Invisible ink, otherwise known as security ink, is a type of substance used for writing messages that look invisible to the naked eye. However, the words will become visible when placed under heat, when exposed to certain chemicals, or when placed under ultraviolet light. Although invisible ink is occasionally used in children’s toys to help little girls hide the secrets they’re keeping in their diaries, it has had important uses throughout history. Here are some of the ways in which invisible ink was used over the years.
Invisible ink can either be made out of organic fluids or synthetic inks. Organic fluids are typically items that a person can find inside of their own home, such as lemon juice, saliva, vinegar, and milk. Meanwhile, synthetic fluids consist of various chemicals that take time, and a great deal of chemistry knowledge, to combine.
The Greeks and Romans were the first people to ever use invisible ink. It was first mentioned by Pliny the Elder in first century AD, who used milk in order to create invisible messages. A little later on, during the Renaissance, a Roman poet named Ovid gave advice to ladies who wished to deceive their husbands and communicate with other men. He told them to include invisible ink in their letters by using milk to write and powdered charcoal in order to read the message later on.
Benedict Arnold was a general for the American army who became a traitor by eventually joining the British army. Before he made the complete transition, he used invisible ink to send letters of treason to the British, providing them with important information about the American army.
During World War I, there were German agents in England known as “lemon juice spies” that tried to use lemon juice as a type of invisible ink to send secret messages to one another. However, eleven of them were sentenced to death after being caught with pens that had pulp stuck to the tips. When World War II occurred, the Germans had created new techniques. They used complicated methods, such as one that involved moistening a paper, sprinkling it with red powder, heating it up, and finally exposing it to ultraviolet light.
Aside from diaries that allow children to use invisible ink to write down their secrets, there are a few other modern uses for invisible ink. At amusement parks, for instance, workers may stamp a person’s hand with an invisible stamp when they leave, and then check it with an ultraviolet light when they try to enter again. There are also UV marker pens, commonly used for security purposes, that check to see if a bill is authentic or to identify belongings. Some invisible ink has even been used in art. Even though the uses for invisible ink have evolved over time, it’s still a sneaky way to communicate without a huge chance of getting caught.
Have you ever used a pen with invisible ink?