Invisible Inks

Test screed developed as part of a research program on invisible inks. November 14, 1945.

By 1944 the oxygen meter and propellant projects were running smoothly with only minimal oversight from Pauling.  With more free time available to him, he began looking into new lines of research.  That year, he was contacted by Arthur Lamb, a Harvard professor, regarding a new line of inquiry.  During World War I, Lamb had developed invisible inks for the U.S. government.  He was restarting his work with inks and wanted Pauling’s help.  And so it is that, in September 1944, Linus Pauling became an official investigator in the Office of Scientific Research and Development’s invisible inks project.

The goal for Pauling and his team was to create a series of inks that were truly invisible and could only be developed by a limited number of chemicals. From September to October 1944, Dr. George Wright, William Eberhardt, and Frank Lanni made preliminary examinations of potential methods for developing invisible inks, the specifications of which were not defined in Pauling’s official reports to the OSRD. Once the preliminary tests were complete, Pauling and his team began a wide range of experiments, testing a variety of potential approaches for creating secret inks.

The team began with possible protein-based inks. They applied various proteins including rabbit serum, human saliva, and homogenized milk to standard typing paper. Then, after steaming and ironing the treated page, the team painted it with a mixture of ink, acetic acid and sodium chloride. The combination of acid and ink caused the protein to darken slightly, rendering it legible in well-lit conditions.

The group also tested non-organic inks such as diluted potassium iodide. After drying, the test screed was painted with gold chloride, rinsed, and then treated with a substance referred to only as “the silver physical reagent,” a compound protected by the Office of Censorship.

Page of test screeds developed as part of a research program on invisible inks. 1945.

Pauling and his team needed to find a better way to protect invisible inks from being identified when intercepted by enemy forces. To this end, the team turned its focus toward substances with high immunological specificity; that is, organic substances that reacted with only a limited number of other compounds. The team began with a polysaccharide gum distilled from a bacterium responsible for lobar pneumonia in humans. (Because the gum was largely non-reactive with other chemicals, the paper it was printed on hid it well.) The ink was then masked with an additional coating of a wax-like substance to prevent all but the most immunologically-specific chemicals from developing it. While tedious, the process was effective.

In addition to the use of polysaccharide gum, Pauling and his group examined antibodies and antigens in the hope that they could be used to create inks. In a report to the OSRD, Pauling explained that when a foreign protein (antigen) is introduced to an animal’s bloodstream, the animal produces a highly specific complementary protein (antibody) to neutralize it. When the two proteins combine, they form a stable protein-protein pair.

Initial tests of the solution suggested that the antibody-antigen combination could be highly effective. Unfortunately, as the researchers began practical testing they found it extremely difficult to develop the protein-protein pair without staining or otherwise rendering illegible the paper on which the ink was printed. What’s more, some of the antigens could be developed with non-organic chemicals, greatly reducing their security. Ultimately, the antibody-antigen ink was impractical. Pauling suggested that a few changes might be made to the process, but no record of additional experimentation appears in the Pauling Papers.

Despite having achieved some measure of success with a variety of inks, Pauling suggested that the project might be pushed even further. As he explained in a report,

From the offensive standpoint, it might be considered that the development by the new techniques of substances which are not detectable by the present methods might be useful as a basis for offensive methods.

While Pauling left no traces suggestive of his engaging in this process, it is at least plausible that he and his team did in fact note and retain a number of potential developers for future scientists to test.

In all Pauling and his team created or enhanced around a dozen different ink-developer combinations, ranging from improvements on existing camphor-based Presto pencils to complex processes using albumin, gypsum, and the catalytic reduction of silver. The project was closed with the publication of the “Final Report on Biological SW” dated December 31, 1945.