CIA File of the Day
German Secret Ink. Formulas used in preparation of.
A copy of formulas used in the preparation of German secret ink, forwarded with the note that the source is "N.G." The first mixture combines a pyramidon tablet and an aspirin tablet or powder with 400 cubic centimeters of pure water. A second formula, for making the invisible ink appear, includes lightly camphorated alcohol, distilled water, nitrate of potash, acetic acid, and carbon tetrachloride, to be applied with wet cotton.
Courtesy of the CIA FOIA Electronic Reading Room. Official page: CIA FOIA Electronic Reading Room. Document PDF: cia.gov reading room PDF. Copy: Internet Archive.