Camera Work: A Photographic Quarterly (1905 (Heft 11))

and washing are required. In ozotype the elimination of the sensitive salt
is accomplished before the picture is made, and if this were not done well we
should soon know it in the pigmenting operation. We may therefore be
perfectly sure that in ozotype all the sensitive salts have been discharged
from the paper and there is no necessity for any subsequent chemical
treatment after the development. Never leave the washing till the following
day; wash as soon as convenient after the batch of prints is made. Of course,
the washing may be done in full daylight, as the prints, as soon as they are
immersed in water, are insensitive to light.
Pigmenting. — The washed initial image will keep for many months
in a pigmentable condition, if kept dry and away from light. I have
succeeded in pigmenting a print four years after exposure. The washed
initial print is reduced in a chemical sense by contact with a damp
atmosphere, by long exposure to light, and by lapse of time. The
chromium is not lost, but the chromates become partly reduced to chromic
oxide, which can be converted into a chromic salt by a continued action
of the acid bath. Therefore, those prints which have been kept six
weeks or two months should be kept in contact with the plaster for
an hour or more, and prints that have been kept six months should
remain in contact with the plaster for two hours, or until the gelatine is
dry. Bearing in mind that the chemical action ceases as soon as the
gelatine becomes dry, plastered prints may be developed the next day,
provided they are not overprinted and the drying action takes place fairly
rapidly. Of course, in all cases where the gelatine becomes dry, or nearly
so, the plastered print should be soaked in cold water for half to one hour
before development.
The Acid Bath.—The acid bath consists of a very dilute solution
of an acid such as acetic acid, a reducing agent such as hydroquinone,
and an accelerator (sulphate of copper). The usual negative-developer
consists of a strong solution of a reducing agent, an accelerator, and a
restrainer. In ozotype the acid acts upon the image and releases chromic
acid, or more probably converts the insoluble chromates into soluble
bichromates; these are reduced again by the hydroquinone to normal
chromates. These chromates exert a strongly insolubilizing action upon
gelatine and the oxidized hydroquinone also has a powerful tanning effect.
But how does the copper act? It is absolutely necessary for quick work.
That copper is an accelerator of the action between an organic developer
and the chromates can be demonstrated by a few experiments. Copper also
obviously acts in another way, for it adds another strongly tanning chromate
(the chromate of copper), which has a powerful insolubilizing action upon
gelatine.
After prolonged and exhaustive experiments with nearly all the true
acids, acetic acid has been found to work the most satisfactorily with
gelatine. The reason seems to be that while the mineral acids release
chromic acid from the chromates, acetic acid does not go so far, but converts
the insoluble chromates into soluble bichromates. Now, it is obvious that

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