Not all inks are meant to last. In 17th-century espionage, a mixture of cobalt chloride and glycerine produced a script visible only when warmed. Letters folded into the lining of coats revealed their secrets beside a fireplace, then faded by morning. This library catalogues those formulations — some deliberate, others tragic accidents of chemistry — and the manuscripts they consumed.
Our earliest record comes from Philo of Byzantium (3rd century BCE), who described a gall-nut ink that could be washed away with a sponge soaked in a particular salt. The tradition resurfaced in the Cold War through "dry-transfer" sheets that left no carbon copy.