A secret ink that passed inspection
Invisible ink had to solve two problems: hide the message from an ordinary reader and survive examination by someone looking for hidden writing. British spies reportedly used semen because a dried message could leave no obvious mark, while routine iodine checks—one way of searching for secret writing—did not reveal it.
Iodine checks were not magic detectors for every substance. They could expose particular inks or residues, but an ink that left no telltale reaction could slip through. The method’s advantage was therefore practical: it evaded the screening available to inspectors, not every possible test.
The weakness was time
A secret message was only useful if it stayed concealed long enough to reach its recipient. Semen is biological material, and biological material breaks down. As it decayed on the paper, it produced an unmistakable odor. A letter that had passed inspection could thus give itself away later, when someone opened or handled it.
That is the awkward tradeoff behind the technique. The ink was hard to spot by the test that mattered at first, but it was not stable. A concealed message could pass through the post unseen and still betray its sender or recipient through a much less sophisticated signal: smell.
