Invisible inks

II Ink colours

The necessity to conceal specific messages led to the invention of methods for writing texts that would remain hidden until they reached their intended recipient. These inks, known as «sympathetic», were made from ingredients that only reacted with certain materials or conditions (the Greek idea of sympatheia refers here to the affinity between the ingredients used for writing and the conditions that allow the message to be revealed).

Greco-Roman sources refer to the use of sympathetic inks in politics and warfare, where they were very useful for clandestine communication and the transmission of secret message. However, the same properties also made these methods very popular among lovers:

«The letter written with fresh milk is also safe and deceptive to the eye; smear it with charcoal powder and you can read it.» (Ovid, The Art of Love 3, 627)

However, these inks also had less honest uses. The Refutatio omnium haerasium (4, 28), attributed to Hippolytus of Rome (3rd century), describes how ritualists requested that their clients inscribe a message with a decoction of oak galls when they sought oracular guidance on a particular matter. Subsequently,

«The magician, while apart, puts vitriol in a vessel filled with water and, after dissolving the chemical, sprinkles the supposedly blank piece of papyrus and forces the hidden and concealed letters to come to light, reading what the worshiper has written.»

It is possible to see how these inks permeated various facets of everyday and cultural life.  As a symbol of inventiveness and sophistication, they reflect the intersection of science, technology and society in ancient times.