Wet-Collodion is a negative-positive photographic process using potassium iodide dissolved with collodion - a solution of gun-cotton in ether. Gun-cotton is made by treating cotton wool with nitric acid, producing nitro-cellulose; nitro-cellulose was later used as a flexible film base.
The potassium iodide/collodion solution is spread onto glass plates, where the ether evaporates. Before the ether completely evaporates, the plates are sensitized in a bath of silver nitrate solution. Plates had to be exposed whilst still wet, so the plate preparation had to be done immediately before use; this meant that outside the studio, photographers had to carry a small tent with a supply of plates, sensitization bath and bottles of the solutions.
The process was invented in 1850, and published in 1851 by Frederick Scott Archer, although a Frenchman, Gustave Le Gray may have discovered the process at around the same time.