Process for Writing and Printing Invisible Characters on Paper or the like and Showing also if they have been Rendered Visible.
Year: 1899

page_text: Date of Application, 28th Mar., 1899-Accepted, 28th Oct., 1899
Process for Writing and Printing Invisible Characters on Paper or
the like, and Showing also if they have been Rendered Visible.
I, ERNST KRETSCHMANN, Doctor of Medicine, of 286, Gadenstedter Strasse,
Gross-Lafferde, Hanover, in the Empire of Germany, Surgeon, do hereby de-
clare the nature of this invention and in what manner the same is to be per-
formed to be particularly described and ascertained in and by the following
statement:-
This invention relates to an improved process for writing and printing invisible
characters on paper or the like and showing also if they have been rendered
visible.
All hitherto known sympathetic inks have the disadvantage that their use
is not always possible as the required chemicals may not be procurable. On
the other hand the handling of the matters generally poisonous of which these
inks are composed, is not without danger. There is no doubt about the
practical utility of sympathetic inks but on account of the above described dis-
advantages they are seldom used.
The present invention relates to a process of preparing paper with a chemical
solution so that it can be written on without showing visible characters; for
this purpose non-poisonous solution is used instead of ink which can be readily
bought or produced. The writing however only becomes visible when the paper
has been treated in the proper manner and it then disappears again.
The
For preparing the paper a solution of chloride of cobalt or of haloid salts
of cobalt is used spread in a suitable manner on the surface of the paper.
best method is to use half rough paper treated with a solution of 1 part of
chloride of cobalt, 12 parts of glycerine, 2 parts of gum arabic and 90 parts
of water, the paper being then glazed in the customary manner and finished.
If the paper so prepared is written on with a solution of rock salt in water, the
writing becomes invisible, but when heated it appears in a pale green color.
Chloride of cobalt although poisonous is used only in such a small quantity
that no danger results as I have proved by experiments, also the solution is so
intimately amalgamated with the paper fibre, that the constituents of same
cannot be rubbed off either in use or intentionally.
The rock salt solution can be made by any person and without difficulty.
In order to prove if an unauthorised person has tampered with the document
before reaching its proper recipient certain signs (lines or crosses or the like)
can be produced on the paper in a sympathetic ink which becomes visible by the
same manipulation but does not disappear again but remains showing. For this
purpose a solution of a small quantity of resorcin and the same quantity of
paratoluidin in 8 drops of water and 6 drops of sulphuric acid can be used.
According to the paper this solution produces pale red or light yellow lines
growing black or brown when heated.
Having now particularly described and ascertained the nature of this invention
and in what manner the same is to be performed I declare that what I claim is:
1
A process of writing in invisible characters on paper or the like, the writing

page_text: being rendered visible however for a certain time by a suitable manipulation,
in which paper or the like is treated with a solution of chloride of cobalt or of
the haloid salts of cobalt and then written upon with a solution of rock salt.
2. A process to prove if unauthorised persons have obtained knowledge of the
contents of a document produced by the processes as described in Claim 1, con-
sisting in the provision of visible or invisible signs on said document in a
sympathetic ink which becomes permanently changed when once rendered visible.
Dated this 28th day of March 1899.
HERBERT HADDAN & Co.,
Agents to Applicant,
18, Buckingham Street, Strand, London, W.C.