Invisible ink is a fluid used to write hidden messages that do not appear unless exposed by a revealing process. Invisible inks can be classified into three main categories: those that are revealed by heat, those revealed by chemical reactions and those that are visible under ultraviolet light. Some common household invisible inks are diluted fruit juices, vinegar and laundry detergent, all which can be applied by a paintbrush, special invisible ink pen or even a toothpick. Historically used in times of war by governments and insurgents alike, invisible ink's peacetime applications include children's invisible ink books and property markers. The chemical processes of invisible inks are well known, so a variety of detection methods exist.
The simplest invisible ink experiment consists of dipping a brush in lemon juice and writing on a piece of blank white paper. When the "ink" dries, it will be invisible to the naked eye, but if the paper is held up to a moderate heat source such as a light bulb, a radiator or an iron, the lemon juice writing will brown before the rest of the paper, exposing the message. Many other mildly acidic liquids also will work, such as apple juice, white wine, cola and even urine.
Spies are more likely to use invisible inks that are developed by a chemical process, because they are more secure and harder to detect. During the Cold war, German secret police used cerium oxalate to write hidden messages and used a solution of manganese sulfate, hydrogen peroxide and other chemicals to reveal the texts. The United States government has kept its formula used during World War I classified. Similar chemical recipes are used for children's entertainment, where hidden texts are printed in an invisible ink book, and an invisible ink pen is used to give clues and to reveal the book's ending. Some well-known chemical ink and developer combinations are vinegar and red cabbage water as well as cornstarch and iodine, both diluted in water.
Invisible inks that can be detected when exposed to ultraviolet or black lights have many modern applications. Using a special marker, invisible ink is applied to money or valuable goods, so that in the case of theft, the article can be identified by pawn shops or police and can be returned to the rightful owner. Many nightclubs use invisible inks to stamp patrons' hands for readmission, and some tattoos use ink that is visible only in black light. Copyrights sometimes are printed on documents with ultraviolet-sensitive ink, preventing photocopies from being mistaken for originals. Desperate prisoners with limited resources might write hidden messages to the outside world using bodily fluids, which contain phosphors that are detectable by ultraviolet lights.
A few reliable detection methods have made invisible inks mostly a part of history. Depending on the ink used, telltale odors and pen scratches might give away the use of invisible inks. When invisible writing that is sensitive to ultraviolet light or heat is suspected, it can be revealed easily. Chemically reactive inks are more difficult to detect, but iodine vapors indicate whether paper fibers have been disturbed by a pen.