Owing to the limited number of colored inorganic compounds available
for toning purposes, it is possible to obtain only a limited range of
tones by the above methods.
In view of the possible wide range of colors obtainable by the use
of dyes, it has long been considered that a satisfactory method of
producing a dye image would be the ideal method for toning purposes.
As a result of recent work in our Research Laboratory, it is now
possible to recommend such a method of toning which depends upon the
fact that certain inorganic compounds, including copper ferrocyanide,
have the property of mordanting certain basic dyes, so that when
they are immersed in a solution of the dye the latter attaches itself
to the mordant, which in turn becomes colored.
The most suitable dyes for dye toning are those which do not readily
dye gelatine so that on immersing, say, a sample of copper toned
film in a weak solution of the dye containing a little acid, the
dye becomes mordanted to the image, while the highlights consisting
of clear gelatine remain colorless, the resulting tone being a
combination of the color of the toned film and the dye.
Full particulars are given under tones J, K, L, M and N.