DURING the hot summer months difficulty is often experienced in keeping the gelatine emulsions of negative making materials in good physical condition during developing, fixing and washing.

It is natural for gelatine to soften and melt in warm water, and while the manufacturer can so harden gelatine that it will withstand a certain amount of heat the photographer himself must take precautions to keep the gelatine hard and tough.

This is accomplished by using an acid-alum rinse bath between developing and fixing and a hardening as well as fixing bath.

Naturally the chemical which produces the hardening action on the gelatine is of great importance. Alum has the property of shrinking and tanning the gelatine and it accomplishes this result most readily when it acts on the gelatine.before it has been allowed to swell and soften.

For this reason developers should be kept as nearly at a temperature of 65° F. as possible. An acid-alum rinse bath should be used directly after developing, as the acid neutralizes the alkali in the developer which remains in the print, while the alum begins the hardening action that is finished in the fixing bath.

The two most common forms of alum are ammonium alum and potassium alum. Either may be labeled "Powdered Alum" but there is considerable difference in their action.

Both will harden gelatine but if alkali from the developer is carried into a fixing bath containing ammonium alum and it becomes alkali, ammonia is liberated and negatives that are being fixed will become chemically fogged. The brilliancy of the negative is destroyed and poor prints are the result. For this reason potassium alum is more desirable.

But of even greater importance is the strength of the alum that is used. All of the alum that is sold under the plain label "Powdered Alum" bearing the Eastman Tested Chemical seal is Potassium Alum of exceptional strength and purity. The formulas for acid hardeners which we recommend are based on chemicals of definite strength and purity.

Powdered alum may be bought in a bag at almost any price, and it may contain almost any amount of impurities and be of any strength.

Impurities in alum, which have no hardening action on gelatine and which are useless in a fixing bath, can only be removed by re-crystalization. This form of purification makes one alum cost more than another. But the pure alum will harden your negatives when the cheap alum will not.

Portrait Film Negative, Artura Print By O.L. Markham Portland, Ore.

Portrait Film Negative, Artura Print By O.L. Markham Portland, Ore.

Of all the complaints of soft gelatine that we receive the majority can be traced to cheap powdered alum. The cheap alum may be all right for some purposes - it is not all right for pho-graphic purposes.