COLLODION GLASS PLATE NEGATIVE
The collodion glass plate negative process is a monochrome silver process invented by the Englishman Frederick Scott Archer in 1851, which was widely used until the 1880s.
It was not the first negative process to make use of a glass support: in fact, following experiments by his cousin Joseph Nicéphore Niépce in the early 1820s, in 1847 Claude Félix Abel Niépce de Saint-Victor developed the first feasible glass process in the form of albumen glass plate negatives. Although, because of its transparency, the images are sharper than the paper negatives used at the time, the process is laborious, specifically requiring a very long time for exposure and development; one reason why it was superseded by the collodion glass plate negative process. The collodion used to produce photographic negatives on glass is obtained by dissolving cellulose nitrate – a chemical product developed by the Swiss chemist Christian Frédéric Schönbein – in alcohol and ether.
The glass plates used are cut to the required size beforehand and thoroughly cleaned. The collodion containing halides – iodides and bromides – is poured onto the centre of the glass plate and then spread uniformly over the entire surface by gently tilting the plate. Once the alcohol and ether have evaporated, the collodion coated plate is sensitized by immersion in a silver nitrate solution and then exposed immediately in a camera while still wet. The latent image obtained is then developed in an ammonium iron sulphate or pyrogallol solution. The image is then fixed in a potassium cyanide or sodium thiosulphate solution and washed. Finally, the plate is dried over a flame and then generally varnished, to protect the collodion layer against abrasion and to prevent tarnishing. Collodion glass plate negatives are printed by contact, in the majority of cases on albumen paper. In order to improve the quality of prints, photographers frequently work on the negatives: portraits are retouched using a graphite pencil and skies in landscapes are covered with inactinic paper or gouache.
Although the collodion wet glass plate negative process takes far less time than the albumen glass plate process it has significant drawbacks associated with the rapid deterioration of sensitized plates, obliging photographers wanting to take pictures outdoors to take their laboratories with them. This process was therefore adapted in order to extend the plates period of use following sensitization. In 1853, Marc-Antoine Gaudin introduced the use of hygroscopic substances such as sugar, honey or glycerine, which, when combined with collodion, allow a certain level of moisture to be maintained within the binder and thereby enable the plates to be prepared in advance. In effect, collodion, which becomes impermeable when dry, prevents all solutions associated with production processes to penetrate as far as the silver salts. Processes, known as "dry collodion processes" were also developed, with the most important being Taupenot's collodion-albumen glass plate negative process of 1855. The following year, Dr. Hill Norris launched the first ready-to-use plates in England. However, dry collodion processes remained rare and mainly used for landscape photography, as their sensitivity is clearly inferior to that of wet plate collodion processes.
Visual glossary of photographic techniques © ARCP / Mairie de Paris, 2013