DRY COLLODION PROCESS. If a collodion plate, excited in the usual way, is allowed to get dry, without having previously washed off the free nitrate of silver, the yellow layer of iodide of silver dis appears, and the film becomes transparent and insensitive. This arises from the fact that iodide of silver is soluble in a concentrated solution of nitrate of silver; so that, as the moisture evaporates from the film, the free nitrate becomes concentrated, and dissolves the iodide of silver, forming a double salt, which has been called iodo-nitrate of silver. In order, therefore, to use the collodion film in a dry state, it is found necessary in the first place to remove the greater part of the free nitrate of silver by washing; and another condition is, that the collodion shall adhere tightly to the plate when dry, and not contract, or form blisters when the solutions are applied to it a second time, after having been once dried.
All these conditions are fulfilled by the simple and excellent pro cess of Dr. Hill Norris, of Birmingham, which is as follows :— The plate is collodionized and excited in the usual way, and then washed with distilled water, until as much as possible of the free nitrate of silver is removed. This done, a solution of gelatine is poured over it, made by dissolving 80 grains of gelatine in 20 ounces of distilled water at the boiling point, filtering while hot, and then boiling down to half the quantity, stirring well with a glass rod, and, when cooled down a little, adding 1 ounces of alcohol. Before applying this gelatine preservative solution, it should be heated by immersing the bottle containing it in hot water, and poured over the plate hot. The plate is then left to dry spontane ously, or it may be dried by artificial heat.
Plates so prepared may be kept sensitive for many months; Dr. Norris says indefinitely.
The exposure in the camera must be much longer than in the wet process ; as long, perhaps, as for waxed-paper.
The development is accomplished thus :—The plate is first im mersed for five minutes in a dish of distilled water; then the usual mixture of pyrogallic and acetic acid is made in a chemically clean measure, and a few drops of nitrate of silver added to it ; this is poured over the plate, and the picture developed in precisely the same way and as quickly as in the wet process.
The picture is fixed either with hyposulphite of soda, or cyanide of potassium, in the usual way, and varnished like an ordinary negative.
When the chemicals are in good order, this process is as certain, and yields as good results, as wet collodion.
Another excellent dry collodion process, due to Mr. Fothergill, is as follows :— The excited plate, after removal from the nitrate bath, is washed once with distilled water ; then albumen is poured over it ; it is then washed again. The remaining treatment is the same as Dr: Hill Norris's. The rationale of this process appears to be that the albu men forms, with the free nitrate in the film, a minute quantity of albuminate of silver. These plates do not keep so well as Dr. Norris's, but are more sensitive. The process was tried by him and aban doned, long before Mr. Fothergill brought it forward.
In all dry collodion processes it is important to use a powdery-non contractile collodion, made with acids at a temperature not lower than and of the minimum strength that is consistent with a struc tureless film.