COLLODION EPIULSION PROCE55.—It is intended under this heading to describe the preparation and manipulations of the dry collodion plates. In other places it has been pointed out that there are two distinct methods of preparing the emulsion—the one termed the "washed " emulsion process, and the other the " unwashed " emulsion process. Under collodion emulsion will be found descriptions of making both these emulsions, the descriptions leading up to the preparation and coating of the glass plates. We will now give the process en detail of preparing the plates, and the developing and finishing of negatives upon them.
The emulsion used can be either the washed or the unwashed ; when any difference is necessary in the manipulation it will be pointed out.
The glass plates to be coated are first soaked in nitric acid and water, and then washed with cold water, rubbed over with tripoli powder and alcohol, and allowed to dry. The plate is then polished with a silk handkerchief or soft cloth, and coated with the following substratum : White of egg t ounce Water too ounces Ammonia (.880. 6 drops This should be well shaken and filtered. The plate is first rinsed in distilled water, and the albumen solution flowed over it and drained off as quickly as possible. Another method is to apply the solution with a Blanchard's brush. Instead of albumen some workers prefer india rubber as a substratum. The formula is i grain of india rubber dissolved in an ounce of chloroform or benzole. This is flowed over the plate in the same way as collodion. When the plate has been provided with the substratum it is ready for coating with the emulsion.
The emulsion thoroughly filtered is placed in a convenient pouring bottle. It is then poured on to the plate in the usual manner. If the collodion " drags," it should be diluted with a small quantity of ether and alcohol in the proportion of two parts of the former to one of the latter.
The collodion is made to flow over the whole of the plate, and the overflow is returned to another bottle, and the plate kept rocking to and fro to prevent the formation of lines and ridges.
If the emulsion be a " washed" one the plates should now be dried and treated with a preservative (see Preservative). An arrangement for the convenient drying of the plates was recommended by Woodbury. It consists of an ordinary iron tripod stand, a flat piece of sheet iron, and a spirit lamp or gas burner. The lamp or burner is placed under the iron tripod, upon which is rested the iron plate. A few pieces of blotting paper are laid upon the iron plate, which is then heated by the burner underneath until the blotting paper is inconveniently warm. Upon this the plates are then laid, and the drying will be very rapid. In applying the preservative to
a washed emulsion the plate should first be washed with water until all trace of greasiness has disappeared. If an unwashed emulsion is used it is first thoroughly washed in distilled water until all the silver nitrate that is in excess be thoroughly eliminated. They are then flooded with a preservative solution and placed in a drying cupboard to dry.
With many emulsions of this description the films are so translucent that to avoid halation it is necessary to provide the plate with a backing (see Backing), which must be removed after exposure.
The exposure necessary with this kind of plate is rather long compared with the modern dry-plate process.
For development perhaps the most suitable developer is that recommended by Colonel Wortley. The film of the exposed plate is first flowed over with— Alcohol I part Water i part This will soften the film. It is then washed under the tap, and the developer applied. This is made up with Crer TT•InfllS T To develop, the following proportions are taken: Solution t 6 minims Solution 2 3 minims Solution 3 3 drams The plate is laid in a flat dish, and the developer poured over it. Its action is not rapid, but after a little time the image should appear slowly and gradually. It should gain strength as it goes on, and be free from veil. If after the image does not appear readily, a fresh developer should be made with only half the quantity of Solution 2.
If the developed negative is lacking in brilliancy it can be intensified with an ordinary acid intensifier.
The plate is first of all thoroughly washed. Solution No. z is then flowed over the plate, and returned to a cup containing five or six drops of Solution 2 dropped into it. This is then re turned to the plate, and then intensification should be carried on until the required density has been arrived at.
With these plates the ferrous-citro-oxalate developer can also be used. Abney's for mula is— Potassium citrate (neutral) zoo grains oxalate 22 grains Water , z ounce The potassium citrate is first dissolved in a flask by heat, and when nearly at boiling point the addition of the ferrous oxalate is made, and shaken up in it. When cold it should have a citrony red color. Its action in development is rather slow.
After development, and, if necessary, intensification, the plates are fixed with— Potassium cyanide 4o grains Water. 1 ounce Or— Sodium hyposulphite z ounce Water • 8 ounces The objection to the former fixing agent is that it is a most violent poison, and no little care must be taken in its use. When thoroughly fixed the plates are dried spontaneously, and then varnished to protect the films.