Collodion process Guide, Meaning , Facts, Information and Description
The collodion process is an early photographic process which gave way in the late 19th century to today's gelatin emulsion process. It was invented by Frederick Scott Archer in 1848 and developed further by others.
The original idea
In 1864 W. B. Bolton and B. J. Sayce published the germ of a process which revolutionized photographic manipulations. In the ordinary collodion process of Scott Archer, a sensitive film is procured by coating a glass plate with collodion containing the iodide and bromide of some soluble salt, and then, when set, immersing it in a solution of silver nitrate in order to form silver iodide and silver bromide in the film.
The question that presented itself to Bolton and Sayce was whether it might not be possible to get the sensitive salts of silver formed in the collodion whilst liquid, and a sensitive film given to a plate by merely letting this collodion, containing the salts in suspension, flow over the glass plate. Gaudin had attempted to do this with silver chloride, and later G. W. Simpson had succeeded in perfecting a printing process with collodion containing silver chloride, citric acid and silver nitrate; but the chloride until recently has been considered a slow working salt, and nearly incapable of development.
Up to the time of W. B. Bolton and B. J. Sayce's experiments silver iodide had been considered the staple of a sensitive film on which to take negatives; and though bromide had been used by Major Russell and others, it had not met with so much favour as to lead to the omission of the iodide. At the date mentioned the suspension of silver iodide in collodion was not thought practicable, and the inventors of the process turned their attention to silver bromide, which they found could be secured in such a fine state of division that it remained suspended for a considerable time in collodion, and even when precipitated could be resuspended by simple agitation.
The rapidity of these plates was not in any way remarkable, but the process had the great advantage of doing away with the sensitizing nitrate of silver bath, and thus avoiding a tiresome operation. The plates were developed by the alkaline method, and gave images which, if not primarily dense enough, could be intensified by the application of pyrogallic acid and silver nitrate’ as in the wet collodion process. Such was the crude germ of a method which was destined to effect a complete change in the aspect of photographic negative taking; but for some time it lay dormant. In fact there was at first much to discourage trial of it, since the plates often became veiled on development.
The addition of the mineral acids such as Carey Lea employed is to prevent the formation of (or to destroy when formed) any silver sub-bromide or oxide, either of which acts as a nucleus on which development can take place. Abney first showed the theoretical effect of acids on the stib-bromide, as also the effect of oxidizing agents on both the above compounds (see below). A more valuable modification was introduced in 1874 by W. B. Bolton, one of the originators of the process, who allowed the ether and the alcohol of the collodion to evaporate, and then washed away all the soluble salts from the gelatinous mass formed of pyroxylin and sensitive salt.
After washing for a considerable time, the pellicle was dried naturally or washed with alcohol, and then the pyroxylin redissolved in ether and alcohol, leaving an emulsion of silver bromide, silver chloride or silver iodide, or mixtures of all suspended in collodion. In this state the plate could be coated and dried at once for exposure. Sometimes, in fact generally, preservatives were used, as in the case of dry plates with the bath, in order to prevent the atmosphere from rendering the surface of the film spotty or insensitive on development. This modification had the great advantage of allowing a large quantity of sensitive salt to be prepared of precisely the same value as to rapidity of action and quality of film.
A great advance in the use of the collodion. bromide process was made by Colonel Stuart Wortley, who in June 1873 made known the powerful nature of a strongly alkaline developer as opposed to the weak one which up to that time had usually been employed for a collodion emulsion plate, or indeed for any dry plate.The basic method
The outline of the method was to dissolve a soluble bromide in plain collodion, and add to it drop by drop an alcoholic solution of silver nitrate, the latter being in excess or defect, according to the will of the operator. To prepare a sensitive surface the collodion containing the emulsified sensitive salt was poured over a glass plate, allowed to set, and washed till all the soluble salts resulting from the double decomposition of the soluble bromide and the silver nitrate, together with the unaltered soluble bromide or silver nitrate, were removed, when the film was exposed wet, or allowed to dry and then exposed.The collodion process becomes widely adopted
M. Carey Lea of Philadelphia, and W. Cooper, jr., of Reading, may be said to have given the real impetus to the method. Carey Lea, by introducing an acid into the emulsion, established a practicable collodion emulsion process, which was rapid and at the same time gave negative pictures free from veil. To secure the rapidity Carey Lea employed a fair excess of silver nitrate, and Colonel H. Stuart Wortley gained further rapidity by a still greater increase of it; the free use of acid was the only means by which this could be effected without hopelessly spoiling the emulsion.