The History of Invisible Ink

As a kid I had a minor obsession with invisible ink after learning about it in a science club. An impressive stack of vinegar and lemon scented papers filled my desk, but the history of invisible ink is much more interesting than a summer enrichment program. Starting with the Roman general Pliny the Elder using milk from the thithymallus plant, to modern inks which are only visible under UV light, the science of invisible ink has grown quite complex. Some of the oldest CIA documents contain recipes for special inks, and MI6 and their very own "adult" mixture during WW1.