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Collodion Dry-Plate

process, emulsion and dry

COLLODION DRY-PLATE PROCESS.—The first published practical formula for a collodion emulsion was that by Messrs. Bolton and Sayce in 1864.

With this process a dry collodion emulsion is used, and before going farther it will perhaps be best to explain the difference between the collodion " dry" and the collodion " wet " process. In the wet process we coat the glass with a collodion containing the bromide or the iodide, or both, and immerse it in a nitrate of silver solution. The effect of this is explained in the description given of that process. The silver combines with the bromide or iodide to form the sensitive compound. In the dry collodion process the effect is similar, though the manipulation is different. To the liquid collodion containing the salts the silver nitrate is added, and the sensitive emulsion thus produced is spread on to glass plates and dried. The advantage of this is that after the preparation of the plates they may be kept for some time previous to the exposure, which is not the case in the wet process.

Dry collodion emulsion processes must be divided into two distinct classes, the " washed " and the "unwashed." When the silver is added to the salted collodion a process of double decomposition takes place by which the sensitive salts are formed. But besides these other compounds or bye-products are produced, which, if they were allowed to remain in the film, would crystallize and spoil the plate. It is therefore necessary that these be removed, and this is done either by washing the emulsion before coating the plates, or by washing the film after the plate is coated. The first process is termed the " washed " emulsion process, and the latter the "unwashed " emulsion process, although it will be seen that in both cases the washing takes place at one stage or another. For description of the two processes see Collodion Emulsion Process, also Beechey's Process, Dawson's Process, Lea's Process.