Any questions in regard to photograph matters will be willingly answered by the Editor of this column, and we should be glad to hear from any of our club who can make helpful suggestions.
THE MYSTERY OF SILVER FEINTING.
Correctly speaking, the image on the sensitive paper is permanent when it is removed from the printing-frame, but the silver chloride which has not been acted upon by the light is still sensitive, and unless it is removed, it would also decompose, and thus the picture would be lost. In order to preserve this picture some chemical agent must be used which shall remove or dissolve the unchanged chloride of silver.
It has been shown that the portion of the chloride of silver which has been acted upon by the light has been changed to a different chemical compound. Any chemical process to which the picture might be subjected would be likely to act on each compound in a different manner. In order to preserve the picture some chemical agent must be employed which shall remove or dissolve the silver chloride, but which shall not affect the chemical compound which forms the picture.
After many experiments a safe, and now cheap,[2] agent was found in hyposulphite of soda. When the print is placed in a solution of hyposulphite of soda a new compound is formed—silver sodium hyposulphite. This double salt dissolves very quickly in water, and is easily washed out of the film. If, however, the hypo solution is not strong enough, another compound is formed, which will not dissolve, and cannot be washed out of the film. It decomposes by degrees, and produces a yellowish-brown deposit, which ruins the paper or film. This is the reason why prints and films are a dull yellowish color; it is the formation of an insoluble salt by using too weak hypo, or not leaving the paper or negative long enough in the solution.
Each atom of nitrate of silver requires three atoms of hyposulphite of soda to form the soluble double salt. Negatives require a solution double the strength of that used for prints. The proportion for negatives is 1 oz. of hyposulphite of soda to 4 oz. of water, while the solution for prints requires 8 oz. of water to 1 oz. of hypo.