Kallitype is a historic photographic printing process developed in 1889 by W.W.J. Nicol, based on the light sensitivity of iron salts and producing a final image formed in metallic silver. It was developed in part as a more affordable alternative to the platinum printing process, which produced images of exceptional tonal quality and archival permanence but required the use of expensive platinum salts that placed it beyond the reach of many photographers.
The kallitype process works by coating a sheet of paper with a light sensitive solution containing ferric iron salts and a silver compound. When exposed to ultraviolet light through a negative - typically a large format negative in contact with the coated paper - the iron salts are reduced by the light, initiating a chemical reaction that causes the silver to be deposited as a visible image in the exposed areas. The print is then developed, cleared, and toned to produce the final result.
One of the appealing characteristics of the kallitype process is its versatility in terms of tonal qualities and colour. By varying the developer chemistry and applying different toning solutions - such as gold, platinum, or palladium toners - a wide range of image colours can be achieved, from warm brown and sepia tones through to cool blue-blacks and neutral greys. Platinum or palladium toning also significantly improves the archival stability of kallitype prints, which in their untoned state can be prone to fading and deterioration over time.
Although largely obsolete as a mainstream photographic process, kallitype has experienced a revival among alternative process photographers and fine art printmakers who value its rich tonal qualities, handcrafted aesthetic, and the unique visual character it imparts to photographic images. It remains one of a number of historic iron and silver based printing processes - alongside cyanotype, Van Dyke brown, and platinum palladium printing - that continue to be practised and appreciated within the alternative and historical photographic community.