Collodion Process - Collodion Emulsion

Collodion Emulsion

In 1864 W. B. Bolton and B. J. Sayce published an idea for a process that would revolutionize photography. They suggested that sensitive silver salts be formed in a liquid collodion, rather than being precipitated, in-situ, on the surface of a plate. A light-sensitive plate could then be prepared by simply flowing this emulsion across the surface of a glass plate; no silver nitrate bath was required.

This idea was soon brought to fruition. First, a printing emulsion was developed using silver chloride. These emulsions were slow, and could not be developed, so they were mostly used for positive printing. Shortly later, silver iodide and silver bromide emulsions were produced. These proved to be significantly faster, and the image could be brought out by development.

The emulsions also had the advantage that they could be washed. In the wet collodion process, silver nitrate reacted with a halide salt; potassium iodide, for example. This resulted in a double replacement reaction. The silver and iodine ions in solution reacted, forming silver iodide on the collodion film. However, at the same time, potassium nitrate also formed, from the potassium ions in the iodide and the nitrate ions in the silver. This salt could not be removed in the wet process. However, with the emulsion process, it could be washed out after creation of the emulsion.

The speed of the emulsion process was unremarkable. It was not as fast as the ordinary wet process, but was not nearly as slow as the dry plate processes. Its chief advantage was that each plate behaved the same way. Inconsistencies in the ordinary process were rare.