Editing Collodion process (section)

==Collodion emulsion==
[[Image:1860 Anonyme Un vétéran et sa femme Ambrotype.jpg|right|thumb|British veteran of the [[Napoleonic Wars]] with his wife, {{circa|1860}}, hand-tinted [[ambrotype]] using the bleached collodion positive process.]]
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 the 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 the 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.

=== Phenotype ===
The ''phenotype'' (from Latin pannus = cloth) is a direct positive that, like the [[tintype]], uses collodion emulsion from an underexposed image that is transferred to a dark surface so that transparent (unexposed) areas appear black and weak precipitated silver (highlights) appear brighter in reflected light, on the same principle as the daguerreotype and ambrotype.<ref name=":0">{{Cite book|last=Eder|first=Josef Maria |translator-last=Epstean |translator-first=Edward |title=History of Photography|date=1945|doi=10.7312/eder91430|isbn=978-0-231-88370-2|oclc=1104874591}}</ref> It was invented in 1852 by French photographer Jean Nicolas Truchelut, a pupil of Louis Daguerre and an itinerant daguerreotypist. Similar images on black waxed linen were displayed at the [[French Academy of Sciences]] by Wulff & Co. in 1853.<ref name=":0"/> 

Various substrates were tried including wood, and Australian photographers Alfred R. Fenton<ref>{{Cite web|title=Alfred R. Fenton :: biography at :: at Design and Art Australia Online|url=https://www.daao.org.au/bio/alfred-r-fenton/|access-date=2021-10-25|website=www.daao.org.au}}</ref> and [[Frederick Coldrey|Frederick H. Coldrey]] patented a version on black leather in 1857 to create an unbreakable photograph that could be sent by mail.<ref>{{Cite news|date=6 November 1857|title=Miscellaneous News|page=6|work=The Age}}</ref> Various practitioners formulated, and some patented, their own recipes with the aim of good adhesion, but a disadvantage of using such supports was that flexing of the surface caused cracking and flaking of the emulsion so few historical examples survive. The process continued to be used until the 1880s but was being gradually displaced by the more durable tintype from the 1860s.<ref name=":0"/>