Invisible ink is a writing fluid that’s invisible beneath regular situations however may 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 right now for a wide range of functions, together with safety, espionage, and counterfeiting.
There are various 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 well-liked recipe for invisible ink makes use of an answer of copper sulfate and water. This resolution is invisible when it’s utilized to paper, however it should flip blue when it’s uncovered to gentle.
