SILVER BATH (for paper).—In the ordinary method of making photograph positives by what is termed silver printing, paper prepared with albumen or gelatine containing a certain quantity of a soluble chloride is floated for varying lengths of time on what is termed a "silver bath." It consists of a solution of silver nitrate with or without the addition of other substances. Upon floating on this solution a certain quantity of it is absorbed by the gelatine or albumen substratum, and combines with the chloride to form silver chloride in a very fine state of division. The effect of the light upon the white surface of the silver chloride is to darken it to a violet color. The action of the light may be stated to be quite superficial, for even if the action of the light be allowed to continue until the color is very deep the amount of reduced silver forming it is exceedingly small. It will be obvious that an important consideration is the strength and com position of the silver bath in order to secure the best results. It will naturally be imagined by the student that the strong bath would sensitize the paper much quicker than a weaker solution, but this is not the case, however. Again, it would be natural to suppose that with a weak bath the paper would take up less silver than with a strong bath. This is also incorrect. A certain quantity of silver nitrate will combine with the chloride and organic substances used in prepar ing the paper no matter whether the bath be weak or A strong solution of silver has the effect of hardening the albumen surface, so that for a • time there is repulsion between the paper and the bath solution ; while the weaker solution not affecting the albumen in this way is readily absorbed, and if the floating of the paper be long continued the solution will penetrate the albumen and dissolve it.* In both these methods of floating, either for a long time on a strong bath, or for a short time upon a weak one, we obtain serious disadvantages. In the first case, although we get bril liancy in the positives, the bronzing of the shadows is with some negatives a serious defect ; and in the second method the prints are usually very weak with a poor sunken-in appearance For general use, a bath of medium strength, 4o grains of silver nitrate to the fluid ounce of water, is the best, as it is suitable for most work, least likely to get out of order, and can be easily kept a required strength.
Silver baths may be divided according to their composition into three classes.t The first class comprises such formula which require the use of nitrate of silver and water only. They usually differ in strength from between 4o to 6o grains of silver nitrate to the ounce of water. It should be kept slightly alkaline in reaction by the occasional addition of a few drops of ammonia or a small quantity of a strong solution of sodic carbonate. A small quantity of silver carbonate will be formed with the addition of the latter solution, but this will settle to the bottom, and tend to keep the bath clear by carrying down with it any floating organic matters.
The second class refers to the old ammonio-nitrate method formerly much used, and still largely, for the production of silver prints upon plain paper. It is prepared as follows: The silver nitrate is first dissolved in the requisite quantity of water, two thirds of the solution are then placed in a separate vessel, and strong ammonia added drop by drop. A precipitate of oxide of silver is at once formed, and the ammonia still added in drops until this precipitate is completely dissolved again. The remaining third of the solution is then added, when a slight precipitate will be again formed. This is removed by adding pure concentrated nitric acid very cautiously drop by drop until the precipitate is just re-dissolved, not another drop being put in after that. Although this bath gives greater sensibility to the paper, and deeper prints, yet it is more likely to discolor owing to the separation of organic matter from the paper.