Wet Collodion Process
The Wet Collodion Process was introduced by Frederick Scott Archer in 1851. The process used a glass plate coated with a layer of collodion that held the sensitive silver halides, it produced high-resolution negatives having good contrast.
The collodion, or wet-plate, process very quickly replaced the Daguerreotype, Calotype and albumen processes and remained the predominant process until replaced by the gelatine dry plate around 1880. It had the advantage of being easier to use than the Daguerreotype and, being a negative/positive process, was able to produce multiple images; having a glass rather than a paper base produced negatives showing finer detail than the calotype process and it was much more sensitive than the Albumen process. It was commonly used in photo-mechanical work into the 1950s.
The drawback of the process was the need to prepare the plate, make the exposure and develop the image whilst the collodion was still moist. This was not a hindrance in a studio but for outdoor work chemicals, water and a portable darkroom, in the form of a tent, had to be added to the travelling photographer's equipment.
From the year of its introduction prepared chemicals were available from leading photographic suppliers such as Horne & Thornthwaite. Collodion could be purchased plain, ready iodised or the photographer could follow instruction manuals and produce his own collodion. Specialised equipment for the process such as dipping baths were also available from the start of the process.
InventionFrederick Scott Archer is rightly recognised as the inventor of the process but at the time Gustave Le Gray and Robert Bingham also claimed to be inventors of similar processes. In a letter published in Notes and Queries Archer points out that although Le Gray in his 1851 book (Traité practique de photographie sur papier et sur verre) mentions collodion it is in association with paper and not with glass, nor was any working method of his process given.
Interestingly Archer goes on to recount that following his (Archer's) article in the Chemist, P.W. Fry called on him for a demonstration of the process and that it is through Fry that Archer met Fallon Horne. Fry was also mistakenly associated with the invention of the process. Horne & Thornthwaite were for a time the exclusive agents for Archer's iodised collodion. John Werge in his book Evolution of Photography (1890) deals with some of these claims and includes copies of letters from Archer's wife.1
Archer's other contributionsArcher was probably the first to use pyrogallic acid as a developer, the credit for this is given by Eder and others to Regnault and Liebig (working independently) who both gave details of its use in 1851. In two letters published in The Chemist in May and June 1850 Archer states that he successfully used pyrogallic acid the previous summer and that it was quicker acting than gallic acid. The photographs were taken using the calotype process where the sensitising fluid applied just before exposure had the same constituents as the developer applied after exposure, in effect the developing agent - gallic acid - was applied before and after exposure. It was known at the time that if a negative were left many hours without developer being applied (after the exposure) the latent image would develop by itself. Archer notes that with pyrogallic acid the second coating of solution was not needed and that the sensitising solution was sufficient to develop the latent image. The date of the photographs, 1849, is interesting as it is the date given elsewhere as the date Archer started to experiment with collodion.2
Archer produced a camera for wet collodion use, essentially a dark-tent and camera combined, two versions were made one rigid the other folding. The camera was only available from Archer directly. He also patented a stripping film; a normal wet collodion plate was processed, gutta percha in benzole was then poured over the image and left to dry, the plate was immersed in water causing the collodion to separate from the glass plate.3
Over the period of its use details of the process and the chemicals used varied. There would be little point in listing all of the variations as, with one or two exceptions, there is no chronology associated with the changes or priority of invention. As a guide the formulae from Charles A. Long's book, Practical Photography on Glass and Paper (1856) are given below.
If the collodion was bought 'plain' then it had to be iodised, at first this was by adding potassium iodide, later cadmium bromide was also included, cadmium or ammonium iodide also tended to replace potassium iodide.4
After development the success of the image could be assessed, if not good enough the collodion could be removed and the glass reused. The plate had to be cleaned before the collodion had set otherwise a ghost image of the original photograph could show in subsequent photographs.
There were seven stages involved in the process:
CleaningOpinions differed from simply washing the plate and drying with chamois leather to cleaning with nitric acid followed by polishing with tripoli powder.
CoatingThe edges of the glass plate might be roughened with emery paper to help the collodion adhere to the surface. The plate was held at one corner while the iodised collodion was poured on near the middle, by tilting the plate to each corner in turn the collodion was made to flow around and cover the plate. The collodion was then allowed to set for a short time.
SensitisingThe plate was sensitised by immersion in silver nitrate which combined with the potassium iodide (in the collodion) to produce silver iodide. Usually the silver nitrate was held in a vertical bath though some photographers preferred a dish. When a bath was used the plate was placed on a dipper and lowered into the bath in one continuous movement. The plate remained in the bath for around two minutes depending on the temperature and age of the bath.
On removal from the bath the plate was allowed to drain and was then put in the camera's dark-slide, the orientation of the plate had to be maintained i.e. the top edge of the plate when in the bath was placed at the top of the dark-slide.5
DevelopingThe plate was either placed on a levelling stand or held in the hand in the same way as when being coated with collodion. The developer was flowed onto the plate and kept in motion by blowing on it or tilting the plate. Development was by inspection, a white sheet of paper being held beneath the plate. A pyrogallic acid developer was used at first but was generally replaced by an iron developer.6
FixingThe plate was washed in water, the remaining silver iodide was then removed by a fixing solution of sodium hyposulphite or potassium cyanide. The fixer was contained either in a bath or the plate was put back on the levelling stand where fixer was poured over it. The plate was then thoroughly washed.7
VarnishingTo protect the negative it was given a coating of varnish, this was poured onto the plate in a similar way to the collodion.
Dark-slidesThe dark-slide normally had silver corner pieces on which the plate rested, in part this protected the slide from silver nitrate staining it also prevented organic matter from the slide touching the plate which could cause oyster-shell markings. The slide often contained a shallow trough coated in varnish to collect any drips from the plate.
Silver nitrate bathThe baths were narrow upright containers in which the plate was dipped vertically. The usual material was glass but ceramic, ebonite and gutta percha was also used. The plate was lowered into the bath using a dipper. This was sometimes of ribbed glass bent at one end to hold the plate, other patterns were of ebonite or metal with sliver prongs to hold the plate. The sides of the bath were bowed to allow the insertion of the dipper. Baths may be held in a simple frame with legs which held it upright, for travelling the bath would have been in a box with an air-tight lid. The vertical bath presented a smaller air surface than a flat dish and helped preserve the solution.
Travelling tentTents were often home-made affairs consisting of poles stuck in the ground with a covering of cloth. More elaborate examples included horse drawn vans, hand-carts, wheelbarrows and many types of commercially available tent. A popular form was a tray resting on a tripod with supports to hold the cloth covering clear of the operator.
References & Notes
Long, Practical Phot. (1856), p. 11. Hardwich, Phot. Chemistry, p. 257. Thornthwaite, Guide to Photography. (1860), p. 36. Monckhoven, Popular Treatise on Photography. (1863).
 Notes and Queries, 25/12/1852, p. 612.
Werge, Evolution of Photography, p. 65.
See Gernsheim, History, p. 197 and Eder, History, p. 200 for a detailed account of these claims as well as the confusion over P.W. Fry's inclusion in the list of inventors.
 Chemist, 1850, pp 360, 450. The solution used by Archer was: silver nitrate 40 grains, acetic acid 1 ounce, pyrogallic acid 4 grains dissolved in 1 ounce of acetic acid.
Information on Regnault and Liebig: Eder, History, p. 330. Brothers, History, p. 274.
 The camera has a registered design number of 3570 (1854). An illustration can be found in Hunt, Man. Phot. Stripping film: BP 1914/1855.
 Pyroxylin or gun-cotton was made by pouring sulphuric acid into a bowl containing potassium nitrate (saltpetre), into this carded cotton was placed, after a few minutes the cotton was removed and washed. Pyroxylin was dissolved in an ether/alcohol mixture to produce collodion.
To iodise collodion: dissolve 4 grains of potassium iodide in 2 drachms of alcohol, after filtering add this to 6 drachms of collodion.
 The sensitising solution comprised: silver nitrate in crystal form, 1.5 ounces; distilled water, 20 ounces and potassium iodide, 10 grains. This was made neutral by adding either acetic acid or ammonia. Alcohol was sometimes included.
 The plate could also be held by a pneumatic holder.
A pyro developer comprised: pyrogallic acid, 3 grains; acetic acid, 1 drachm; distilled water, 2 ounces. To which was added 10 drops of silver nitrate immediately before use.
An iron developer comprised: ferrous sulphate, 4 ounces; alcohol, 4 ounces; acetic acid, 3 ounces; water, 65 ounces.
 The silver iodide had a yellow colour, fixing would continue until this had disappeared. Fixer: Sodium hyposulphite, 8 ounces; distilled water, 20 ounces.