George Washington to Benjamin Tallmadge, February 5, 1780
In this letter, Washington expresses concern about the blank sheets of paper he receives from the Culpers, suggesting it would be better for them to write something on the sheet rather than simply carry around single blank sheets of paper. This suggests Washington does not know how the Culpers are sending their blank sheets, but it documents that they are sending their messages as blank sheets.
Samuel Culper to John Bolton, April 18, 1780
This letter from Woodhull also refers to "blanks" as the Culpers called the sheets with their messages in invisible ink.
George Washington to Benjamin Tallmadge, October 30, 1779
Washington describes the use of counterstain, the substance applied to make the invisible message appear, in this message. He also indicates the chemical solutions are not easy to procure.
Samuel Culper to John Bolton, October 3, 1780
Woodhull suggests that the quality of the paper influences the reliability and invisibility of the ink, and also suggests paper is readily available behind British lines when he complains about the paper Tallmadge used in his last letter. In this letter Woodhull even claims he is sending Tallmadge a new ream of paper to fix the problem, which would be easy if the Culpers were using reams of blank paper to disguise their messages.
George Washington to Benjamin Tallmadge, April 30, 1779
Washington responds to a request that he has no "ink" but will forward some to the Culpers when he gets more.