Long exposures
This early photographic technique invented by Frederick Scott Archer in 1851 involves adding a soluble iodide to a solution of collodion (cellulose nitrate) and coating a glass or metal plate with the mixture. In the darkroom the plate is then immersed in a solution of silver nitrate to form silver iodide. While the plate is still wet, it can be exposed to light in the camera. A solution of pyrogallic acid is poured over the plate which acts as a developer. Additional ‘dipping’ in the developer creates effects of waves (see below). Washing it off acts as a stop bath. The back side of the plate is then blackened to make the image visible and to enhance the contrast.
