Present day practice is more favorable towards preventive than to remedial measures and the prevention of any trouble is safer, more satisfactory and in the end much cheaper than the application of a remedy after the trouble has occurred.

It is quite true that in many uses of chemicals it is difficult to anticipate trouble but in photography there are such simple rules to follow and so much practical information at hand that there is little excuse for trouble if simple preventive measures are used.

Blisters may be produced on any gelatine paper if the gelatine is abused and when you think of the treatment the gelatine receives you can readily see that it is under a constant strain from the time it goes into the developer until it comes out of the final wash water.

Immediately a print is placed in the developer the gelatine begins to swell as it fills with water and this swelling is increased by the alkali in the solution and by heat. The developer should not be too hot and while it must contain alkali, there should not be too much.

These precautions should be observed, not because prints are likely to blister in the developer but because such precautions will help to prevent blisters further along.

If prints do contain an excessive amount of alkali and are carried from a warm developer into a strongly acid short stop solution or fixing bath, there is likely to be trouble at once. The action of the acid on the alkali in the gelatine forms a gas and immediately there are thousands of miniature volcanic eruptions on the surface of the print.

Normally, the pores of the gelatine are open, the gas blows off and no harm is done. But if an excess of alkali and warm developer has softened the gelatine, the pores or small canals running through it have been made smaller by the swelling mass - the vents have been closed and a blister forms.

Rinsing after developing removes a considerable amount of the alkali in the gelatine, reduces the amount of gas formed and not only helps to prevent blisters but prevents the fixing bath from becoming alkaline.

The blisters we have mentioned are not air bells but gas bells. Correctly speaking an air bell is caused by dissolved air in the water and this condition is encountered when the water used has been under high pressure.

Heat such water and you will see the air expelled and the bubbles form on the side of the vessel. Used in a developer, the aerated water penetrates the gelatine and if the solution becomes warmer the air is either expelled or the soft gelatine is blown up into an air bell. The only safe way to use such water is to draw it in a barrel and allow it to stand over night before using. This allows the air to escape.

Portrait Film Negative, Artura Print By J. L. Rivkin Tulsa, Okla.

Portrait Film Negative, Artura Print By J. L. Rivkin Tulsa, Okla.

So far as we have gone the preventive measures are simple but there is one thing left which is probably the most frequent cause of blisters - worn out fixing baths. Use fresh fixing baths, make them properly from good chemicals and don't overwork them. It must be remembered that either acid or alkali will soften gelatine and it is the alum which has the hardening action. A worn out acid fixing bath will more likely soften gelatine than harden it and the real strain on the gelatine comes when the print leaves the fixing bath and goes into the wash water.

This may seem strange but it is true. The gelatine is filled with hypo in solution and this solution is of high concentration while the wash water, free of chemicals, is of low concentration. There is an equalizing force which causes the water to rush into the gelatine faster than the hypo can diffuse out and as this force is greater than the resisting power of soft gelatine, if there is a weak spot in the gelatine caused by softening or swelling, that spot will develop a blister filled with water.

The means of prevention is a fresh fixing bath which will harden the gelatine emulsion uniformly. Don't attempt to renew a fixing bath. It can't be done practically. A bath that is ready to be discarded is contaminated by developing chemicals and some of its own properties have ceased to function. You can't build it up. Use a fresh bath.

There is one other general precaution to be observed. Keep the temperature of solutions as nearly uniform as possible. If the developer is 70° F. don't have the hypo 50° F. and the water 80° F. A sudden change from warm to cold, or cold to warm solutions will often produce blisters.

If prints are inclined to blister during toning the remedy is to treat the prints with a 3% solution of formalin before toning. If prints have not been properly hardened during developing and fixing, blisters may be caused by the hypo alum bath being too hot, or if they are re-developed, by an acid bleaching bath or an excessively strong sulphiding bath. The fact that the great majority of printers are never troubled with blistered prints leads one to believe that only ordinary care is necessary to prevent the trouble and in this case it is certain that prevention is always the safest measure, as there is no really satisfactory cure. If a blistered print must be saved it may be immersed in equal parts of water and alcohol, followed by a bath of alcohol.