ALUM BATH.
The following bath is one which is often omitted by some of the most particular workers, and yet others consider that it is almost indispensable for several reasons. It is known as the hardening bath, and is best made up as follows :—Chrome alum, 20 grains ; common salt, 1 oz. ; water, 20 oz. Instead of chrome alum, the o•di nary variety of powdered alum may be used, in which case take VI oz. The prints, after aluming, are much more easily and safely handled in the subse quent stages ; which is a strong point when time and clean work are considera tions. It is best to alum the prints before toning, as the slightest contact with the fixing agent will be fatal, and cause yellow stains. For enamelled prints, or those to be hot. rolled and burnished, this bath is absolutely necessary, since the film is toughened, and therefore better able to bear the heat and pressure. On the other hand, those which are to be mounted in optical contact should not be so treated. The salt is added to the bath,
not only for the purpose of converting any free silver salts in the paper into chloride, as before explained, but because it is also a. great preventive of black spots, clue to water charged with metallic impurities. Rust from iron pipes through which the water flows is a constant source of such troubles in old houses, or where the water is allowed to stand in the pipes for long periods. The prints must be kept con stantly on the move, and should remain soaking in the alum for five to ten minutes (of course, in winter the time may be much shorter than in the warmer season), care being taken that they do not stick to one another. On removal from this bath, again well wash in three or four changes, or, better still, in gently running water, for ten minutes, by which time they will be ready for the toning bath.