In order to duplicate any particular tint with a given dye-bath the
film may be dyed either by time or by inspection. Dyeing by time is
reliable if the dye-bath does not contain acid, though if acid is
present, in time the acidity decreases, causing a slowing down of the
rate of dyeing so that it becomes necessary to judge the progress of
dyeing by inspection.
If two or more tints of the same color are required, in order to
reduce the number of individual dye-baths to a minimum, it is better
to vary the time of dyeing rather than to vary the dilution of the
bath, providing the time of dyeing for the lighter tint is not
less than one minute, which time is considered a minimum for the
production of uniform results and for complete control of the dyeing
operations.
The time of dyeing also depends somewhat on the previous handling of
the film. Film which has been fixed in a bath containing ordinary, or
chrome alum, dyes more quickly than that treated with plain hypo and
hardened with formalin.
It is probable therefore, that small traces of alum are left in the
film even after prolonged washing, which serve as a mordant for the
dye.
The film samples herewith shown were fixed in the regular acid hypo
bath, so that if for any reason the tints indicated are not obtained
in the time stated, then either the time of dyeing or the dilution of
the dye-bath should be altered accordingly.
Should the film for any reason be over-dyed, a small portion of the
dye may be removed by washing for 10 to 15 minutes, though the nature
of the dyes will permit only slight mistakes to be rectified in this
manner.