treated paper. This process was widely adopted in the 1890s and remained popular, especially in art photography, until film photography was largely replaced by digital photography.
Layers
There are four layers that make up a silver gelatin photo print. There is a paper base on the bottom, made of rags originally, but made of wood pulp after World War I. On top of that is a "baryta," which is a white coating made of barium sulfate suspended in gelatin. Above that is a gelatin binder that holds silver particles. And on top is a hardened gelatin layer, called the overcoat (or supercoat or topcoat), that protects the print surface.
Silver
The silver that is present in a silver gelatin print is in silver salt particles of about 0.5 micrometers in diameter. The paper appears to be white, but when light reacts with the silver salt particles, each bit of silver turns into a dark speck. All of the dark specks create an image when light is filtered through photographic film onto the photographic paper. The paper is then rinsed clean of the silver particles that did not react with light.
Gels
Gelatin is used in these prints because it both protects the photographs when dry, and swells when immersed in developing chemicals. This allows the silver to react with the developers when the artist chooses, and then stop reacting when the photograph is no longer exposed to developer. Gelatin is an organic substance, made from animal skin and bones.
Baryta
The baryta layer that sits between the silver gel and the paper base is only present to provide a smooth white surface. In art photography, the baryta layer can have added dyes to bring out certain colors. It can also have an embossed surface to provide an additional layer of texture to the print.
Black & White Film
When taking a picture on black and white film, the image is an inverse of the image that will be printed. The parts of the image that are especially dark will not allow light to pass through the film, and so light will not fall on the photographic paper. Black and white film is an important part of producing silver gelatin photographs because it allows for sharper resolution in the images than other types of film.
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