This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Keeping The Sensitizing Bath In Working Condition. From constant use the silver bath will in time become charged with soluble salts, resulting from the numerous collodionized plates which have been sensitized in this bath. It is advisable, therefore, to have two baths in working order, and while one bath is in use the second may be doctored and put in condition. Unless the bath has been worked hard, ordinarily pouring it into a large bottle and placing in the sun for a clay or so will precipitate all the impurities. At the same time a certain amount of the solution will evaporate, and it will be necessary, therefore, to bring up the bath to its normal quantity by adding distilled water to the bath, after which it should be tested for strength and brought to the normal condition.
932. The sunning of the bath will also evaporate the excess ether which has accumulated from the collodion, and all organic matter will, as a rule, be precipitated to the bottom of the bath. Usually, the bath is again in condition for use after a good day's sunning. If the bath is very much overworked, however, it may have become seriously charged with organic matter. Under such conditions you will need to resort to the boiling of the bath. To do this pour the solution into an enamel or earthen dish and place over a lighted gas or oil stove. Before placing over the fire, however, first neutralize the bath - make it alkaline - by adding a few drops of ammonia, and test with red litmus paper. When the red litmus paper turns blue, the bath is alkaline and may be placed on the fire to boil. The object of neutralizing the bath is to precipitate all organic matter.
933. When the bath comes to a good boiling stage, you will find the color of the bath to be black and turbid. The heating will cause evaporation and when evaporated, or boiled down to about one-third its former quantity, remove it from the stove and allow to cool, then filter. Before filtering, however, you should test the bath with the hydrometer, when you will find it very strong, the strength being increased as the bulk of the bath decreases by evaporation. You will, therefore, need to add sufficient distilled water to bring it to its original quantity, then finally test with the hydrometer.
934. As a certain amount of silver is deposited upon each plate sensitized, the bath when brought back to its original quantity, naturally will be weaker and it will be necessary to add more nitrate of silver to bring it up to its normal strength. All this should be done before the bath is filtered.
Fusing The Bath. There is still another method of doctoring a bath, which is termed fusing. This method is similar to the one just described, only that you carry the work farther. You continue the evaporation to dryness, allowing the bath to remain on the stove until all the frothiness has disappeared. This process, however, is best done in an earthen dish, which can be purchased from any photographic supply house.
936. When the bath is brought to the proper stage and the frothiness has disappeared, remove it from the stove and gather the mass into a lump in the center of the dish; then apply to this a diluted solution of nitric acid prepared in the proportion of about 1 ounce nitric acid to 12 ounces of water, which will redissolve the silver. By placing the dish over the fire again the bath is once more evaporated to dryness, after which the sediment is dissolved in distilled water. Additional distilled water is then added until the bath is brought to its normal quantity and sufficient additional nitrate of silver crystals added to bring it to the proper strength. By this latter method all organic matter will have been thoroughly carbonized, and after acidifying the bath with a few drops of nitric acid and testing with blue litmus paper it is again in good condition for use.
937. Special Formula. Negative Collodion. Good for Ferrotypes also. -
Negative Cotton ..............
Iodide of Ammonium
Iodide of Cadmium ...........
Bromide of Cadmium
Dissolve in the order given.