Invisible ink is a writing fluid intended for secret communication. The invisible writing is developed by heat or a chemical. Milk, whey, sugar solutions, or any colorless vegetable juice may be used as invisible ink. These fluids turn brown when exposed to heat not quite high enough to damage the paper.
Many pairs of chemicals are also used as invisible ink. One chemical is used for writing and the other for developing. For example, marks made with a dilute solution of potassium ferrocyanide or tannic acid can be developed by dipping the paper in a dilute solution of ferric chloride or ferric alum.
Scientists can easily detect secret writing. Photography with infrared or ultraviolet light often reveals invisible ink.
