As distinct from tinting, a toned image consists of a color image
embedded within a layer of colorless gelatine, so that while the
highlights are clear, the shadows are colored.
The coloring matter may consist of an inorganic colored compound or
a dye, or of a mixture of both, and by wholly or partially replacing
the black and white silver image by one or more of these substances,
a colored image is produced.
It is, of course, important that the colored compound should be as
transparent as possible so that the full tone will be apparent on
projection. In this respect, while some samples of toned film appear
colored in the hand, they produce a substantially black and white
image on projection, so that when judging any particular tone, it is
absolutely necessary to view the image on the screen.
Of the various inorganic colored compounds, the metallic
ferrocyanides are the most suitable for toning purposes of which:
Iron (ferric) ferrocyanide is blue,
Copper ferrocyanide is red, and
Uranium ferrocyanide is reddish brown.
It is the object in toning to replace the metallic silver composing
the image more or less by one of the above compounds or by a mixture
of these. In this way intermediate tones are obtained.
Toning may be effected either by a single solution process or a two
solution process.