COLLODION PROCESS (WET), OR WET PLATE PROCESS (Fr., Procede d in col lodion humide, Collodion mouill ; Ger., Kollodiumverfahren, Nasses hollodiotwer fahren) Collodion was introduced into England in 1847 ; immediately afterwards R. J. Bingham (one of Faraday's assistants) suggested its use for photography. Gustave Le Gray, a noted French worker, also suggested that collodion might prove useful. The actual invention of the first workable process is due, however, to F. Scott Archer, who made the first collodion nega tives in the autumn of 1848, and who published his perfected process in the Chemist for March, 1851. So popular did Archer's process become that it practically displaced daguerreotype and calotype, and it was almost exclusively used between 1855 to 1881. It is largely used to-day by process workers and by itinerant photo graphers ; while for certain other branches of photography—lantern slides, for example—it is considered by many to be unsurpassed. Its chief advantages are a structureless film, an ex tremely fine grain, and clear whites. The fixing agent is easily washed out of the film, and the negative can be dried by heat. Wet-plate nega tives can also be easily reduced and intensified.
Wet plates are of low sensitiveness, their speed being about the same as that of lantern plates.
The photographer must prepare his own plates. A deep porcelain dish for the silver bath, a size larger than the plate to be sensitised, is required ; it must be scrupulously clean, and if it has previously contained other chemicals it must be very thoroughly washed. Place 48o grs. of silver nitrate in a clean pint bottle, and add 15 oz. of distilled water (rain or tap water will not do). Shake until dissolved, and add 3 drops of pure nitric acid. Keep a day or two before using.
Each ounce of the silver bath contains 32 grs. of silver nitrate, and the strength should be kept as near this as possible, using, if necessary, an argentometer for testing the density. The silver bath is filtered, poured into the porcelain dish, covered with a piece of cardboard, and placed in the corner of the dark-room that is farthest from the developing sink. Adjust the
dark-room lamp so that a good light is thrown on the dish. Wet plates will stand far more red or yellow light than dry plates.
For coating the glass the following are neces sary :—(a) A 4-oz. bottle of Mawson's collodion, with iodiser in a separate bottle. Before use the iodiser is poured into the collodion, and this iodised collodion will keep in good condition for several months, becoming deep red in colour, the plates then requiring a much longer exposure. Iodised collodion can be purchased ready for use, but the plan here described is preferable. (b) Rubber solution for edging the plate.
For developing, fixing, etc., obtain :— Pyrogallic acid . . . i oz. Glacial acetic acid Ferric protosulphate . . 1 lb.
Mercuric bichloride . • 1 oz. ' Liquor ammonim • • • 1 PPLead nitrate Potassium ferricyanide . . 1 „ Sodium hyposulphite or potas sium cyanide . . 1 lb.
Before coating with collodion, the plate is " edged " to prevent the film from leaving the glass. A drop of rubber solution is taken up on a piece of cotton-wool and run round the edge of the glass, and the plate is then ready for coating.
Take the glass at one corner between the finger and thumb, and pour a small pool of collodion upon it. Carefully tilt the glass so that the collodion flows to the corner farthest from the fingers, tilt again into the other top corner, next bring the collodion to the corner nearest the thumb, and then pour the surplus back into the bottle via the remaining corner. While the collodion is being poured into the bottle the plate must be kept moving to and fro laterally, or the collodion will set in ribs. This movement of the plate must be continued for several seconds, till the collodion is set. Replace the stopper in the collodion bottle, dose the door of the dark-room, and immerse the plate in the sensitis ing solution.