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.
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