Collodium

Collodium, more commonly spelled collodion in historical contexts, is a solution of nitrocellulose (cellulose nitrate) dissolved in volatile solvents such as diethyl ether and alcohol. When applied as a film and dried, it forms a clear, flexible nitrocellulose sheet. Some formulations include plasticizers or stabilizers to improve strength and flexibility.

The collodion process was developed in the 1850s and became a dominant method in wet plate photography. A thin coating of collodion is made on a cleaned glass plate, then sensitized in a silver nitrate bath to produce a light-sensitive emulsion. The plate must be exposed and developed while the film remains wet, enabling fine detail and relatively short exposures. Variants yielded glass negatives, ambrotypes or tintypes, and the practice remained popular until the advent of dry plates in the 1880s.

In addition to photography, collodion has been used as a protective coating, adhesive film, and wound dressing in medical and industrial contexts, particularly in its flexible varieties. Today its use outside photography is limited, with alternatives widely preferred due to safety and handling concerns.

Safety and handling: The solvent system is highly flammable; ether vapors can form explosive mixtures, and the dried film is easily ignitable. Proper storage in cool, ventilated areas away from heat sources is essential. Nitrocellulose is also flammable. Work with collodion under fume hoods or well-ventilated spaces, away from open flames.