Instead of the two separate baths used above, a single solution may
be employed consisting of a metallic ferricyanide dissolved in a
suitable solvent (say, an alkali salt of citric, tartaric, or oxalic
acids) in presence of an acid and certain other salts.
On immersion of the positive film in this solution the silver image
is converted to silver ferrocyanide, while the colored ferrocyanide
is formed simultaneously and combined with it.
In either of the above cases in view of the fact that the metallic
ferrocyanide is deposited in a colloidal condition in presence of the
gelatine of the film, its state of division and the nature of the
tone are usually affected by the presence of certain salts, changes
of temperature, concentration of the baths and other factors which
must be maintained constant in order to obtain uniform results.
With such single baths it is possible to secure tones which are
unobtainable by a two solution process, though as such single
solutions are sensitive to light and rapidly attack foreign metals
which may be present in the shape of faucets, etc., such solutions
are comparatively unstable and require care in their use.