The ferrotype and how to make it - The Grian Press

COLLODION. 61
Those that are generally used are the iodides
and bromides of potassium, ammonium, cadmium,
and sometimes zinc and sodium ; of these, all but
the potassium have the great advantage of being
readily soluble in any Collodion, and may, there-
fore, be added at once to the solution of gun-
cotton, but iodide of potassium requires to be
dissolved in water, and even then must be added
to the alcohol before mixing with the ether, or it
will be precipitated in the form of a fine powder,
insoluble except by the addition of more water.
To produce a film that is best adapted to the
Ferrotvpe-plate, it is recommended to avoid the
use of potassium as an excitant; it has been
found that the film of Collodion prepared with
this iodide possesses too much transparency ;
that
the smallest defect on, or discoloration of, the
surface of the plate may be observed in the
shadows of the picture. With a Collodion pre-
pared with ammonium and cadmium this is not
the case; the film is more opaque, and covers
many defects in or on the surface of the plate.
The iodides of zinc and ammonium give Collo-
dions that are equal to any others in sensitive-
ness, and in gradations of tone, and in all other
respects when first prepared, but after a short
time they lose those good qualities to a great
extent from the decomposition and liberation of
free iodine, while the iodide of cadmium is a cor-
rective of this tendency, and retains its original
qualities long after the others have lost theirs.