Invisible ink was once a key part of espionage. While it may seem quaint and low-tech now, it’s still used by people who want to communicate secretly. Since centuries, people have used paper to write messages that could only be read by special glasses or a flashlight. But how do they work? In this Wonder of the Day we will look at how invisible ink functions, and some fun ways you can use it.
The longevity of invisible pens varies widely depending on the quality and type of ink, as well as how it is stored. It can last anywhere from a few days to years. To reveal a hidden message, you must either expose the paper to heat (like placing it over an iron or over a candle flame) or apply a chemical liquid like lemon juice or vinegar to it. Many invisible inks also have telltale signs that can alert a censor to their presence, such as pen scratches or unusually smooth paper or a change in the reflectivity of the paper.
Invisible ink will not protect you from someone who sneaks a peek into your TikTok or text messages, but it can prevent you from having to discuss something too scandalous with your friends. It’s also helpful when you need to confide in someone but don’t want anyone else to know what you’re saying. You can also use it to write secret notes to your friend or to give spoilers for a movie or show without giving away the surprise.