The third class of silver baths includes those which in addition to the silver nitrate contain alkaline nitrate, such as the ammonium, sodium, or potassium salts. The action of these salts is to absorb the free chlorine, and further to prevent the paper from becoming too dry in very hot weather. There are a great variety of formula, one of the most generally used is— Silver nitrate 5o grains Ammonium nitrate 3o grains Water z ounce Made slightly alkaline with the addition of ammonia or sodium carbonate. A few grains of alum are also added sometimes, for preventing blisters.
In sensitizing albumen or plain salted papers upon the silver bath they are always floated. For full instructions see Sensitizing.
The strength of the silver baths should always be greater in the winter and less in the summer.
Never attempt to use the smallest possible amount of solution. The depth in the bath should be at least half an inch.
Considerable care and attention must be taken in the preparation and care of the silver bath. It must be remembered that each sheet of paper floated upon the solution absorbs a quan tity of the silver, and unless this be replaced the bath will soon refuse to work. For strengthen ing the solution, a stock solution of 6o grains of silver nitrate to the ounce of distilled water. A practice adopted by some operators is to allow about so grains of silver nitrate for each sheet of paper, and the amount of silver corresponding to the number of sheets used is dissolved in a few drachms of distilled water and added to the bath.
With regard to the water used in making up a strengthening silver bath, there is no doubt that distilled is the best; ordinary cistern water can, however, be used if the solution be exposed in a clear glass vessel to the sun or bright daylight. It will then become quite brown, owing to a precipitate formed, which will, however, fall in a few hours to the bottom of the vessel, when the clear liquid can be poured off and filtered.
The alkalinity of the bath should not be too decided. When it shows an acid reaction a few drops of a diluted solution of ammonia are added until the red litmus paper slowly changes to blue.
Some operators prefer to use an acid bath acidified with citric acid. This is useful when the paper is required to be kept for some days, but otherwise it is not recommendable, because the results are not so fine, the action is slower, and the bath is not so easily purified.
The usual method of determining the strength of the silver solution is by means of the hydrometer or argentometer, the latter being specially marked, showing at a glance the number of grains of silver nitrate in each ounce of the solution. With pure solutions of silver nitrate the strength can be correctly estimated, but after a time the bath becomes contaminated with soluble salts, organic matter, etc., which tend to increase the density of the solution, and to render the working of the argentometer less accurate. The principal source of contamination to the solu tion is from the dissolution of the albumen from the paper floated on it. This, in time, causes the bath to become yellow and then brown. This discoloration must, of course, be removed, otherwise the paper will be unevenly sensitized and darkened.
There are several methods of purifying the bath. A simple one is to add a little kaolin, and then shake it up well and place in glass vessel in the sun. The organic matter is carried down with the kaolin as it subsides. It is then filtered and is ready for use. Kaolin is the purest form of disintegrated felspar; it sometimes contains chalk, which renders it unfit for decolorizing acid solutions of silver citrate. It should then be treated with a weak acid and washed.
Another method of purifying the bath is by the addition of a little of a strong solution of potassium permanganate. A stock solution is made up of— Potassium permanganate. 20 grains. Water ounce.
The usual method after the paper has been all sensitized for the day is to add a few drops of this solution with a glass rod, and stirring up the bath to well mix it. The solution takes a rose tint, which gradually dies out; a few more drops are added until the rose color does not die away. The tinted solution is then put into a clean glass bottle and exposed to sunlight until the color disappears. It is then filtered until clear. With a few drops of the permanganate solution, the bath can be decolorizing during the sensitizing operations by well mixing it up. It must be used with caution, however, as evil effects can also arise from it.