Invisible ink has been used for over 2,000 years. Methods of secret writing were used by the ancient Greeks and Romans. During the American Revolution, the chief British intelligence officer put letters in the corner of invisible messages as a code to show how secret messages should be read (F to reveal by fire, A to reveal by acid).

During World War II, the Allies and the Axis powers developed and deployed invisible inks. Germany's military intelligence agency, the Abwehr, developed inks at multiple levels. The agency gave less trusted German agents lower level inks while the best German agents received the most complicated inks. During this time period, the American government screened mail entering and leaving the country. When a suspicious message was detected, the message was sent to the FBI for testing. Out of 4600 suspicious messages, 400 items revealed secret writings and codes. The Germans responded by developing increasingly complicated inks. One ink required three applications of a reagent spaced three hours apart.

- Lemon
- Water
- Cotton swab
- Gloves
- Eye protection
- Chemical jar labeled 'Lemon ink'

Squeezed lemon juice into the chemical jar
Added a few water drops
Dipped cotton swab into the lemon juice and wrote a message on a piece of paper

Once dry, the message is difficult to detect

Fruit juice contains Carbon. Lemon juice mixed with water is difficult to detect when applied to paper (invisible ink).

When placed near heat, the message becomes much easier to detect.

When the dried lemon juice is heated, the molecules break apart and the visible black or brown Carbon is visible. The writing turns from clear (invisible) to visible.

Due to the proximity of the flame, this method is extremely dangerous and may result in the loss of the secret message. Or worse, a dangerous fire.

The Agent and Reagent

- Ferric ammonium sulfate
- Sodium ferrocyanide
- 2 Test tubes with caps
- Test tube rack
- 2 chemical jars
- Measuring spoon
- Funnel
- 2 cotton swabs
- Measuring cup
- 2 stirring sticks
- Dechlorinated water
- White paper
- Permanent marker

Added one teaspoon of dechlorinated water into a test tube.
Added one teaspoon sodium ferrocyanide to the water in the test tube.

Mixed well and put the formula into a chemical jar labeled "invisible ink".
Added one teaspoon of dechlorinated water into the second test tube.
Added one teaspoon ferric ammonium sulfate to the water in the second test tube.

Mixed well and put the formula into a chemical jar labeled "reagent".

Wrote a message with the formula in the "invisible ink" container and let dry.

When reagent formula is swabbed over the message, the secret message is revealed.

The invisible ink formula is an agent. Since one chemical is reacting with the invisible ink, it is called a reagent.