Wet Plate Collodion Process
The collodion process is said to have been invented almost simultaneously by Frederick Scott Archer (English) and Gustave Le Gray (French) in about 1850.
The collodion process replaced the first practical photographic process, the daguerreotype. By the 1880s the collodion process itself was replaced almost entirely by gelatin dry plates (glass plates with a photographic emulsion of silver halides suspended in gelatin). The dry gelatin emulsion was far more convenient and was much more sensitive, which greatly reduced exposure times from the collodion process.
The collodion process, or wet plate collodion process, is inconvenient in that it requires the photographic material to be coated, sensitized, exposed and developed within the span of about fifteen minutes ... which means a portable darkroom is required for use outside of the studio.
Although collodion was normally used in its wet form, the material could also be used in both a humid, or 'preserved' form as well as 'dry' form ... both of these forms greatly increased exposure times however, and these long exposures make these forms unsuitable for portraiture. Their use is better suited to landscape photography and other special applications, where very long exposure times are tolerable.
The advantage of the collodion processes is its ability to record exceptionally fine detail, so their use for some special purposes continued long after the advent of the gelatin dry plate, and is actually still used to this day by many fine art photographers. Commercially the wet plate collodion process was still in use within the printing industry into the 1960s for line and tone work. For large work it was much cheaper than gelatin film. One collodion process, the tintype, was still in limited use for casual portraiture as late as the 1930s, by which time tintypes were already regarded as quaint and old-fashioned.
Many photographers still to this day use the collodion process ... see Silver & Light by Ian Ruhter to the left for one of many examples.