Uranium Nitrate (Neutral) 16½ ozs. 500 grams
Potassium Oxalate (Neutral) 16½ ozs. 500 grams
Potassium Ferricyanide 6½ ozs. 200 grams
Ammonium Alum 2½ lbs. 1200 grams
Hydrochloric Acid 10% 1 quart 1000 cc.
Water to 50 gals. 200 liters
Mix in the order given. The solution obtained should be perfectly
clear and pale yellow in color.
In view of the fact that the nature of the tone is influenced largely
by the acid content, it is very important that the uranium nitrate
should contain no free acid, and this may be assured by neutralizing
a solution with dilute ammonia until a slight permanent precipitate
is obtained.
It is convenient to keep stock solutions of the above (say, 10%
solutions) from which a new bath may be expeditiously compounded. A
10% hydrochloric acid solution is one containing 10 parts by volume
of the concentrated acid per 100 volumes of the final solution.
_Temperature of Toning_—65 degrees to 70 degrees F.
_Time of Toning_—Ten minutes.
Since this and the following single solution methods of toning
produce a marked intensification of the silver image—which
intensification increases with the time of toning—it follows that the
nature of the tone changes with time also.
The composition of the bath has been so adjusted that the maximum
effect is produced in about 10 minutes, the tone passing through a
series of changes from brown to red during this time.
It is therefore possible to obtain intermediate tones by withdrawing
the film from the bath at shorter intervals, though in such cases
the contrast of the original black and white film should be adjusted
accordingly, since with slight toning very little intensification
takes place. (See tones C and D.)
_Time of Washing_—Ten to fifteen minutes.
The highlights will become clear in the above time, though a thin
yellowish brown veil may remain in the clear gelatine as a result of
the intensification of minute traces of fog, but this has no effect
on projection. If the bath is working correctly the yellowish veil is
only just perceptible, but if appreciably visible, then either the
film was fogged during development, or the bath was not compounded
correctly. Washing should not be carried out for too long a period,
especially with water inclined to be alkaline, because the toned
image is soluble in alkali.
_Life of Bath_
Fifty gallons of solution will tone about 5000 feet of film without
any appreciable change in the tone, after which the rich tone tends
to flatness as a result of a deficiency of acid in the bath. At this
point the bath may be revived by the further addition of acid to the
extent of the original amount, when a further 5000 feet may be toned.
At this stage the richness of tone falls off rapidly and the bath
should be thrown away.
Used intermittently over a period of several days the life of the
bath is approximately the same.
With continued use a slight brownish flocculent precipitate may
form in the bath, but this should only be slight, otherwise it is
caused by incorrect mixing, the action of light, or by contact with a
metallic surface.