For a bromo-iodised collodion the following sensitizing bath may be used: Silver nitrate (re-crystallized) 4o grains Distilled water z ounce Potassium iodide grain This is made up by dissolving the silver nitrate in a quarter of the quantity of water. The potassium iodide is next added. This will form silver iodide, and will be partially re dissolved when the solution is agitated. The remaining quantity of water is then added, and a re-emulsifi cation of the silver iodide will take place. It is then filtered and tested for acidity or alkalinity. A piece of blue litmus paper after immersion for a minute in the solution should redden slightly. If it redden immediately on immersion a small quantity of sodium carbonate should be added. This will form a precipitate, which is then filtered out, and a few drops of nitric acid solution (1 to 12 of water) added. If the litmus paper remains blue on immersion a small quantity of the nitric acid solution must be added. Instead of nitric acid acetic acid is sometimes used. With this, however, silver acetate is sometimes formed, which is not easily eliminated.
The silver bath will after a time become supersaturated with iodine or ether and alcohol. The presence of iodine is indicated by " pinholes " in the negatives. To eliminate, the bath should be diluted to about one-half its strength, and silver nitrate added to make up to its original strength. It should then be filtered. If ether and alcohol be present, unequal sensitizing and streaky development will be the effect. If the bath be gently heated, both the alcohol and ether will be driven off. The presence of organic matter in the silver bath will result in foggy, dirty, veiled negatives. The removal of this is not easy. The solution should first be made distinctly alkaline with sodium carbonate, and then placed in the sun for some days. A black deposit will be formed in the bottle, and the liquid should be filtered, acidified with nitric or acetic acid, and again filtered. The bath prepared, we have next to sensi tize our plates, coated with the iodized collodion, by immersing them in it.
Sensitizing the have already stated that the glass coated with the collodion is; when in a tacky state, ready for immersion in the sensi tizing bath. This is usually done by employing what is termed a dipping bath (figs. i34, 135). This is made of glass, enclosed in a wooden box sup ported in an upright position. Into this the silver solution is poured. The collodionized plate is laid upon a dipper made of ebonite or porcelain, and plunged into the silver solution, the lower end of the plate being immersed first, as the collodion is probably thicker at that part. This operation is,
of course, done in non-actinic light. In a few seconds the plate will be seen to change to a grey, or bluish grey color, according to the salts in the collodion. This is due to the double decomposition of the silver haloids giving a layer of silver bromide or iodide, thus : NH,I + AgNO, = AgI + NII,NO, CdBr, + zAgNO, = 2AgBr + Cd(NO,), After the plate has been immersed in the bath for about forty-five to fifty seconds it should be gently raised. When there is no longer any appearance of greasiness on the plate the sensi tization is complete. It is then raised slowly from the bath, and the solution at the lower end blotted off on to clean blotting paper. The back is then wiped with the same material, and the plate placed into the dark slide of the camera. A special kind of slide is necessary, in which the plate is kept in position by glass or silver corners. A piece of damped red blotting paper is next placed behind the plate, and the slide closed, and the plate is ready for exposure. After exposure the plate is returned to the dark room, and is ready for development.
The Developer.—There are two methods of development, i.e., iron and pyrogallic acid. The latter is but rarely used at the present time, since the former has been discovered to be such a superior reducing agent. If, however, a collodion is employed containing an iodide only, the gyro developer may still be used. It gives a very dense image, and for this reason is specially useful for copying purposes. A longer exposure is necessary than with the iron developer. The pyrogallic developer is made up in the following manner : Pyrogallic acid t grain Glacial acetic acid 20 minims Alcohol quant. suf.
Water t ounce The most suitable developer in this process is undoubtedly the ferrous sulphate ; a con venient formula runs : Iron protosulphate 12 grains Acetic acid (glacial) 20 to 25 minims Water 1 ounce Alcohol quant. suf.
Instead of acetic acid, i minim of nitric acid may be used. No definite quantity of the alcohol can be given, as it must be regulated by the amount of alcohol in the bath. For instance, a new bath requires a very small quantity of alcohol in the developer, but as it grows old the quantity of alcohol in the developer must be gradually increased, or streaky development will be the result.
The iron should be quite fresh, and of a light green color. Iron which has been exposed to the atmosphere for some time becomes oxidized, having a yellow coating, which, if used for development, acts as a powerful restrainer.