In a previous edition of our Man Knowledge series, we discussed the fascinating history of invisible ink. In doing the research for that post, we came across an equally interesting tool in the spy’s bag of tricks: the concealment device.

Invisible ink was handy for sending secret messages, but sometimes spies and soldiers needed to hide other kinds of objects, or simply wanted a double-layer of protection for their coded missives. Concealment devices or CD’s looked like normal, everyday objects but actually contained a secret compartment or cavity, inside which could be placed film, notes, eavesdropping equipment, and various other types of contraband.  They were used to smuggle escape aids to prisoners of war, exchange information with friendlies, monitor the enemy, store secrets for safe keeping, and transport items without arousing suspicion.

The earliest quasi-concealments were used by the ancient Greeks and Romans. The Greek general Histiaeus wrote a message on the head of his servant, waited for his hair to grow back, and sent him on his way. This was, of course, not a very effective method of communicating something that was even remotely time-sensitive.

Roman generals placed secret messages in the bandages wrapped around the limbs of wounded soldiers or sowed a message into the sole of a courier’s sandal.