Rebecca
SGF, Supreme Grumble Framer
Last night I went to a class on the identification of photographs by a very knowledgeable archival conservator. It was great. Here are some things some things that should be added to the photographs 1,2,3 posts.
1) Glass negatives from 1879-1915 almost certainly have a gelatin emulsion, not collodian.
2) There is more than one kind of cellulose nitrate – the degree of nitration can vary. This will affect stability. Collodian is a pretty stable form of cellulose nitrate. Cellulose nitrate negatives are more unstable (although some that have been stored in good environments are in surprisingly good shape). Cellulose nitrate movie film needed to be flexible, and so had yet a different (very unstable) formulation.
3) “Mirroring” is most common with gelatin emulsions (negatives or prints). When exposed to high humidities it softenens, allowing silver ions to migrate to the surface. When that happens, and they are exposed to oxidative pollutants, they can turn into silver metal particles. This is called “mirroring”.
4) The image in modern silver gelatin developing out papers (most commonly found 1905 to the present) is made with filamental silver particles. These are quite large when compared to the very tiny photolithic silver particles used in other 19th century silver based photographs (collodian, albumen, silver gelatin printing out papers). Because photolithic silver particles are so small, they are very reactive chemically, and so are more prone to damage from poor quality contact materials and atmospheric pollutants.
5) Modern spool film. Professional film is on stable polyester. Amateur film is on unstable triacetate. The problem with polyester is that it has “spool memory”. Because professionals use their film quickly, curl isn’t a problem. Amateurs however, tend to leave the film in the camera for a fairly long time. This would give polyester film a permanent curl. Triacetate is curl resistant.
My posts are just an overview. This subject is really endless. For those who like this stuff very good reference is: James M.Reilly. Care and Identification of 19th Century Photographic Prints. Rochester: Eastman Kodak Company (1986).
Rebecca
1) Glass negatives from 1879-1915 almost certainly have a gelatin emulsion, not collodian.
2) There is more than one kind of cellulose nitrate – the degree of nitration can vary. This will affect stability. Collodian is a pretty stable form of cellulose nitrate. Cellulose nitrate negatives are more unstable (although some that have been stored in good environments are in surprisingly good shape). Cellulose nitrate movie film needed to be flexible, and so had yet a different (very unstable) formulation.
3) “Mirroring” is most common with gelatin emulsions (negatives or prints). When exposed to high humidities it softenens, allowing silver ions to migrate to the surface. When that happens, and they are exposed to oxidative pollutants, they can turn into silver metal particles. This is called “mirroring”.
4) The image in modern silver gelatin developing out papers (most commonly found 1905 to the present) is made with filamental silver particles. These are quite large when compared to the very tiny photolithic silver particles used in other 19th century silver based photographs (collodian, albumen, silver gelatin printing out papers). Because photolithic silver particles are so small, they are very reactive chemically, and so are more prone to damage from poor quality contact materials and atmospheric pollutants.
5) Modern spool film. Professional film is on stable polyester. Amateur film is on unstable triacetate. The problem with polyester is that it has “spool memory”. Because professionals use their film quickly, curl isn’t a problem. Amateurs however, tend to leave the film in the camera for a fairly long time. This would give polyester film a permanent curl. Triacetate is curl resistant.
My posts are just an overview. This subject is really endless. For those who like this stuff very good reference is: James M.Reilly. Care and Identification of 19th Century Photographic Prints. Rochester: Eastman Kodak Company (1986).
Rebecca
