To fuse a bath pour the bath into one-fourth its volume of water, and filter out the precipitated iodide; place the solution into an evaporating-dish on the stove, and evaporate to dryness, without neutralizing with ammonia or any other agent. As the solution becomes dry, a series of phenomena takes place; minute bubbles commence forming in the centre of the dish and gradually spread out to the sides. They increase in size and become violent in action, finally breaking open and emitting masses of vapor. "When this has all ceased, scrape the mass into the centre of the dish, where it will commence to liquefy again; or, in other words, to melt. It will soon have the appearance of a heavy oil or syrup, and may be considered fused. "When cool, it can be dissolved in pure water, diluted to any extent, and used as required. - Elbert Anderson.
Boil down in an evaporating-dish the refractory bath, and, when nearly dry, it will froth up very much; therefore take care to have the vessel large enough. After a time the frothing will cease, and the contents of the dish will settle down into a crystalline mass. Continue the heat, taking care not to have too much, or the silver will again froth up, and after
166. And yet the means of rectifying the bath - of saving it from being utterly cart out and aside - are not yet exhausted.While ingenuity has been greatly exercised to find a substitute for it as much thought has been given to means of wring it and making it do its duty.
Then are those whose time is so valuable that they can better afford to make up a new hath than spend time in doctoring an old one; all cannot.
The notes which follow will be found very helpful to all who are in the midst of bath trouble a time become perfectly liquid. It is now fused, but the operation should be continued until all bubbles have disappeared. The dish should now be set aside to cool. When cold, a gray cake, not unlike toffee, will be formed at the bottom of the dish. The fused silver should be dissolved in distilled water, but it will be quite a work of time, for the cake will be found as hard as a stone, and very insoluble. When in solution, sufficient water should be added to make forty grains per ounce. On trying a plate after the addition of a drop of dilute nitric acid, the result will be quite equal, if not superior, to that obtained in a bath of new silver. - Valentine Blanchard.
Put the solution to be clarified into a porcelain dish over a slow fire or a Bunsen burner, and, when boiling, drop by intervals into it pieces of caustic potash, stirring all the time with a glass rod. Continue this operation until the solution ceases to precipitate in the form of a brown-black powder, which is an oxide of silver. In order to be certain that the reaction is terminated, take a few drops of the solution and put it into a glass with a little distilled water; in adding a small quantity of hydrochloric acid, if the solution remains clear, the operation has been well conducted; if it turns of a milky appearance, more potash is required. The oxide of silver obtained must be well washed; it can then be dissolved in nitric acid, in order to form pure nitrate of silver. - Prof. E. StebbinG.
166. I will now describe my process of treating an old bath, by which it may be made n<>t as "good as new," but better; that is, as good as old. First. Filter the old bath into a bottle. Add silver, if necessary, until it is forty grains strong. Second. Add an excess of cyanide of potassium (about one grain to each ounce of silver nitrate is sufficient); shake thoroughly until no more cyanide of silver will dissolve; let settle; decant the clear liquid, and filter the residue. Third. Add three fluid drachms of No. 8 acetic acid to each quart of solution. Fourth. Neutralize with bicarbonate of soda, and set in the sun until it settles clear. Filter. Fifth Add pure nitric acid, beginning with one - half fluid drachm to the gallon, until all tendency to fog is removed. When more bath is needed, proceed in the same manner with the new, after having dissolved the silver and iodized it as usual; add the new to the old, and your experience will be different from mine if you do not find it better than a new bath.
The composition of this bath is peculiar. The addition of the cyanide alone causes mat silver stains in abundance; then, after adding the acetic acid, the most intense fogging will follow. The next step is also remarkable; for, in neutralizing the acetic acid, a considerable quantity of acetate of soda is introduced, the usual result of which would be precipitation of acetate of silver. Instead of this, a double salt is probably formed, more soluble than acetate of silver. Finally, the addition of nitric acid liberates a portion of acetic acid, and forms nitrate of soda.
167. As has been hinted already, the abuse of the bath solution is oftenest the cause of its refusal to do its duty. It should not be overworked. It should be kept at a proper degree of warmth in winter and of coolness in summer. All the other chemical solutions should be made to work in harmony with it. These things being properly attended to, there should be no unusual trouble with it.
The developer which I use with this bath, and recommend, is composed as follows:
Protosulphate of Iron,...........................................................................
Acetic Acid, No. 8,.........
10 fl. ounces.
W. H. Sherman.
When the bath yields dull negatives without vigor, or fogged pictures, it is usual to render the solution alkaline by the addition of carbonate of soda until a permanent precipitate is formed, and then to expose it for several days in the sun, to filter it, and finally to add a few drops of nitric acid to bring about a slightly acid reaction. Thus doctored, the bath usually becomes good; but in winter, when there is no sun, the bath sometimes freezes externally, and several days are necessary before it is again in good order.
The method I am about to indicate is not only very rapid and very effective in removing the iodide of silver, but likewise puts the bath into such order that it is capable of producing negatives as clear and brilliant and free from fog as a new bath in good condition.