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Section 1 Invisible Ink Techniques

In this section, we examine different types of materials used to send invisible messages.

Subsection 1.1 Invisible Ink Spy Letters.

Examples of some spy letters written in invisible ink from the International Spy Museum in Washington DC. These date to the 1960s.

Figure 1.1. Invisible Ink postcard (photo taken at the International Spy Museum, Washington D.C.)
Figure 1.2. Invisible Ink postcard (photo taken at the International Spy Museum, Washington D.C.)
Figure 1.3. Invisible Ink letter (photo taken at the International Spy Museum, Washington D.C.)

It was dangerous to carry bottles of secret ink, eventually spies were taught to hide the ink in articles of clothing, e.g. socks, handkerchiefs, scarves, buttons, ties, etc. The ink would be released by applying water.

Subsection 1.2 Lemon Juice

In video Figure 1.4 we demonstrate writing a message using pure lemon juice. When this dries it is invisible.

Figure 1.4. Writing in Lemon Juice

To reveal the lemon juice, we activate the lemon juice with heat. In video Figure 1.5 we use a standard hot plate from a chemistry lab to provide the heat.

Figure 1.5. Revealing Lemon Juice

So what happened?

Answer

Lemon juice contains carbon-based compounds, such as sugar and citric acid, which are absorbed into the paper’s cellulors fibers. The acids in the lemon juice also promote the partial breakdown of the cellulose fibers in the paper. Heating the sugars and acids in the lemon juice and the partially broken cellulose molecules causes the release of oxygen and hydrogen from the compounds, leaving carbon behind, a process similar to making charcoal from wood. And just as charcoal is black, the carbon-enriched material left behind by the heat-caused breakdown of these compounds is brownish/blackish, and we see the message. Nearly any organic material can work in asimilar manner.

Lemon Juice/Organic Material History.
Figure 1.6. Invisible Ink letter with milk

Two German spies, Carl Muller and John Hahn, in England in 1915 were caught sending letters using lemon juice for invisible ink. They would either write the secret message between the lines of a regular message, or on the back side of a regular message. Evidence at their trial consisted of lemons, pens (pen nibs), and testing the pens for cellular matter from the lemons. One of these lemons still exists in the British National Archives.

Subsection 1.3 Iron Sulfate and Tannic Acid

In video Figure 1.7 we demonstrate writing a message using iron sulfate (or ferrous sulfate). In this case we use a fairly low concentration of iron sulfate (.1 gram to 6 mL of water). When this dries it is invisible.

Figure 1.7. Writing in Ferrous

We activate the iron sulfate with tannic acid in video Figure 1.8. Again we have a fairly low concentration of tannic acid in distilled water. (10mM)

Figure 1.8. Revealing with Tannic Acid

So what's happening here?

Answer

Essentially we are separating the ingredients for iron gall ink, the standard ink formulation used in Europe from 5th century until the 19th century. (It is still available today, but not widely used.) Iron gall ink has two main ingredients iron sulfate and gallotannic acid. Any iron ion donor source can be used to create the iron sulfate solution. The gallotannic acid was usually extracted from oak galls or galls of other trees. Galls are any kind of growth on the external tissues of a plant. Oak galls are caused by chemicals injected by the larva of certain kinds of gall wasps.
Figure 1.9. Oak galls on a Pyrenean oak (photo from wikipedia)
Chemically, what's happening is that when tannin reacts with iron sulfate, a water soluble ferrous tannate complex is formed. This complex turns black from oxidation as the iron ion changes charge from Fe\(^{2+}\) (ferrous) to Fe\(^{3+}\) (ferric). This new complex is no longer water-soluble and is an excellent permanent ink.

Iron Sulfate History.

The oldest reference to using the components of iron gall ink dates to a Greek engineer, Philo of Byzantium in 280-220 B.C. Onew writes this letter on a new hat or on human skin with crushed gallnuts dissolved in water. When the writing is dry it will become invisible. Then soak a sponge with vitriol [ferrous sulfate] and it appears.

It was used by both sides during the American Revolution. It appears to have been brought to Washington by James Jay and was used extensively by the Culper Spy Ring. James Jay never revealed the solution he used, although he later asked congress for money for his contribution to the Revolutionary War. But later deductive work has revealed it as iron sulfate and tannic acid.

A letter from George Washington to Benjamin Tallmadge about the ink https://www.loc.gov/resource/mgw4.060_0590_0591/?sp=1

A typed version of the letter https://founders.archives.gov/documents/Washington/03-21-02-0539

A letter where the invisible ink remains that was tested to determine its composition.

Figure 1.10. Benjamin Thompson Invisible Ink Letter (photo from William L. Clements Library, University of Michigan)

Subsection 1.4 Phenolphthalein

In video Figure 1.11 we demonstrate writing a message using phenolphthalein (1% solution).

Figure 1.11. Writing in Phenolphthalein

We activate the phenolphthalein with ammonia in video Figure 1.12. Ammonia is available as a standard cleaning product.

Figure 1.12. Revealing Phenolphthalein

What happened?

Answer

Phenolphthalein is a chemical compound that is colorless when it is in a neutral or acidic environment, but when it’s exposed to a basic compound, such as ammonia (NH\(_3\)), it changes structure slightly and turns pink. It is often used as an acid/base indicator for this reason. This reaction is reversible and will naturally fade back to invisible after being removed from the ammonia.

Phenolphthalein History.

Phenolphthalein used to be a common ingredient in laxatives so it was easily available and easy to carry without suspicion. A German spy group was sent to the US in 1942 for the purposes of sabotage (blowing up factories, railroads, ships, etc). They used silk handkerchiefs for sending messages. Two members of the saboteur ring got cold feet and turned themselves and their ring into the FBI. One of them turned in a silk handkerchief with the names and addresses of their contacts, but it was in invisible ink and he didn't know how to reveal it. The handkerchief was subjected to ammonia fumes at a military commission revealing red writing and the use of phenolphthalein. The eight saboteurs were all convicted.

Figure 1.13. An Invisible Ink Handkerchief from the 1960s (photo taken at the International Spy Museum, Washington D.C.)

Subsection 1.5 Cobalt

In video Figure 1.11 we demonstrate writing a message using cobalt chloride. In this case we use a fairly low concentration (.25 gram to 10 mL of water). Higher concentrations result in prettier ink, but do not dry completely invisible.

Figure 1.14. Writing in Cobalt Chloride

To reveal this ink, we activate the cobalt chloride with heat. In video Figure 1.15 we use a standard hot plate from a chemistry lab to provide the heat.

Figure 1.15. Revealing Cobalt Chloride

What happened?

Answer

The pink solution used to write with contains cobalt chloride hexahydrate: CoCl\(_2\)·6H\(_2\)O. When this compound is heated, it loses the associated water molecules, forming CoCl\(_2\text{,}\) which is blue. If you let it sit, especially in humid conditions, the CoCl\(_2\)·6H\(_2\)O will reform, and the message will fade away.

Cobalt History.

The use of cobalt for secret ink and secret art seems was very well known in the 1700s. It was studied extensively by Jean Hellot (1685-1766, of France) though he credits an unnamed German artist for the discovery. A blue sympathetic ink (invisible ink) was developed from acetate of cobalt and a green sympathetic ink was developed from muriate of cobalt. Other colors can be created with different solutions and additives. By 1746 it was very fashionable in Paris to make sympathetic ink and many recipes existed. It also became popular to use in various artistic application.

Figure 1.16. Fire Screen which depicted barren winter landscape until heated by a fire

Edgar Allen Poe's The Gold Bug, published in 1883, is a story which involves a cipher written in invisible ink which is activated by fire but disappears when removed from the fire with a discussion of the use of cobalt.

"At this stage of my reflections I endeavored to remember, and did remember, with entire distinctness, every incident which occurred about the period in question. The weather was chilly (oh rare and happy accident!), and a fire was blazing on the hearth. I was heated with exercise and sat near the table. You, however, had drawn a chair close to the chimney. Just as I placed the parchment in your hand, and as you were in the act of inspecting it, Wolf, the Newfoundland, entered, and leaped upon your shoulders. With your left hand you caressed him and kept him off, while your right, holding the parchment, was permitted to fall listlessly between your knees, and in close proximity to the fire. At one moment I thought the blaze had caught it, and was about to caution you, but, before I could speak, you had withdrawn it, and were engaged in its examination. When I considered all these particulars, I doubted not for a moment that heat had been the agent in bringing to light, on the parchment, the skull which I saw designed on it. You are well aware that chemical preparations exist, and have existed time out of mind, by means of which it is possible to write on either paper or vellum, so that the characters shall become visible only when subjected to the action of fire. Zaire, digested in aqua regia, and diluted with four times its weight of water, is sometimes employed; a green tint results. The regulus of cobalt, dissolved in spirit of nitre, gives a red. These colors disappear at longer or shorter intervals after the material written on cools, but again become apparent upon the re-application of heat.
http://www.easylit.com/poe/comtext/prose/goldbug.shtml

Subsection 1.6 Spy Museum Pen

Live Demo!!

Figure 1.17. Spy Pen from International Spy Museum

What happened?

Answer

Visible light is a mixture of light of various wavelengths, ranging from long (red) to short (violet). The flashlight on the pen emits light of slightly shorter wavelengths than the violet light that we can see. Compounds in the ink absorb this ultraviolet light and then emit longer wavelength light that we can see, a process called fluorescence.

Ultra Violet History.
Figure 1.18. UV Based Invisible Inks (photo taken at the International Spy Museum, Washington D.C.)
Figure 1.19. Invisible Ink Detection Kit(photo taken at the International Spy Museum, Washington D.C.)

Subsection 1.7 Iodine Magic

In video Figure 1.20 we demonstrate writing a message using starch water. The starch water solution has been heated for a faster reaction.

Figure 1.20. Writing in Starch Water

To reveal this ink, we place the paper into an iodine chamber as in video Figure 1.21

Figure 1.21. Revealing Starch Water

What happened?

Answer

The starch water provides a material that binds the iodine, which is brown. Because it’s a better surface for binding the iodine than the untreated paper, the writing turns brown. This is similar to an old technique used to develop fingerprints. The surface with the prints was exposed to iodine fumes and the iodine bound to the oils in the fingerprint. Even more impressively, iodine can be used to reveal nearly any invisible ink which alters the surface of the paper. Any fluid applied to paper will alter the paper surface fibers or sizing. Fumes created from heating iodine crystals will develop the writing, which will appear brown because the iodine sticks preferentially to the altered areas of the paper. Exposing the paper to strong sunlight will return the writing to its invisible state, as will using a bleach solution. Slightly dampening paper with a sponge or by steam and then drying it before writing a message will prevent writing from being developed by this method, but overdoing dampening will result in telltale paper cockling.

Iodine Chamber History.

Iodine enters into invisible ink history in the attempt to develop a 'universal reagent'. That is, a reagent that will develop any secret writing. French, British and German scientists claimed to have developed the iodine vapor test during World War I but, in fact, it was discovered by Belgian legal chemists in the late 19th century in attempts to uncover fraud.

Any fluid applied to the paper, or even writing with a stylus or pen with no ink, will alter the paper surface fibers or sizing. Fumes created from heating iodine crystals will develop the writing, which will appear brown because the iodine sticks preferentially to the altered areas of the paper. Exposing the paper to strong sunlight will return the writing to its invisible state, as will using a bleach solution.

However, nearly as soon as this method was discovered, so was a way to defeat it. Slightly dampening paper with a sponge or by steam and then drying it before writing a message will prevent writing from being developed by this method. One must be careful, because overdoing dampening will result in telltale paper wrinkling.

Subsection 1.8 What do you think?

Figure 1.22. National Treasure's Take on Invisible Ink

Subsection 1.9 Acknowledgements

Teresa Longin and Dan Wacks from the University of Redlands Chemistry department graciously volunteered their time to help develop these materials.

Prisoners, Lovers and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda, by Kristie Macrakis