Secret Writings & Invisible Inks

Secret Writings & Invisible Ink

A note with secret writing written
in invisible ink revealed.

Count Jaggi writing his reports to Berlin using secret writing and invisible ink is one of the key plot points in the novel.

Secret writings, stenography, has been used for thousands of years. References to invisible inks have been found in ancient Roman text. Secret writing is an attempt to mask any message or communication from unwanted eyes. The embedding of Easter eggs in software, by programmers, is a form of stenography.

Until the appearance of microdots and electronic communications there was an arms race between intelligence agencies in the development of various invisible inks formulas and methods to avoid detection.

One interesting formula tested by MI6 during the WW1 was the use of semen. It didn’t react to the invisible ink detection methods being used at the time. Also, depending on the agent’s gender, was readily available. Unfortunately, the concept caused problems for the officer who developed the idea. He had to move departments when teasing became to much.

Until 2011, the CIA refused to declassify WW1 formulas for invisible ink arguing that they were still to sensitive for release. Until they were finally made available to the public, they were the oldest secret documents in the CIA archives.

In my novel Count Jaggi would drop a coat button that was impregnated with invisible ink crystals into a cup of water to create an invisible ink solution. Similar ingenious methods were developed to disguise invisible ink. MI6 issued store bought ball points pens with invisible ink refills. The CIA issued their invisible inks to their agents disguised as aspirin tablets.

Invisible ink types

Invisible Inks come in three different types:

heat based,
chemical based, and
ultraviolet light or fluorescent inks.

Heat based invisible inks

Heat based inks require that a heat source be applied to the paper for the writing to be revealed. Everyday household items can be used as ink, for example:

milk;
sugar dissolved in water;
lemon, orange, grapefruit, onion, and apple juices;
honey; and
vinegar.

The best source of heat is a steam iron that allows you to control the amount of heat you apply to the paper. You can use a candle but you have to be careful since the paper could burst into flames destroying the message.

Chemical based invisible inks

For chemical based invisible inks a chemical developer is needed to reveal the secret writing. Some of these inks are based on iron sulphate and copper sulphate diluted in water. An iodine solution would be dabbed or sprayed onto the paper to reveal the writing. Some German developed inks required the multiple application of a developer, spaced three hours apart, to reveal the writings.

During WW11 the U.S. Government had a concern that German prisoners of war would send valuable information back to Germany in the letters they were allowed, under the Geneva convention, to send home. The writing papers that were handed out to the prisoners were chemically treated to turn green if any invisible ink were used on them.

Ultraviolet light or fluorescent inks

Certain inks invisible to the naked eye become visible under an ultraviolet (black) light. These inks are used today mainly for security reasons such as a hand stamps for patrons entering a night club. Under a black light, the stamp will glow slightly.

Some business will mark their products with invisible inks pens for identification purposes, to prevent fraud and to help detect stolen property.

You can buy invisible ink pens and black lights from various commercial vendors. Also, some companies sell invisible ink for fountain pens if you are so inclined.