This section is from the "Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1922 " book, by Aristo Motto. Also see Amazon: Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1922 .
If sulphur has already been precipitated in the fixing bath, however, further addition of sodium sulphite will not dissolve or re-form it into hypo. This is because a cold solution of sodium sulphite is only capable of dissolving sulphur at the exact time it is released from hypo, when it is in a very finely divided form. So it is important that there be sufficient pure sulphite in the solution at all times.
Aside from the exhaustion of the hypo, practically all of the trouble encountered with the acid fixing bath is due to the releasing of sulphur and its consequent action on the print that is being fixed.
Sodium sulphite that is impure, that is old or that has been exposed to the air will contain considerable sulphate, which has no action as a preservative. Even the purest sulphite will oxidize if left exposed to the air. If such soda is used in making a bath and it becomes milky it is due to the lack of sufficient pure sulphite. Sodium Sulphite (E.K. Co.) is 98% pure, but like an inferior sulphite it will lose some of its purity by oxidation if the bottle is left open to the action of the air.
Sodium sulphite oxidizes more readily in solution than in its dry form, so the stock solution of hardener should be kept in a bottle tightly corked and the prepared fixing bath poured into a bottle or jug if it is in condition to use a second time.
Oxidation will destroy an acid fixing bath that has never been used if it is allowed to stand in an open tray for some time.
In hot weather a bath will become warm and release sulphur, even though it has been properly prepared, which is another argument for the making of fixing baths only for immediate use.
The first important thing in hot weather is to be sure the print is thoroughly fixed. The next is to be sure it is thoroughly washed. Prints should be kept separated in the wash water in warm weather, especially as the fixing solution must be eliminated from the gelatine emulsion quickly and thoroughly.
If prints lie matted together in warm water they may begin to tone in spots where water does not reach the emulsion. If they are removed from the water before the hypo has been entirely eliminated that portion of the print containing hypo may turn brown after the prints have been laid out to dry, or even after they are in the hands of your customers and the chemicals left in the emulsion are acted upon by the light and air.
Acetic acid (28% pure) is specified in our formulas because the fixing bath requires a considerable volume of a weak acid and 28% is the proper strength. A stronger acid should not be used. Acetic acid (28% pure) can be secured at any photographic stock house. If you depend upon a local source of supply and can only secure glacial acetic 99% pure, this can be used but only after it has been properly diluted. To make a 28% solution add 3 ounces of 99% acid to 8 ounces of water. Do not add 99% acid to a fixing bath under any conditions.
If you know the action of the acid fixing bath, keep it as near a temperature of 65° F. as possible and take the other precautions we have mentioned to prevent sulphurization, you will never have any trouble in making permanent prints even in the hottest weather. And with a stock solution of hardener it is certainly easy to make a fresh bath whenever needed.
The expense is not in discarding an old bath and making a fresh one - it is in using the old bath, spoiling prints, losing time and material and, possibly, customers.
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