We may thus summarize:—
1. A paper floated on a strong solution may require long floating.
2. A paper floated on a weak solution requires short floating.
3. And the strength of the solution may be between the 30 grains and 80 grains to the ounce of silver according to the amount of soluble chloride dissolved in the albumen on the paper when the negative is really good as regards opacity and delicacy.
The knowledge of the amount of chloride in the paper supplied by dealers has to be arrived at somehow, and the following method will answer. Cut up a quarter sheet of the paper into small pieces, and place them in a couple of ounces of methylated spirit. This will dissolve out most of the chloride, and should be decanted off. Another two ounces of spirit should be added to the paper, and, after thoroughly soaking, should be decanted off, and added to the other spirit. The spirit containing the chloride may then be placed in a glass vessel standing in hot water, when it will evaporate and leave the chloride behind. It may be weighed; but since it is better to know how much silver chloride (AgCl) would be formed, the residue should be dissolved in a few drops of water, and a little silver nitrate added. The silver chloride will be precipitated, and should be carefully washed with water, and then be filtered, the paper being opened out and dried before the fire on filter paper. The chloride is then detached and weighed; 31/2 grains of silver chloride would show that a weak bath should be used, whilst 10 grains would show that a strong bath was required.
With most brands of albumenized paper directions are issued as to the best strength of silver nitrate solution for sensitizing, and a fair estimate of the chloride present can be gained from such directions.
A weak solution loses much of its strength by each sheet of paper floated, much more proportionally, in fact, than a strong solution, since the same amount of fluid is absorbed by the paper in each case, whilst the amount of silver abstracted from the whole is also equal, which reduces the strength per ounce more with the former than with the latter. A weak sensitizing solution, therefore, requires much more attention than a strong one: crystals of silver nitrate must be constantly added to the former. In practice and for general work, then, we recommend a moderately strong bath, the method of making up of which we shall describe.
To make up 2 pints of solution with a strength of 50 grains to the ounce, we shall require 2,000 grains of silver nitrate. This is carefully weighed out in the scales, a piece of filter paper being placed in each pan. By adopting this plan freedom from all impurities that may cling to the pans will be avoided, and the silver nitrate will be perfectly pure. Place the silver salt in a large clean bottle, and add half-a-pint of water to it, and shake it to dissolve it. The best water for the purpose is distilled water; but filtered rain, pure spring, or river water answers well. If the water contain any chlorides, it will be shown by a milkiness due to a formation of silver chloride. This must be filtered out when the remaining pint and a-half of water is added. The solution is now ready for use, and, being of the simplest character, is not to be excelled, though the addition of some soluble salts may be advantageous, particularly in dry climates or in very dry weather. Such salts are found in sodium nitrate, or ammonium nitrate, as much as equal weights of either of these substances being added. Thus our formula would stand as follows were these additions made:—
| Original Solution. | |||
|---|---|---|---|
| 1.— | Silver nitrate | 50 | grains |
| Water | 1 | ounce | |
| Modified Solution. | |||
| 2.— | Silver nitrate | 50 | grains |
| Ammonium nitrate or sodium nitrate | 50 | " | |
| Water | 1 | ounce | |
The reason of the addition of the ammonium or sodium nitrate is that prints are better obtained on paper which is not absolutely free from water. When very dry, the liberated chlorine (see [page 32]) is apt to attack the albuminate, whereas it is deprived of much of its activity when it is able to be absorbed by water, which, in the presence of light, is decomposed into hydrochloric acid and oxygen.[10]