Invisible ink is invisible either on application or soon thereafter, but can later be made visible by some method. The need to send private messages has existed since time immemorial, and invisible ink is for instance mentioned by Aeneas Tacticus, a Greek author who lived in the 4th century BCE and wrote about the art of war. Among other things, he published a guide for secure military communications. Regrettably, his surviving treaty does not include any detailed information about how invisible ink is made or from which ingredients. In the 3rd century BCE, the Greek writer Philo of Byzantium – who lived most of this life in Alexandria – described…
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