Gelatin-silver process
Encyclopedia
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.
is utilized in molecular biology to visualize DNA or proteins after gel electrophoresis
, usually SDS-PAGE
. The latent image is formed by the DNA or protein molecules (i.e. the reduced silver selectively precipitates onto those molecules). It is known for being nearly as sensitive as autoradiography, the "gold standard" technique, but one not widely used due to the use of radioactive materials.
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
process used with currently available black-and-white films and printing papers. A suspension of silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
salts in gelatin
Gelatin
Gelatin is a translucent, colorless, brittle , flavorless solid substance, derived from the collagen inside animals' skin and bones. It is commonly used as a gelling agent in food, pharmaceuticals, photography, and cosmetic manufacturing. Substances containing gelatin or functioning in a similar...
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
Richard Leach Maddox
Richard Leach Maddox was an English photographer and physician who invented lightweight gelatin negative plates for photography in 1871.-Early life:Richard Leach Maddox was born at Bath, England, on August 4, 1816....
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
Silver bromide
Silver bromide , a soft, pale-yellow, water insoluble salt well known for its unusual sensitivity to light. This property has allowed silver halides to become the basis of modern photographic materials. AgBr is widely used in photographic films and is believed by some to have been used for making...
and silver chloride
Silver chloride
Silver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water . Upon illumination or heating, silver chloride converts to silver , which is signalled by greyish or purplish coloration to some samples...
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
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
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
Sodium thiosulfate
Sodium thiosulfate , also spelled sodium thiosulphate, is a colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3•5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.”...
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.
In molecular biology
An essentially identical procedure called "silver staining"Silver stain
Silver staining is the use of silver to selectively alter the appearance of the target.-Use in medicine:It is used to stain histologic sections. This kind of staining is important especially to show proteins and DNA. It is used to show both substances inside and outside cells...
is utilized in molecular biology to visualize DNA or proteins after gel electrophoresis
Gel electrophoresis
Gel electrophoresis is a method used in clinical chemistry to separate proteins by charge and or size and in biochemistry and molecular biology to separate a mixed population of DNA and RNA fragments by length, to estimate the size of DNA and RNA fragments or to separate proteins by charge...
, usually SDS-PAGE
SDS-PAGE
SDS-PAGE, sodium dodecyl sulfate polyacrylamide gel electrophoresis, describes a collection of related techniques widely used in biochemistry, forensics, genetics and molecular biology to separate proteins according to their electrophoretic mobility...
. The latent image is formed by the DNA or protein molecules (i.e. the reduced silver selectively precipitates onto those molecules). It is known for being nearly as sensitive as autoradiography, the "gold standard" technique, but one not widely used due to the use of radioactive materials.