2. Neutralise with ammonia till just alkaline; boil till black; let cool, filter, acidify with pure nitric acid, and evaporate to crystallisation, then fuse. When cool, add distilled water, shake and let stand exposed to light. Filter and add drained crystals. Dissolve and make solution acid with pure nitric acid. Expose again to sunlight, filter, and bath is ready for use.
3. Add potassium permanganate, expose to sunlight, filter, acidify, put in clean bottles four-fifths full, cork, and freeze in a tray; thaw gradually till ball of ice one-eighth size of bottle remains. This contains impurities to be reduced as III, below. Use rest for new bath at once.
II. Hyposulphite Bath.
1. Precipitate as silver sulphide by potassium sulphide. Reduce as III, below, or dissolve in nitric acid.
2. Precipitate by hydrosulphuric acid, and reduce as III.
3. Decompose hyposulphite by waste nitro-sulphuric acid from manufacture of gun-cotton for collodion. Have silver sulphide and sulphur, with sodium nitrate and sulphate in solution. Suspend zinc in solution, then boil two or three hours; wash on filter, dry, fuse with borax and sodium carbonate.
4. Suspend sheet copper in bath.
5. Add hydrochloric acid, which sets free sulphur and precipitates silver chloride. Oxidise sulphur by aqua regia, and reduce silver chloride as in III.
6. Add sodium hypochlorite to the alkaline solution. Wash, precipitate, and fuse with mixed carbonates. This gives sodium bisulphate and chloride as by-products, no sulphur, no fumes.
III. Reduction of Silver Chloride, Oxide, or Sulphide.