Document from Spies of the Revolution
While serving in Paris as an agent of the Committee of Secret Correspondence, Silas Deane is known to have used a heat-developing invisible ink, compounded of cobalt chloride, glycerin and water, for some of his intelligence reports back to America.
Even more useful to him later was a "sympathetic stain" created for secret communications by James Jay, a physician and the brother of John Jay. Dr. Jay, who had been knighted by George III, used the "stain" for reporting military information from London to America. Later he supplied quantities of the stain to George Washington at home and to Silas Deane in Paris.
The stain required one chemical for writing the message and a second to develop it, affording greater security than the ink used by Deane earlier.
Once, in a letter of John Jay, Robert Morris spoke of an innocuous letter from "Timothy Jones" (Deane) and the "concealed beauties therein," noting "the cursory examinations of a sea captain would never discover them, but transferred from his hand to the penetrating eye of a Jay, the diamonds stand confessed at once."
Washington instructed his agents in the use of the "sympathetic stain," noting in connection with "Culper Junior" that the ink "will not only render his communications less exposed to detection, but relieve the fears of such persons as may be entrusted in its conveyance . . ."
Washington suggested that reports could be written in the invisible ink "on the blank leaves of a pamphlet . . . a common pocket book, or on the blank leaves at each end of registers, almanacs, or any publication or book of small value."
Washington especially recommended that agents conceal their reports by using the ink in correspondence: "A much better way is to write a letter in the Tory stile with some mixture of family matters and between the lines and on the remaining part of the sheet communicate with the stain the intended intelligence."