_Recovery of Silver._--As many correspondents of "N. & Q." have asked how
to recover the silver from their nitrate baths when deteriorated or
spoiled, perhaps the following hints may be acceptable to them. Let them
first precipitate the silver in the form of a chloride by adding common
salt to the nitrate solution. Let them then filter it, and it may be
reduced to its metallic state by either of the three following methods.
1. By adding to the wet chloride at least double its volume of water,
containing one-tenth part of sulphuric acid; plunge into this a thick piece
of zinc, and leave it here for four-and-twenty hours. The chloride of
silver will be reduced by the formation of {477} chloride and sulphate of
zinc, and of pure silver, which will remain under the form of a blackish
powder, which is then to be washed, filtered, and preserved for the purpose
of making nitrate of silver.
2. The chloride of silver which is to be reduced is put into a flask with
about twice its volume of a solution of caustic potash (of one part of
caustic potash to nine of water), in which a small portion of sugar has
been dissolved. Let it boil gently. The operation is complete when the
blackish powder which results from this process, having been washed in
several waters, is entirely soluble in nitric acid, which is easily
ascertained by experimenting on a small quantity. This powder is to be
preserved in the same way as the former for the purpose of converting it
into nitrate of silver.
3. The metallic silver is obtained in the form of a button, by mixing
thoroughly 100 parts of dried chloride of silver, 70 parts of chalk or
whitening, and 4 parts of charcoal. This mixture is to be exposed in a
crucible to a fierce red heat for at least half an hour. When completely
cold the crucible is broken, and a button of pure silver is the result. The
first two processes are those which I should most strongly recommend to
your correspondents.