Invisible ink

Procedure

• Take a stick, dip it in milk (or lemon juice, vinegar, sugar solution, apple juice), and write whatever we want on the paper.
• Let it dry.
• Bring it close to a heat source, in this case a candle.
• Observe how everything we have written appears.
• Another way: mix 60 g of baking soda with 60 mL of water. The result will be a kind of baking soda paste. With this paste, write a message on a piece of paper. Then, to make what we have written visible, paint the paper with grape juice using a brush.

Theoretical explanation

“Invisible ink” is the term applied to a number of liquids that remain invisible to the eye after being written on paper until certain means are used to make the writing visible. Employed in espionage, invisible ink is another form of steganography (the art of hidden messages in writing).

The liquid for writing invisibly is applied on blank paper. After drying, the paper remains blank. The writing becomes visible by bringing the written paper close to a heat source, ultraviolet light, or by applying the appropriate chemicals. These methods cause the ink to become visible again. Some invisible inks rely on chemical reactions called acid-base reactions. In this case, a liquid is used as a developer to bring the ink to a visible form.

As a general rule, any acidic fluid can work as invisible ink (usually diluted with water). Writing in these liquids becomes visible as a brown color, which is the result of the oxidation of the organic substances present in them. These substances include milk, onion extract, lemon or orange juice, apple juice, diluted honey, sugar solution, soapy water, vinegar, or wine. Heat can be provided by a radiator, an electric iron, or an oven.

Lemon juice contains citric acid. When brought close to a heat source, the acid reacts with the heat, and a chemical oxidation reaction occurs, causing the citric acid to transform into another substance that is now brown and visible.

Did you know…? This type of invisible inks was used in World War I to pass messages without the enemy noticing and also among lovers with forbidden loves.