The permanency of the tones produced largely depends on the care
exercised throughout the various operations.
The silver sulphide image may be considered permanent, though in the
other cases, where more or less silver ferrocyanide is present, the
toned image is not absolutely permanent, being somewhat soluble in
hypo and affected by fumes of sulphuretted hydrogen.
In the case of the iron and uranium tones, the colored ferrocyanides
are dissolved by alkali such as sodium carbonate, so that the toned
film should not be washed for too long a time in water which is at
all inclined to be alkaline.
In no case however, if the instructions are carefully followed
will the toned image deteriorate during the active life of the
film. Moreover, so far as can be ascertained, the wear and tear of
film which has been toned by the methods recommended is in no way
impaired, though by virtue of the hardening action of most of the
toning baths on the gelatine it is advisable, especially during the
winter months, to immerse the film for three or four minutes in the
usual 2% glycerine bath after toning.
Should film have to be stored for long periods of time, toning
is inadvisable, nor is it advisable to tone valuable film unless
duplicates are available.
The _life_ of the toning bath has been carefully investigated in
each case, the term “life” being considered as the total length of
film capable of being toned by a given volume of fresh solution when
toning is conducted continuously and without interruption.
In all cases it is false economy to exhaust a toning bath to the
limit and thus obtain inferior tones, since the cost of the chemicals
employed is insignificant compared with the value of the film being
treated.
Since the figures given represent the capacity of the bath for
toning under the best conditions, they only apply providing the baths
are kept covered as far as possible when not in use, to exclude
light, and providing no foreign metallic surface, however small, is
allowed to come into contact with the solution.
As mentioned above, single solution baths are not intended for
occasional use. In such cases two-solution methods should as far as
possible be employed, although it has only been possible to recommend
the latter for the production of sulphide and blue-green tones.