In order to keep a fixing bath from discoloration and from staining the material fixed in it, it is necessary that the bath should be acid, and since developer is always carried over into the fixing bath with prints or films, it is necessary that a considerable amount of acid should be present in the fixing bath in order to neutralize the alkaline developer carried over. Unfortunately, hypo is decomposed in the presence of strong acid, the hypo being converted into sulphite and free sulphur, and this sulphur which appears as a milky precipitate in the fixing bath is injurious to prints.

This change of hypo into sulphite and sulphur is reversible, that is to say, if we boil together sulphite and sulphur we can get hypo, so that while acids free the sulphur from the hypo, sulphite combines with the sulphur to form hypo again. Consequently, we can prevent acid decomposing hypo if we have enough sulphite present since the sulphite works in the opposite direction to the acid. An acid fixing bath therefore is preserved from decomposition by the addition of sulphite.

Now, acids are of various chemical strengths. Sulphuric acid is a strong acid, and a small quantity of it will liberate the sulphur from the hypo, but acetic acid is a chemically weak acid, and a good deal of acetic acid can be added to a fixing bath without any sulphur being liberated. The reason, therefore, for using acetic acid in fixing baths is that it forms a sort of reserve of acid which can neutralize a large amount of alkali carried over from the developer. By using acetic acid with sulphite we can make a fixing bath containing a large amount of chemically weak acid which can neutralize the alkaline developer but which is not strong enough chemically to precipitate sulphur from the hypo in the presence of the sulphite.

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By Eugene Hutchinson

Middle Atlantic States Convention

At the present time there is a great shortage of acetic acid owing to its use for military purposes and it is necessary to find a substitute. Citric acid can be substituted, but it is more expensive and the supply is insufficient to replace all the acetic acid which has been used, so the only substitute which appears to be available is bisulphite.

Bisulphite will not decompose hypo provided that it is pure. It makes a satisfactory acid fixing bath, but, unfortunately, it does not give as good a reserve of acid as acetic acid does. As a consequence a fixing bath made up with bisulphite and alum will become milky after some quantity of developer has been carried over into it and it has lost its acidity. At the first glance this milkiness looks like sulphur, but the precipitate is basic aluminum sulphite and is due to the fixing bath losing its acidity.

In order to avoid this precipitation the only satisfactory method is to use some other hardening agent than alum, and for this purpose the use of chrome alum is suggested. Chromium does not precipitate as easily as the aluminum and will remain clear even when appreciably alkaline. A bath without acetic acid recommended for fixing prints is made up as follows:

Water .... 64 ounces Hypo .... 16 ounces

When dissolved add the following solution:

Chrome Alum . 100 grains Sodium Bisulphite 1 ounce Water .... 5 ounces

Quickly but thoroughly rinse prints in fresh water, immediately after developing and before placing them in the fixing solution. Fix for about ten minutes, during which time prints should be well separated to receive uniform action of the solution.

Exhaustion of the Chrome Alum bath will be noticed when prints lose a certain leathery feeling and appear slimy to the touch. When this condition is present, the bath should be discarded and a fresh one prepared.

Our Research Laboratory is at present working on a similar tank fixing bath for films which we hope to publish before long.

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By Eugene Hutchinson

Middle Atlantic States Convention

Due to a greater use of Hypo in several of the industries there has been a trifling advance in the price of this most essential photographic chemical, but this of itself should not be a sufficient reason for anyone to stint the fixing of prints or plates, though one of our correspondents has suggested such a possibility.

One would have to be a wizard to be able to say, merely by the process of inspection, how near to exhaustion any particular fixing bath might be. It can't be done, so we suggest that a method be used to keep account of the number of prints fixed in a given amount of bath, or better still, that only the amount of bath necessary to fix a given number of prints be made up at one time and the bath discarded when the batch has been fixed.

Some readers will immediately say that it takes too much time to prepare a fresh bath daily - but does it? Acid hardener may be made up in any quantity, and keeps indefinitely in a well stoppered bottle. With such a stock solution the hard part of making up a fixing bath is the dissolving of a pound of Hypo in sixty-four ounces of water. Some may contend this is too much trouble, but surely the careful worker will not. To him, good results are the all important thing.

Sixty-four ounces of solution will fix two gross of cabinet size prints or one-half gross 8x10. It is quite safe to figure a gross of 8 x 10 prints to the gallon of bath, and surely this is not expensive. If you must make up a given amount of bath, keep an account of the prints fixed in it and don't over-work it.

An advantage of the fresh bath in hot weather is that the temperature of the solution is materially reduced as the Hypo crystals dissolve, and this advantage is lost once the bath has become warm. It soon becomes a toning bath. You may have had prints begin to tone in a warm fixing bath and you know that the loss of even a few prints, and the time and trouble of making them over, is much more expensive than making a fresh bath and being sure that every print you deliver is a perfect one.

You have no means of judging how fast the fixing action takes place in a print or when the hypo in the bath has become used up and fixing stops. You can get some idea of the condition of a paper fixing bath by noting the time it takes for an unexposed plate to clear in it, but even this will not tell you whether or not it will properly fix any given number of prints.

When you are in doubt, use a fresh bath and be certain of results.

Dad knows something of war from experience, so, safe to say, there will be pictures a plenty for his boy's pocket.

Your photograph for your soldier.

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Line cut No. 252. Price, 50 cents.

Substitute For Acetic Acid Fixing Bath StudioLightMagazine1918 116


Charles Brandenburg New York, N. Y