THE REMOVAL OF THE SILVER IMAGE
BLEACHING.—The bleaching process has the purpose of making the bromide
print, correctly prepared according to the previously described method,
suitable for the bromoil process. To this end the silver image must
be made to disappear and in its place that condition of the gelatine
produced which renders it possible for it to take up the greasy ink.
_The bleaching solution has, therefore, two functions: it must remove
the metallic silver, imbedded in the gelatine film, which forms the
bromide image, and at the same time cause a tanning of the gelatine film
corresponding to the image that disappears. In the place of the silver
image there then exists an invisible tanned image in the gelatine film._
There are a large number of chemical compounds known to photographic
technique, which enable us to dissolve out the metallic silver imbedded
in the gelatine film. Such are, for example, the many reducers which have
found practical application. Many of these chemicals also cause changes
in the gelatine simultaneously with the solution of the silver. But not
one of the hitherto known bleaching solutions possesses the double power
required of it: solution of the silver image and corresponding tanning
of the film. Some produce too great a tanning which acts upon the whole
film, and the result in inking-up is muddy flat prints, which do not lend
themselves to artistic modification. With other bleaching solutions a
differential tanning of the gelatine is produced, but at the same time
they so alter the surface of the gelatine that it becomes glossy all
over, and only takes even soft inks with difficulty.
My experiments have led to the compounding of a bleach which completely
fulfils the requirements set for it; the silver image is quickly and
completely removed, while simultaneously a tanning of the film, strictly
analogous to the disappearing image, is effected; easier and more
certain inking-up is rendered possible, and besides this the advantage
is obtained that the differences of relief, produced in the gelatine by
the bleaching process, can be influenced to a wide degree by varying the
temperature of the water. The composition of this bleaching solution,
which prepares the gelatine film in the most perfect manner for the
bromoil print, is as follows, three stock solutions being required:
I. Cupric sulphate 200 g 2 oz.
Water 1000 ccm 10 oz.
II. Potassium bromide 200 g 2 oz.
Water 1000 ccm 10 oz.
III. Cold saturated solution of potassium bichromate.
A concentrated bleach is made by mixing:
Solution I. 3 parts
Solution II. 3 parts
Solution III. 1 part
To every 100 ccm of this mixture should be added 10 drops of pure
hydrochloric acid (10 drops to 3½ oz.). This concentrated bleach will
keep indefinitely and should be diluted before use with three to four
times its volume of water. The use of a more concentrated solution is not
advisable, as irregularities frequently occur in consequence of too rapid
bleaching, especially towards the margins of the prints.
The color of the concentrated bleach is green, or when diluted,
yellowish; the solution must be absolutely clear. When the stock
solutions are mixed there is usually some cloudiness, but this is
cleared up by the hydrochloric acid. By standing for a long time at low
temperatures a precipitate is sometimes formed, but this is of no moment.
The compounding of this bleach should be made with the greatest accuracy.
Inaccuracies or modifications in its composition are serious, because
although the solution does not lose in bleaching power, yet the invisible
tanning action is then often not completed in the desired manner. Too
great an addition of hydrochloric acid for example, accelerates the
process of bleaching, but the inking-up of prints thus bleached is
frequently difficult. If the bleaching of the shadows of the bromide
prints goes on slowly, the reason as a rule lies in the fact that the
prints were overdeveloped and have an excessively dense silver deposit.
The bromide prints should be immersed in this bleaching solution,
after previous soaking in cold water. If they have been correctly
made, the image rapidly grows weaker and after a few minutes its
greyish-black color changes into a pale citron yellow. If the bromide
print was developed too far, the bleaching takes rather longer, as
the shadows, developed right through to the base, require a lengthy
period for solution. If several prints are to be bleached at once,
the best procedure is to place one print in the solution and turn it
film side down when the first traces of bleaching are noticeable.
Then the next print should be immersed with the film up and by thus
proceeding gradually it is possible to bleach a large number of sheets
simultaneously in the one dish. Continual movement will prevent the
formation of air bells. If air bells adhere to the film, they protect
those places from the action of the bleach and dark points or spots of
unchanged metallic silver remain, the subsequent bleaching of which
naturally prolongs the process. The same applies to prints which lie on
top of one another.
With too slow bleaching, the hydrochloric acid may be gradually
increased, _at the most_ to double that prescribed; one should not hasten
the bleaching process by warming the solution. The bleaching is rapidly
effected in warm solutions; yet generally the film of moderately hardened
papers is so altered that they swell up too much even in cold water and
take the ink badly or not at all. The dilute bleaching solution will keep
and may be used repeatedly as long as it acts; when it becomes exhausted,
the slowing up of the bleaching cannot be hastened by the addition of
hydrochloric acid. The chemical reactions in the bleaching bath are,
according to Dr. P. R. von Schrott, as follows:
2CuBr₂ + Ag₂ = 2AgBr + Cu₂Br₂
The cuprous bromide, Cu₂Br₂, which is formed, reduces the bichromate as
follows:
3Cu₂Br₂ + 6CrO₃ = 3CuBr₂ + 3CuCrO₄ + Cr₂O₃.CrO₃
_It sometimes happens that bromide prints, in spite of long immersion in
the bleaching solution, apparently will not bleach and only change their
color to brown._
The reason for this usually unimportant phenomenon is, as a rule, that
such prints have not been sufficiently washed and still contain hypo.
It may also happen that prints which have lain on top of each other in
washing are badly washed in parts; then the image bleaches, _but the film
shows dark patches or streaks at those places which still contain hypo_.
Such apparently unbleached prints should be left for about 10 minutes in
the bleaching solution; _the disturbing coloration, whether of the whole
picture or only of parts, disappears completely in the subsequent baths_,
even when the image had apparently remained at full strength.
If such a print, apparently not bleached or spotty, is immersed in the
sulphuric acid bath mentioned below, the discoloration of the film is
quickly removed by its action; the print then often passes through a
phase in which it appears to be a negative, the secondary image becoming
visible on the yellow ground, and then bleaches out completely. With such
prints it may also happen that it is only noticed after removal of the
stain that unbleached traces of the silver image still remain. Then the
bleaching must be repeated.
If the color of the bromide print only changes to brown even after
protracted immersion in the bleaching solution, otherwise retaining full
gradation, and remaining unchanged even in the sulphuric acid bath,
though it bleaches out in the hypo, the print cannot be inked. The reason
for this difficulty is improper composition of the bleaching solution, or
occasionally improper development and fixation of the bromide print. It
may also be due to excessive use of the bleaching solution; 3 to 4 ccm
(50 to 70 minims) of concentrated bleaching solution should be allowed
for every 13 by 18 cm (5 by 7) print.
Obviously all these processes may be carried out by diffused daylight.
The bleached-out prints should be repeatedly washed, until the drainings
are quite clear, and should then be immersed in the following bath:
Sulphuric acid, pure 10 ccm 77 min.
Water 1000 ccm 16 oz.
_In this bath any remaining color disappears quickly and completely_, and
prints, which have apparently wholly or partially resisted bleaching,
are also very rapidly decolorized in this bath. Any spots and streaks
also disappear. If, however, there is anything left, then the bleaching
was not complete, and unreduced metallic silver remains in the film.
_After the sulphuric acid bath the prints should show the pure color of
the paper base; the film side ought to be hardly different from the back
in color._ With prints that have been overdeveloped, a certain slight
variation of color remains in the film, which, however, in no wise
prejudices the inking-up. If there are still some spots, they are usually
due to a slight precipitate lying on the surface of the film, which can
be easily swabbed off. When this point of colorlessness is reached, and
it usually requires only a few minutes, it is useless to leave the prints
longer in the acid bath. They should be washed in repeated changes of
water and immersed in the following fixing bath:
Hypo 100 g 1 oz.
Water 1000 ccm 10 oz.
The use of this fixing bath is essential and is based on the following
considerations. During the bleaching process a secondary silver bromide
image is formed in the gelatine film. This secondary image is not visible
on white and yellowish bromide papers, because it is whitish-grey. If a
bleached print, which has not been fixed, is exposed for a long time to
daylight a distinctly visible blue-grey image is formed, which naturally
is troublesome in the further operations. This secondary image of silver
bromide is completely removed, however, by the fixing bath.
The ordinary acid fixing baths can also be used without disadvantage
for fixing. If the sulphuric acid is not sufficiently washed out,
decomposition of the fixing bath may ensue, which will be made apparent
by the unpleasant smell, and which is prejudicial to the action of the
bath. Care should be taken that the prints do not stick to one another
in the fixing bath and that they are thoroughly fixed out, as the
secondary bromide image that is not removed will make its appearance in
insufficiently fixed places and may cause darker patches.
Washing then completes the preliminary preparation of the prints.
For the sake of completeness it should be mentioned that the prints may
be immersed in the bleaching solution in the darkroom after the first
development, and can be fixed after the solution of the silver image.
This shortened process is, however, uncertain and can not be recommended.
THE INTERMEDIATE DRYING.—_After the bleaching process outlined in the
previous section the print must be dried without fail._ While drying
after the development and fixation of the bromide print is advisable but
not absolutely necessary, _the intermediate drying after bleaching is
of the greatest importance_. It is possible that the later operations
may be successful in spite of neglect of this recommendation. As a rule,
however, various mishaps occur when the intermediate drying is omitted.
In many cases the ink can only be caused to adhere with difficulty, in
others, not at all; sometimes the inking will proceed up to a certain
point and then suddenly completely stop. Sometimes the image appears
as a negative, that is to say, the ink is taken up by the high lights
and rejected by the shadows. All these failures will be obviated by the
intermediate drying at this stage. Whether this intermediate drying takes
place rapidly or slowly is practically immaterial; naturally it ought not
to be so prolonged that the gelatine suffers.
The prints thus prepared can either be again soaked in water and
immediately worked up, _or kept and treated at any time_. It is very
convenient, especially for an amateur, to have a stock of such ready
prepared and dry prints, because he is then in a position to work when
he finds time and opportunity. The prints, prepared and dried as has
been described, will keep indefinitely. With correct treatment there can
be seen on the gelatine film of the dry print scarcely a trace of the
bleached-out image; only in the very deepest shadows a slight coloration
of the film, tending to grey, can sometimes be noticed. It is advisable,
therefore, to mark the print on the paper side before bleaching, as
otherwise it is subsequently difficult to distinguish this.
Before we go any further, the whole preliminary process is summarized
once more:
Development,
Fixation,
Washing,
Bleaching,
Short washing,
Sulphuric acid bath,
Short washing,
Fixation,
Washing,
Intermediate drying.