PRESERVATION OF 19TH-CENTURY NEGATIVES IN THE NATIONAL ARCHIVES
3 THE WET COLLODION NEGATIVE PROCESS
THE WET Collodion process was employed to create the Brady and Western Survey negatives held by the National Archives. Introduced in 1851 by Frederick Scott Archer in England, the collodion process was the forst practical negative system. The collodion negative was ideally suited for use with the albumen printing process, which predominated during the 19th century. The collodion process was commonly used in the United States from 1855 until about 1880, when the more convenient gelatin dry-plate negative became popular (Reilly 1986). The collodion process required the photographer to prepare a light-sensitive plate just prior to exposure in the camera. Collodion is a solution of cellulose nitrate dissolved in ether and alcohol. To prepare the plate, a glass sheet was carefully cleaned and flowed with a viscous solution of brome-iodized collodion. To coat the collodion negative, the plate was normally held at one corner between finger and thumb. The viscous collodion was poured onto the center of the plate, which was then tilted so the plate would be evenly covered with it. The excess collodion was then poured off the plate, usually from the corner opposite to that which was held. The plate was then immersed in a bath of silver nitrate, which created the light-sensitive silver halide. While still damp, the plate was placed in the camera for an exposure that ranged from several seconds to several minutes. Once the collodion layer was made light sensitive, only the areas that were coated and exposed to light would form an image. Before it was allowed to dry, the plate was developed in an iron sulfate or pyrogallic acid developer. The plate was then rinsed and fixed using a solution of potassium cyanide or sodium thiosulfate, which left metallic silver as the final image material. The plate was again rinsed with water and, when dry, was varnished to protect the easily abraded collodion layer.