Improvements in Papers for Photographic Purposes.
Year: 1895

page_text: Date of Application, 24th July, 1895-Accepted, 21st Sept., 1895
COMPLETE SPECIFICATION.
Improvements in Papers for Photographic Purposes.
I, HERMANN WANDROWSKY, Chemist of Ehrenfeld near Cologne on the Rhine
in the Province of Rhineland and Kingdom of Prussia do hereby declare the
nature of this invention and in what manner the same is to be performed, to
be particularly described and ascertained in and by the following statement :-
The sensitive papers used for photographic purposes, which are prepared in such
a way, with silver nitrate solution or emulsion that there is still free silver nitrate
present in the sensitive layer, suffer from the disadvantage that they become very
quickly brown on the reverse side of the paper. The reason of this is, that the
silver-nitrate, which penetrates from the sensitive layer into the layer of paper
lying under it attacks and destroys more or less the paper fibres as well as all other
organic stuffs that may be present in the paper layer.
It is true that attempts have been made by the addition of acids to prevent the
paper becoming brown; these acids however either formed no silver combination
at all or only very week ones and thus on the one hand did not fulfil their object,
on the other hand they produced a number of minor noxious effects, which rendered
their use still less desirable.
By the new process a paper is produced which in every respects fulfils all
requirements.
This process consists in introducing into the paper intended for photographic
purposes, at the time of its manufacture or later on, a salt which by a double
decomposition turns the silver nitrate into a preparation of silver, which is
insoluble or very nearly so in water, but easily soluble in sulfite of soda, and is
active in the development of the picture.
In sensitive papers which consist of barytes paper it is advisable to add the salts
to the layer of barytes as well as to the raw paper.
Salts suitable for the purpose in question are the salts of oxalic acid, tartaric
acid, acetic acid, citric acid, benzoic acid and succinic acid, which with silver
nitrate give the corresponding silver combinations by double decomposition. The
salts must be present to such an extent in the paper layer, that they are capable
of decomposing entirely the silver nitrate.
In the picture-layer of the copying paper the presence of free silver nitrate is
absolutely necessary for obtaining a good picture; this is also obtained by my
process, for only the silver nitrate, that actually penetrates into the paper layer
itself, is changed by the salts, thus preventing the former from becoming brown,
while a surplus of silver nitrate remains behind in the picture layer.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed, I declare that what
I claim is:
Paper, which is to be made sensitive for photographic purposes by solutions and
emulsions containing silver nitrate, thereby distinguished, that by suitable salts
incorporated in the paper layer, the silver nitrate penetrating from the sensitive
layer into the paper layer is changed into another compound, that does not turn the

page_text: paper brown by double decomposition, thus preventing the sensitive paper from
becoming brown when it is warehoused.
Dated this 18th day of July 1895
For HERMANN WANDROWSKY,
Bernh. Brockhues.