Submitted by Al …
(Adapted from The Encyclopedia of Photography ©1963)
The collodion process today is virtually obsolete, its place taken by special high-contrast emulsions coated on film base.
The distinguishing feature of the collodion process is the fact that the plate had to be coated, sensitized, and exposed while still wet. For this reason, it is known as the wet-plate process. It was first introduced in 1851 by Frederick Scott Archer and was used by Mathew Brady for his great Civil War photographs, T.H. O’Sullivan in photographing the West, and by Margaret Cameron and many other photographers.
Collodion is a solution of pyroxylin or guncotton in a mixture of alcohol and ether. When poured out on a glass plate and permitted to dry, it forms a thin, tough, transparent film, which is not soluble in water, but can be penetrated by various water solutions. In making a collodion plate, a special mixture of collodion and potassium iodide is flowed onto a glass plate and permitted to dry. (It does not actually dry, but sets to a thin film.) At this stage, the entire plate is dipped in a tank of silver nitrate. This reacts with the iodide in the coating to form a white silver iodide which is sensitive to light only as long as it remains wet.
The plate must be exposed in the camera before it has a chance to dry. As soon as the plate has been exposed, it must be developed using a form of physical developer containing pyrogallol and silver nitrate. Since silver iodide is not very soluble in ordinary hypo, a solution of sodium cyanide is used for a fixing bath, after which the plate is washed and dried in the ordinary way. The finished negative can be printed in the same way as any other negative.
The tintype photographic process—a later use of collodion emulsion.
A later development was the collodion emulsion, which was coated on plates and used dry (tannin was used as a preservative). The appearance shortly afterward of the faster gelatin emulsion plate largely eliminated the collodion emulsion from the market. However, collodion emulsion printing-out papers, especially those containing both silver and gold salts (‘self toning’ papers), were used until shortly before World War II.
A collodion glass negative is comprised of a glass support with a collodion binder and silver image. When viewed in reflected light, the image tone ranges from warm light brown to dark brown. Thick glass plates were coated by hand, which resulted in an uneven thickness of the collodion that is particularly apparent at the corners.
Another early use for wet collodion plates was the ambrotype, which was simply a glass plate coated with collodion and silver sensitized. The image (which was deliberately underexposed), was developed and bleached in mercury salts to make a whitish negative. This was then backed with black enamel or velvet to make an apparent positive.
A gelatin glass negative is comprised of a glass support with a gelatin binder and silver image. Image tones range from a neutral to black. Unlike earlier glass plate negatives, gelatin plates were fully coated by emulsion and were cut to standard dimensions on factory-cut glass less than 2 mm thick. A thin blank strip can be seen near the border. The gelatin and silver emulsion was applied by machine, resulting in a thin, smooth, even coating that also covered the edges.
Until fairly recently, photomechanical workers used wet collodion, especially for complicated jobs requiring the final image to be stripped off the glass plate, either for image reversal or for combination with images from other plates. With the availability of photomechanical films on special backings which allow for stripping the emulsion, the collodion process ceased to have any further use.
An albumen negative is comprised of an albumen and silver image on a glass support. The image tone ranges from ocher-orange to olive green. Emulsion will appear creamy in dark areas. The image resolution is relatively sharp. Albumen negatives are typically made with thick, irregularly sized rough-cut glass. Corners may not be coated with emulsion.
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