This section is from the "Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1913" book, by Aristo Motto. Also see Amazon: Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1913.
Many photographers have trouble during the warm summer months, with milky fixing baths, and as a good reputation may be very easily damaged by turning out a few prints of inferior quality, it is well to use all possible care and take all necessary precautions to insure perfect prints, even if it is just a trifle more trouble.
Prints must be hardened in the fixing bath, and alum is the best hardener. Alum in combination with hypo will release sulphur, but acetic acid and pure sulphite of soda forms a gas which prevents this releasing of sulphur, making a perfectly balanced acid fixing bath.
There are three reasons for sulphur being released, which causes the bath to become milky, but the effect on the print is practically the same in each case. Impure sulphite of soda contains sulphate, and the same is true of sulphite which has deteriorated through exposure to the air. Sulphate of soda added to hypo solution in very small quantities will cause sulphurization. Buy pure sulphite of soda and keep it in a tightly corked bottle.
If you are sure about the quality of your sulphite of soda - are sure it contains no sulphate there is still another precaution to take in mixing your chemicals for an acid fixing bath. Be sure the hypo is thoroughly dissolved before adding the hardener. This is important. If any hypo remains undissolved, the addition of the hardener will make the bath milky, which indicates that sulphur has been released.
A properly made acid fixing bath should never be allowed to become very warm. Even an unused fixing bath, if allowed to stand until it becomes warm and the gas formed by the acetic acid and sulphite of soda has partially evaporated, will become milky, even though a print has never been fixed in it. It becomes, in reality, a toning bath. For this reason, it is much better in warm weather to mix a fresh fixing bath each time prints are fixed. By making up a stock solution of hardener, it is only necessary to dissolve the proper quantity of hypo in the water and add the hardener. The use of glacial instead of No. 8 Acetic Acid will also cause sulphurization.
A print fixed in a milky acid fixing bath really begins to take on a sulphur tone while fixing, and while it may not be noticed at the time prints are taken from the bath, or while in the wash water, nevertheless the toning process has begun and will continue even after the prints have been laid out to dry, especially if the air is warm. The result is a print with brown or yellowish spots and sometimes a brown tone fairly even over the entire print. The customer would say his pictures were fading.
From An Artura Iris Print By The Kidd Studio Roanoke, Va.
Use tested soda of certain purity, being careful not to expose it to the air; keep a stock solution of hardener and mix fresh fixing baths as needed; do not add the hardener to the hypo until it is thoroughly dissolved and do not allow the bath to become warm after it has been mixed. These are all important. Of course, everyone knows a fixing bath should not be overworked, as prints will neither be properly fixed nor hardened in a bath that has had most of the chemicals worked out of it.
With these precautions carefully observed, no one should have any trouble in fixing prints, even in extremely warm weather.