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 Silver Bath In Working Condition. There is considerable likelihood of the bath deteriorating, therefore, it requires frequent examining, or testing. The bath should be filtered back into the original bottle after using, and tested for strength, as each sheet of paper silvered takes up a certain amount of silver and thereby alters the condition and strength of the bath. As the bath becomes charged with organic matter, which may be liberated from the paper sensitized, it gradually turns to a dirty brownish color, and when in such a condition it is impossible to produce good results. This must be remedied at once, and it can only be done by removing the organic matter and strengthening the bath by the addition of silver.
41. The first step to take in clearing the bath is to expose it to strong daylight; better still, to sunlight for an entire day. The strong sunlight will cause the organic matter to precipitate to the bottom of the bottle. The clear solution must then be decanted. When using this method it is, of course, necessary to have two silver baths, although one can be made to do if you will sensitize all the paper that you will need for the day early in the morning and then immediately return the bath to the bottle and place in the sunlight. It is advisable, if possible, to have two baths. A more rapid way of removing this organic matter is to add a very small quantity of hydrochloric acid. After adding this the solution should be tested with blue litmus paper. If it shows an acid reaction sufficient carbonate of soda must be added to neutralize it.
42. Still another method, and the most practical and simple, being used by most photographers in winter, is to add a little Kaolin (china clay). This will carry down the finely divided precipitate. You must be absolutely sure that the Kaolin is pure; if adulterated it is not only useless but injurious.
Testing Strength Of Silver Bath. In order to ascertain the strength of your silver bath you should test it with a hydrometer. The ordinary hydrometer is not always accurate, however, and should be first tested by placing in plain water. If the bulb sinks to zero, or 0, it is correct; if it sinks below or above it is just that much off from being accurate. A safe instrument is what is known as an argentometer. This instrument is not unlike the hydrometer and is used in taking the specific gravity of the solution. It is made in the form of a glass tube and has three compartments. The top one, which resembles a rod, contains an ivory scale, graduated downward, from zero to 100. The second compartment is an air chamber, while the third contains mercury. When you place your solution into your graduate, and then place your argentometer into it, should this sink to the Fig. 50, it would show that each ounce of solution contains 50 grains of silver. If the meter is in perfect condition it will float at a number exactly equal to the number of grains of silver contained in an ounce of water.