Invisible ink is a writing fluid that’s invisible underneath regular situations however might be revealed by making use of warmth, gentle, or a chemical reagent. It has been used for hundreds of years for secret communication, and it’s nonetheless used at this time for quite a lot of functions, together with safety, espionage, and counterfeiting.
There are lots of completely different recipes for invisible ink, however the most typical substances embody lemon juice, vinegar, and milk. These liquids are invisible when they’re utilized to paper, however they may flip brown when they’re heated. One other standard recipe for invisible ink makes use of an answer of copper sulfate and water. This answer is invisible when it’s utilized to paper, however it should flip blue when it’s uncovered to gentle.
