Invisible ink isn’t just in the movies, it was a regular fixture throughout history and can even be made at home too! Utilising certain chemical reactions and properties, hidden messages can be made in no time.
For instance, brushing lemon juice onto a piece of paper and then heating it can reveal messages. The acid from the juice burns before the paper and voilà the message is revealed! Baking soda (sodium bicarbonate) and even milk are other things that use a heat reaction to make the hidden ink.
UV light is used in another type of invisible ink where the ink is not visible in daylight and can only be under UV light. This method is popular in marking items and products in manufacturing.
Invisible ink has been important in sending secret messages in the past. A more complex method using Cerium Oxalate was used by the Stasi in post-war East Germany. The chemical would be sandwiched between two sheets of paper. The top piece of A4 would then be written on and the text would be revealed using a mixture of manganese sulphate and hydrogen peroxide. Invisble ink was also used frequently in both World Wars and the Cold War and as far back as the Roman Empire when philosopher Pliny the Elder mentioned using milk from the tithymalus plant.