A substance very much used by photographers of late years--in fact, so
much used that no well-appointed laboratory could be considered complete
without it--is the substance known is common alum, or potash alum, being
a double sulphate of alumina and potash; but it is interesting to note
that much of the commercial alum met with at the present time is ammonia
alum, or the double sulphate of alum and ammonia. It is quite a matter
of indifference to the photographer whether he uses potash alum or
ammonia alum.
Besides its great value to the autotype, Woodburytype, and mechanical
printers as an agent for hardening the gelatine films, it has been
recommended for all sorts of ailments photographic. The silver printer
adds a small portion to his sensitizing bath to keep it in working
order, and to prevent blistering of the albumen; then, again, silver
prints are soaked in a dilute solution of alum, having for its object
the thorough elimination of the last traces of the fixing salt. A very
good proportion to use for this latter purpose is four fluid ounces of a
saturated solution, diluted with one gallon of water, the prints being
well agitated during an immersion of ten minutes.
Of all the uses to which alum is put, perhaps not in any single instance
can so much satisfaction be derived as when it is used to
arrest frilling of gelatine plates. This it has the power to do
instantaneously, and many of the most careful workers, both amateur and
professional, or at least those who do net care to run any unnecessary
risks with negatives which have cost them a good deal of anxiety and
trouble to secure, but prefer to make assurance doubly sure--such
individuals may be numbered by the hundred--make it a point in every-day
practice to immerse all their plates in a solution of alum, either
before fixing, or immediately afterward. In fact, some operators have
two alum baths in use, one a normal bath, as above mentioned, for
immersing the plates in when of the ordinary printing intensity; and the
other a saturated solution strongly acidified by means of a vegetable
acid (such as citric) or a mineral acid (such as sulphuric), for use
when there is too much printing density, since it has been found
in practice that an acid solution of alum in contact with sodium
thio-sulphate on the gelatine image (after fixing, but before washing)
not only removes the color or stain caused by the alkaline or
pyrogallol, but perceptibly reduces the strength of the image. Moreover,
the color does not again reappear after washing, as it does sometimes
when the fixing salt has been partially washed away. In cases where
there is great tendency to frill--such, for instance, as when a soft
sample of gelatine has been employed, or old decomposed emulsion worked
in with the fresh emulsion--it will in such cases be safer to put the
plates in the normal-bath for a few minutes previous to immersing them
in the acid bath.
Potash alum is obtained tolerably pure in commerce in colorless
transparent crystalline masses, having an acid, sweetish, astringent
taste. It is soluble in 18 parts of water at 60° F., and in its own
weight of water at 212° F.; but the excess crystallizes out upon
cooling. The solution reddens litmus paper, and, when impure, usually
contains traces of oxide of iron. Upon the addition of either caustic
soda or potash, a white gelatinous precipitate is formed (hydrate of
alumina), which is soluble in excess of the reagent employed. The
precipitate thus obtained has much of the character of the opalescent
film sometimes observed on gelatine plates, when dry, which have been
soaked in alum, and not well washed afterward.
Alkaline carbonates--such as washing soda, for instance--precipitate
hydrate of alumina, which does not dissolve in an excess of the
reagents, and carbon dioxide is evolved.
Ammonia hydrate produces a precipitate in a much finer state of divison,
which does not dissolve in excess when examined in a test-tube, it
somewhat resembles thin starch paste.
The presence of traces of iron may be known by adding a few drops of
hydrochloric acid to a small quantity of a saturated solution of alum
in a test-tube, to which add strong liquid ammonia; should any iron be
present, the mixture will have a reddish-brown tinge when examined over
a sheet of white paper. Other alums exist, such as the double sulphate
of alumina and sodium, and sodium or aluminum and ammonium; but hitherto
their uses have been confined to the experimental portion of the
community rather than the practical.--_Photo. News_.
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