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Dr Jb Obernetter's Process 1863

Copper chloride 100 parts
Ferric chloride, sol. sp. 13 parts
gr. 1.5
Hydrochloric acid, conc. 12 parts
C. P.
Water 1,000 parts

Float the paper on this solution for about two minutes and hang it up to
dry. The keeping quality of the prepared paper is remarkable; it has been
kept for two years without apparent change; its sensitiveness is at least
one-third greater than that of silver albumen paper. Unless developed
within an hour or two, the vigor of the proof is much impaired; after
twenty-four hours a print can be taken over on the same.

When exposed, only a faint image is visible. It should be fixed in the
following solution:

Potassium sulphocyanate 12 parts
Sulphuric acid, conc. 1 part
Sensitizing solution 10 to 12 parts
Water 1,000 parts

A print is floated on this solution, face downward, for three or four
minutes, taking care to agitate the liquid as little as possible; the
print is afterwards immersed and another one floated in its place, thus
proceeding until all the prints are immersed or the solution can hold no
more. A fresh solution is then added to strengthen it: the older the
solution the more rapidly and better it works. In this developer copper
cyanide is precipitated on the parts acted on by light, and this exactly
in the proportion to the luminous action. The time of immersion depends
on the method selected to finish the proofs; it its from five minutes to
half an hour. If the proof is immersed for, say, twenty-four hours, the
image comes out in a relief which may bring the shadows to two lines in
depth. When well developed and thoroughly washed, the proof can be dried
and the subsequent operations made at any convenient time.

Various processes may be employed to give to these proofs the tone
required; thus: the prints well washed are placed in a solution of
ferricyanate of potassium at 6 to 12 per 100 of water, where they take a
red color increasing in intensity. If left over night the color becomes a
splendid velvet deep red with perfect clear whites. To obtain the color
of silver photographs one hour's immersion is sufficient. After this
operation the proofs are washed until the water is no more tinged yellow.

By immersion in
Ferrous sulphate 100 parts
Iron sesquichloride 40 parts
Hydrochloric acid 80 parts
Water 200 to 300 parts

the proofs undergo the following gradation of colors: red, reddish violet,
blue-violet, black and greenish black. As soon as the desired color is
obtained, the proofs are washed in acidified water and dried.

The most beautiful purple violet is obtained by leaving the proofs in the
iron solution until green-black, and then washing for a moment in a dilute
solution of sub-acetate of lead.

A brown-black may be produced by treatment, after washing, with an
ammoniacal solution of hypermanganate of potash.

A weak solution of nitrate of silver also yields very fine pictures, but
the exposure should be very short, and the proofs must be fixed in water
containing a small quantity of oxalate of ammonia.

In order to impart to the proofs the gloss of silver photographs, they
should be albumenized in the ordinary manner, and the albumen
insolubilized by well known means.

The chemical actions in this process I explain in the following manner: On
the paper there are Fe2Cl3 and CuCl, the latter in excess. By the action
of light, and according to the transparency of the negative, Fe2Cl3 is
reduced to FeCl, while CuCl suffers no alteration.

If the paper be immediately placed in an absolutely dry room after
exposure, the picture remains unchanged. In a moist atmosphere FeCl
attracts moisture and, with a part of CuCl, is so decomposed that Fe2Cl3
is formed together with Cu2Cl.

After this action has commenced, if the proof be not immediately immersed
in a solution of sulphocyanate of potassium, Cu2Cl passes over to a higher
combination of chlorine, and the paper is again fit to be impressed anew
by the action of light.

As long as FeCl or even Cu2Cl is present, if the print is immersed in the
sulphocyanate solution, sulphocyanate of copper is immediately formed on
the reduced parts, while on the others the sulphocyanide of copper, formed
and dissolved by the sulphocyanide of potassium in excess, becomes decom-
posed with water in soluble sulphocyanide of copper and deposited as such
on the parts already covered with the salt.

Frequently the prints appear yellow from formation of the double
sulphocyanide of copper, but the color disappears by washing in water.
Red coloration is due to decomposition into ferrocyanide of copper.





Next: L Liesegang's Process 1865
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