Invisible ink can be split into two categories, organic and synthetic. As children, we’ve all made invisible ink from various organic substances, including lemon juice, vinegar and milk, developing them with the use of heat or ultraviolet light. Synthetic invisible inks contain chemical that require a specific ‘reagent’ in order to develop, such as another chemical. Modern technology might have made invisible ink practically redundant, but it has a fascinating history nonetheless. It’s almost always been used during times of war and espionage, surrounding it with mystery and intrigue.
The ancient Greeks and Romans were using invisible ink over 2,000 years ago, and its first mention is by Pliny the Elder in 1BCE. He recorded the use of the milk of the tithymalus plant in his Natural History.
Invisible ink continued to be used throughout the Renaissance, with many politicians and powerful lords using it to send correspondence- Ovid mentions the use of it in his work, Art of Love. An Italian polymath named Giovanni della Porta developed a formula from alum and vinegar to be painted onto the shell of a hardboiled egg. It would seep through the shell and transfer the message onto the inside, making it visible only when the egg was peeled.
The American Revolution
Both British and American forces used invisible ink during the American War of Independence. Britain used a mixture of both organic and synthetic inks, writing a letter in the corner to inform the recipient how to uncover the message, such as an F for fire.
When George Washington had his forces use invisible ink, he got them to write mundane letters with visible ink in between the lines, giving us the famous saying “read between the lines”.
At this point, American forces used natural inks such as vinegar and lemon juice and synthetic inks, whereas German forces resorted to making invisible inks from various sulphates and cobalt salts.
After Allied forces found that iodine vapour turned all ink brown, German forces found that steaming the paper would alter it, and thus avoid suspicion. Both sides came up with ever more imaginative ways to create and conceal invisible ink.
The Cold War
During the 1960s, the threat of war forced intelligence agencies to come up with new ways to use invisible ink, with the CIA managing to embed chemicals in credit cards, pen lids, glasses frames, toothpicks and key fobs in order to create writing objects with invisible ink. The CIA employed 36 secret writing specialists working in the USA and abroad, it was that essential an espionage tool.