Invisible Ink - For Real

This is the actual project that I did for the 2013 Ice Dragon Pentathlon as both a research paper as as Other. I couldn't resist the previous post. I am disappointed that no one left a comment on my joke.

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Invisible Ink from Le Menagier de Paris

In 1393 a book was written providing instructions, for a young bride, on how to run a household. In between recipes, cleaning instructions, wedding plans and medicinal cures is the following statement: 

TO WRITE ON PAPER A LETTER NO-ONE CAN SEE UNTIL THE PAPER IS HEATED, take sal ammoniac and moisten and dissolve it in water: then write with this and let it dry. And it will last about eight days. [1]

While medieval invisible ink isn’t surprising; records of such can be dated back to the first century AD when "Pliny the Elder explained how the "milk" of the thithymallus plant could be used as an invisible ink". [2]

John Carey directs us to Ovid’s advice, in Ars Amatoria 3:627-28: "Tuta quoque est fallitque oculos e lacte recenti / Littera: carbonis pulvere tange, leges" [A letter too is safe and escapes the eye when written in "new milk": touch it with coal-dust and you will read]. G. P. Goold glosses the passage with references from Pliny and Ausonius on the "herba lactaria" used by the ancients as invisible ink (see the revised Loeb edition of Ars Amatoria, 162). Ovid adds, in 3:629-30, that writing with a stalk of wet flax works the same way as the milk plant. (Cowley, p243)

Susan Zimmerman wrote about the escape of John Arden and the Jesuit priest John Gerard from the Tower of London on the 5th of October, 1597; an escape that was planed with the help of secret messages written in orange juice. "Orange juice was the preferred form of invisible ink in the period because, as Gerard explains in his memoir, while characters written in lemon juice disappear again after the letter is moistened or exposed to heat, characters written in orange juice do not, making it a more secure form of writing, less subject to interference if intercepted." [3]

I cannot say thou writ’st instead of Ink

With juice of lemons, that’s too sharp a drink

And quick; but yet as that conceals what’s writ,

   writes well enough, but then

   Blots out as fast agen;

   And so by riddling play

   Brings Night in midst of Day,

   And none must hope to see

   What’s written, though there it be,

Just so thou jugglest, speak’st good sense and wit

Yet so obscure in every part of it

    As that it dissappears

   From all our eyes....

(Abraham Cowley, Upon The Author’s Mystery Of Babylon - sig C2) [4]

They all get Porta, for the sundry ways

    To write in cypher, and the several keys,

To ope the character.  They’ve found the slight

    With juice of lemons, onions, piss, to write;

To break up seals and close ’em.  And they know,

    If the states make peace, how it will go

With England.  All forbidden books they get,

    And of the powder-plot, they will talk yet. 

[Ben Jonson, Epigrams, 92, II 25-29] [5]

The amazing thing was that this one sentence, from Le Menagier de Paris, was actually written down between instructions on how to make salt white and how to make lime, [6] without a single word as to why the young bride might ever want to write a secret message. Was it included so that the bride could write secret love letters? Or hidden instructions on financial matters? It was important enough that the instruction was written down, but not important enough to explain why it was included. While I was aware of using the juice of lemons, onions, cabbages and artichokes, as well as some tree sap, vinegar and urine, as invisible ink, I was unaware of the use of sal ammoniac, [7] known today as NH4Cl: ammonium chloride. [8]

Sal ammoniac was also used as an insect bite treatment, a leather cleaner, a copper cleaner, as smelling salts, in ink and paint production, a soldering flux and to "age" bronze [9] (was used to give new bronze casting the appearance and patina of old bronze) and in much, much more. It appears to have been the universal ingredient in most household cleaning methods as well as a major staple in industrial work. [10] I searched through my resources looking for other references to sal ammoniac as an invisible ink. I found many references from the American Revolutionary War, and later, for invisible inks made with ammonium chloride and an additional chemical (e.g.: zinc powder, iron or cobalt sulfate), but few from the SCA time period... Until I came across an English translation of the 1653 edition of Gargantua And His Son Pantagruel, and discovered that I had found a gold mine.

A letter which a messenger brought to Pantagruel from a lady of Paris, together with the exposition of a posy written in a gold ring. When Pantagruel had read the superscription he was much amazed, and therefore demanded of the said messenger the name of her that had sent it. Then opened he the letter, and found nothing written in it, nor otherwise enclosed, but only a gold ring, with a square table diamond. Wondering at this, he called Panurge to him, and showed him the case. Whereupon Panurge told him that the leaf of paper was written upon, but with such cunning and artifice that no man could see the writing at the first sight. Therefore, to find it out, he set it by the fire to see if it was made with sal ammoniac soaked in water. Then put he it into the water, to see if the letter was written with the juice of tithymalle. After that he held it up against the candle, to see if it was written with the juice of white onions. 

Then he rubbed one part of it with oil of nuts, to see if it were not written with the lee of a fig-tree, and another part of it with the milk of a woman giving suck to her eldest daughter, to see if it was written with the blood of red toads or green earth-frogs. Afterwards he rubbed one corner with the ashes of a swallow’s nest, to see if it were not written with the dew that is found within the herb alcakengy, called the winter-cherry. He rubbed, after that, one end with ear-wax, to see if it were not written with the gall of a raven. Then did he dip it into vinegar, to try if it was not written with the juice of the garden spurge. After that he greased it with the fat of a bat or flittermouse, to see if it was not written with the sperm of a whale, which some call ambergris. Then put it very fairly into a basinful of fresh water, and forthwith took it out, to see whether it were written with stone-alum. But after all experiments, when he perceived that he could find out nothing, he called the messenger and asked him, Good fellow, the lady that sent thee hither, did she not give thee a staff to bring with thee? thinking that it had been according to the conceit whereof Aulus Gellius maketh mention. And the messenger answered him, No, sir. Then Panurge would have caused his head to be shaven, to see whether the lady had written upon his bald pate, with the hard lye whereof soap is made, that which she meant; but, perceiving that his hair was very long, he forbore, considering that it could not have grown to so great a length in so short a time. [11]

I was interested in seeing how effective sal ammoniac was as an invisible ink, and how long lasting it was. Le Menagier states that the ink is good for eight days. I remember using lemon juice, as a kid, and having hidden messages last for weeks. George Washington’s "Sympathetic Stain", which was used during the American Revolutionary War, could remain hidden for years; one such hidden message, from one of the Culper spies, was revealed in a book during the 1930s: all but invisible for over a century. [12]

The purpose of this project is to compare the effectiveness of sal ammoniac with that of ordinary lemon juice and of white onion juice. Solutions of sal ammoniac, of lemon juice and of onion juice will be applied to paper and then photographed every few days. The elapsed images will show if any of the chemicals becomes visible. Furthermore, additional hidden messages will be provided for the judges, along with a heat source, to show which chemical was more effective as invisible ink. I am using lemon juice because most of us has had some experience with using lemon juice as invisible ink, through the boy/girl scouts or through science classes in school. The white onion is being included as they were more common than lemons; and I am interested in the effectiveness of the juice when compared to that of lemons.

Procedure:

Sal amonic

While sal ammoniac, in period, was quarried or produced by the distillation of animal dung, hooves and horn, [13] I purchased a container of animal grade ammonium chloride. I had to use animal grade ammonium chloride, [14] which is 99.9% pure, because chemical grade ammonium chloride, 99.9999% pure, is a restricted product; one would have to work for a school, university or laboratory in order to purchase it. This is because the chemical can be used to make explosives. So, because of this, I was unable to purchase 50g of chemical grade NH4Cl, but, for the same price, I was able to purchase 1135g of the animal grade version of the same chemical. [15]

I was not too concerned with the purity of the chemical; for my purposes, there was no difference between chemical grade and animal grade ammonium chloride. What I secured was a more pure chemical than what was available via mining or from animal remains. I dissolved a tablespoon in just enough water to eliminate all traces of the crystals. The solution was left alone for a good twenty minutes while the other two solutions were produced. The test messages were written next to the number one.

Lemon Juice

One lemon was cut in half and the juice of each half was squeezed out into a glass. The juice was not watered down.

Onion Juice

I will freely admit that I had no clue on how to make onion juice. Jack LaLanne’s Juicing Machine might predate the SCA, but I was unable to find any period sources for electric juicers. A hand-held citrus reamer produced little to no juice from half of a white onion. Also, the onion was too solid to yield any juice when a potato masher was applied. In the end, I dropped half an onion into a food processor and converted it into a fine mush. I then squeezed the liquid out of the mush with my bare hands.

Inks, ready to be applied

Test Cards

Several cards were prepared, all from 100 pound, hot-pressed, cotton paper. [16] The Ammonium Chloride was written next to the number one; the lemon juice next to number two and the onion juice next to number three. Each chemical was written on each card with the same pen nib with the nib being cleaned between each chemical. The important thing to remember about invisible ink is that you cannot see it. [17] I need to point this out as it was very difficult to see what I was actually writing. I had to hold the paper at an extreme angle so that the ink would shine in the light. When viewed straight on, the ink was all but invisible: just a wet shine.

After each card was prepared, it was set aside and left to dry. The lemon juice and onion juice soaked into the paper and dried within a few minutes, while the ammonia chloride, after an hour, was still thick and wet. [18] Even after a few days, the thickness of the ammonium chloride ink could still be seen, and felt, on the paper. While the juices could not be felt (as either raised text or a different texture) and could only be seen as something shiny when held at an angle to bright light: the lemon juice far more visible than the onion juice.

Two cards are marked Age Test and were left alone to see if any of the chemicals would darken and reveal itself over time: Le Menagier said that the hidden, ammonium chloride message is good for eight days.

Age Test 1, after drying.

Two cards were marked Sunlight Test and each card was placed in two different windows in such a way that one would bask in the rays of the morning sun and the other in the afternoon sun. This is to see if plain sunlight could reveal the hidden messages.

Sunlight Test 1, after drying

Two card were marked Reveal Test. Each card was aged for three days and then exposed to heat to test which of the three chemicals could be read after different levels of heat. 

One card was marked Body Heat Test and was carried in my shirt pocket everyday at work. This was to test to simulate a message being carried within a doublet. I wish to see if any of the chemicals could be revealed by simple body heat.

Four days into the experiment, the three inks were visible (shiny) when held at the right angle to bright light and the Ammonia chloride was very tactile. A chance conversation about this project brought up the definition of the word "melt". I had used cold water to dissolve the ammonium chloride and only enough to dissolve the powder. Reproducing the same mixture in a clear glass, [19] I could see that not all of the ammonium chloride was dissolved, as I first thought. There was a thin film of wet powder on top of the solution. And it might be that the thick writing was due to the undissolved power sticking to the pen nib. Another possibility was that the solution had too much ammonium chloride. 

I prepared two additional paper cards, titled Consistency Test and Consistency Test 2. I also acquired a scrap of pergamenta to see if the invisible ink would hide better. I prepared a watered down solution of ammonium chloride (one tablespoon mixed in with a quarter cup of cold water), as well as a hot solution (one tablespoon of ammonium chloride mixed with about four tablespoons of hot, but not boiling, water). I also juiced another lemon to act as a control.

The cold solution was written down next to the number one, the hot solution next to the number two and the lemon juice next to the number three. I used lemon juice for the control as I had another lemon available and I did not want to gooify [20] another onion. The hot ammonium chloride solution dried faster than the cold solution (about a hour faster).

Results and Observations

Inks

In preparation, the lemon juice was the easiest to prepare. The ammonium chloride, both cold and hot methods, were the next easiest and the onion juice was the most difficult to prepare. For aroma, the onion juice had the strongest smell in the cup, followed by the lemon juice and then the ammonium chloride. However, once the inks were on paper, all odors disappeared within a few minutes. No detectable aroma could be discerned after the inks had dried. For the reveal test: the lemon juice and ammonium chloride were about equal darkness, after being heated, while the onion juice was not quite as dark: it required a longer exposure to heat to fully develop. The onion juice was the best in being invisible: it was not very detectable when the cards were held up to the light or when held at an angle to the light. The ink was detectable as a rough surface by one’s fingers, but could not be identified as text as could the ammonium chloride and the lemon juice. The ammonium chloride and the lemon juice could be seen as shiny text when held up at an angle to the light and almost read at this angle. The ammonium chloride could even detectable when the cards were held up with a bright light shining behind them.

Pergamenta vs. Paper

As I had noted above, the lemon and onion juices soaked into the paper, and dried, within minutes, while the ammonium chloride took over any hour before it dried and set up. While on the pergamenta, both ammonium chloride solutions caused the scrap of pergamenta to start to curl and bend where it was moistened. The lemon juice did the same, but not as severely. Plain water did not cause any similar reaction.

Within minutes, all three liquids had soaked into the pergamenta and were quite invisible, even when held up at an angle to bright light. While the hidden text on the paper had a certain shine to it, on the pergamenta, if one did not know that that was a hidden message, it would have been almost completely undetectable, particularly to the casual glance. In comparison, the hidden text on the paper might pass a casual glance, but would be detected with a detailed search. However, it was obvious that the pergamenta had gotten wet at some point.

Pergamenta Test, after drying

Age Test

This test was done to see what would happen after eight days; the original source gave a fixed time limit for the invisibility. However, after 20 plus days, there is no visible difference between the three inks. Comparing the cards with the photos taken at the start of the project, it does not appear that visible changes have taken place. I do not know why the author of Le Menagier de Paris stated that the ink would only last eight days, but since there is no context for the instructions, we might never know why it was listed as a temporary solution, [21] or, indeed, why the instruction was provided at all.

While I was finishing up this documentation, I lit a candle and held Age Test 2 to the flame. The hidden text was revealed with any problems, except for when I brought the card too close to the flame and burned a hole in the card.

Age Test 1, after two days

Age Test 1, after twenty days

Body Heat Test

I am ashamed to say that the body heat card was misplaced after only three days. However, judging by the length of time it took for a candle flame to reveal the hidden messages, I do not think that normal body heat would be capable of sparking the chemical changes necessary to darken the text.

Body Heat Test, after one day

Sunlight Test

Like the Age Test, there is no visible difference between the cards and their initial photos after three weeks of sitting in sunshine. The exposure to sunlight appears to have no effect on the ammonium chloride, lemon juice or the onion juice. Like Age Test 2, I also brought Sunlight Test 2 near a candle flame to see if age and light exposure would prevent the hidden messages from being revealed. However, this time I did not set the card on fire. The only noticeable difference was the the last two characters written in lemon juice did not fully appear. I will put that down to not enough lemon juice was in the pen nib.

Sunlight Test 1, after twenty days

Reveal Test

After four days, the two paper cards marked Reveal Test were exposed to fire. I acquired a candle and a lighter and sat down on the sofa, ready to reveal the hidden messages. I picked up the lighter and the candle and realized that paper is, potentially, flammable. I could set the paper on fire and ruin the experiment. And, more importantly, I could set the sofa on fire. [22]  I moved the candle, lighter and paper to the less flammable bathroom and cleaned off the counter; removing anything that could catch on fire. I then got the fire extinguisher from the kitchen and set it by the sink, just in case. There are no pictures of this portion of the experiment as I only had two hands, one for the paper and one to steady the candle.

For the test, I held one card at an angle to the candle flame (Reveal Test) and the other (Reveal Test 2) directly over the flame. This was a test to see which of the three chemicals appeared the fastest and the easiest to read.

Of the three chemicals, the ammonium chloride was the darkest, the lemon juice the next darkest and the onion juice the faintest. However, the onion juice was revealed the quickest, then the lemon juice and finally the ammonium chloride.

With the card held at an angle to the candle flame, I started at the bottom of the card, with the onion juice, and moved the card so that the lemon juice and finally the ammonium chloride was exposed to the flame. Even with added heat of exposure, as the heat moved up the card, the lemon juice took about twice as long as the onion juice to reveal (estimated two seconds for the onion juice and four seconds for the lemon juice). [23] It took an estimated nine seconds for the ammonium chloride to reveal, while I moved the paper back and forth to keep the paper from burning. As it was, the paper is darkened around the text, where the paper was held to the flame too long.

Reveal Test 1 and 2 after exposure to heat

With Reveal Test 2, with the paper held directly over the candle, the Onion Juice was revealed in one and a half Mississippis and the lemon juice in three. The ammonium chloride was revealed in four and a half seconds, but, the paper trapped the rising heat from the candle and darkened the paper considerable more than Reveal Test.

Reveal Test 2 twelve days after exposure to heat

Since I had the candle burning, and as I had a safe environment already prepared, I decided to test the pergamenta; I had another scrap that I could use for any further testing. I brought the pergamenta close to the candle flame and it almost instantly started to bubble, pop and then catch on fire. On the plus side: the heat of the fire did reveal the lemon juice and both ammonium chloride solutions. On the minus side: the sample was on fire!

Pergamenta Test after being set on fire

Dear readers, if you only take away one thing from this project, it should be that pergamenta is flammable and I highly recommend that you do not expose it to fire. There are plenty of other things one can do with pergamenta; setting it and your house on fire should be low on your list of priorities. So, remember, only you can prevent scroll fires.

Pergamenta Test four days after being set on fire

Fortunately, the fire was not very big and I was able to put it out without too much difficulty, and without losing too much of the pergamenta. However, due to the flammability of the material, I decided to discontinue testing with the pergamenta, other than observing if the charred remains of the ammonium chloride text would fade.

Consistency Test

The ammonium chloride applied to these cards were less visible than that applied to the previous cards. However, the text could be felt as raised text with one’s fingers. The lemon juice could also be felt on the card’s surface, although not as severe. Like the previous cards, all three could be detected by their sheen, when held up at an angle to the light and the two lines of ammonium chloride could be seen when held up to a bright light.

Consistency Test 1 after 17 days

Like the previous cards, no visible changes could be seen after three weeks of aging. One of the cards, from this test, was exposed to the heat of a candle. The warm and cold solutions of ammonium chloride were revealed in about the same amount of time (about nine seconds).

Unfortunately, I do not have any pictures of any of the additional cards, after they were exposed to heat. I left them for the judges to experiment with. After Ice Dragon, I had packed the cards and paperwork away someplace safe. If I manage to find them, I will photograph them and update this blog.

Conclusion

Sal ammoniac appears to be as effective as an invisible ink as lemon juice and onion juice. In period, it would have been easier to acquire than lemon juice and, probably, easier to prepare than onion juice. [24]. I was pleasantly surprised at how effective it was and can easily see how messages can be secretly passed between two people: if not for bright light, and the knowledge that there was writing present, my hidden text would be easily overlooked by anyone who was not actively looking for invisible ink.

[1] Hinson, Other Odds and Ends

[2] Singh, p6 and Kahn, p522

[3] Zimmerman, p104

[4] Cowley; Calhoun, p242-3

[5] Ben Jonson, William Gifford, p203

[6] Not the fruit

[7] Salt of ammonia

[8] Eliade, p216; Cotnoir, Wasserman, p75, et al

[9] Vert Antique

[10] Hiscox, p137-9, 184, 206, 209; Coolet, Tuson, p68, 88, 255, 268, 270; Merrifield, p55-59; Longman, Hurst, Rees, Orme & Brown, no page numbers

[11] Rabelais, Chapter 2.XXIV

[12] Allen, p170

[13] Myers p27

[14] Used to treat and prevent urinary tract infections in goats

[15] And they gave me free shipping.

[16] Bristol

[17] Duh.

[18] That’s what she said.

[19] I first used a ceramic mug

[20] Yes, it is a real word

[21] Pun intented.

[22] That would be bad.

[23] Using the Mississippi method.

[24] And most likely easier to find than the dew from alcakengy, the gall of a raven or ambergris.

Bibliography:

Agricola, Georgius. De Re Metallica, Translated From the First Latin Edition of 1556. Library of Alexandria, 1950. (Google eBook)

Allen, Thomas B. George Washington, Spymaster: How the Americans Outspied the British and Won the Revolutionary War. National Geographic Books, Jan 2007.

Cooley, Arnold James; Tuson, Richard Vine. Cyclopaedia of practical receipts and collateral information. J. & A. Churchill, 1872. (Google eBook)

Cotnoir, Brian; Wasserman, James. The Weiser Concise Guide to Alchemy. Weiser Books, May 20, 2006. (Google eBook)

Cowley, Abraham; Calhoun, Thomas O. Collected works of Abraham Cowley: Poems (1656) ; Pt. 1 : The mistress, Volumes 1-2. University of Delaware Press, 1993. (Google eBook)

Eliade, Mircea. The Forge and the Crucible: The Origins and Structure of Alchemy. University of Chicago Press, Mar 15, 1979

Grant, Edward. A Source Book in Medieval Science. Harvard University Press, 1974

Hiscox, Gardner Dexter. Henley’s Twentieth Century Forrmulas, Recipes and Processes: Containing Ten Thousand Selected Household and Workshop Formulas, Recipes, Processes and Moneymaking Methods for the Practical Use of Manufacturers, Mechanics, Housekeepers and Home Workers. Norman W. Henley, 1914. (Google eBook)

Jonson, Ben; Gifford, William. The works of Ben Jonson...: with notes critical and explanatory, and a biographical memoir, Volume 8. G. and W. Nicol, 1816. (Google eBook)

Kahn, David. The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet. Simon and Schuster, Dec 5, 1996. (Google eBook)

Le Menagier de Paris. Translated by Hinson, Janet. http://daviddfriedman.com/Medieval/Cookbooks/Menagier/Menagier_Contents.html

Merrifield, Mary Philadelphia. Medieval and Renaissance Treatises on the Arts of Painting: Original Texts With English Translations. Courier Dover Publications, 1999. (Google eBook)

Myers, Richard L. The 100 Most Important Chemical Compounds: A Reference Guide. ABC-CLIO, Aug 30, 2007. (Google eBook)

Rabelais, Francois. Five Books Of The Lives, Heroic Deeds And Sayings Of Gargantua And His Son Pantagruel: Translated into English by Sir Thomas Urquhart of Cromarty and Peter Antony Motteux. The text of the first Two Books of Rabelais has been reprinted from the first edition (1653) of Urquhart’s translation. Footnotes initialled ‘M.’ are drawn from the Maitland Club edition (1838); other footnotes are by the translator. Urquhart’s translation of Book III. appeared posthumously in 1693, with a new edition of Books I. and II., under Motteux’s editorship. Motteux’s rendering of Books IV. and V. followed in 1708. Occasionally (as the footnotes indicate) passages omitted by Motteux have been restored from the 1738 copy edited by Ozell. (Project Gutenberg Ebook)

Rees, Abraham. The Cyclopædia: Or, Universal Dictionary of Arts, Sciences, and Literature, Volume 19. Longman, Hurst, Rees, Orme & Brown, 1819. (Google eBook)

Rose, Alexander. Washington’s Spies: The Story of America’s First Spy Ring. Random House Digital, Inc., Dec 18, 2007

Singh, Simon. The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography. Random House Digital, Inc., Jan 26, 2011. (Google eBook)

Zimmerman, Susan. Shakespeare Studies, Volume 34. Fairleigh Dickinson Univ Press, Oct 1. 2006. (Google eBook)

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Observations from 2015:

I really enjoyed conducting this experiment. I like researching oddball things and this was a very odd ball. And, I got to go all "Mythbusters" on the cards. I only posted a small selection of the pictures: for most of the cards, I have between 20 and 40 pictures. I did not want to flood this post with almost identical photos. Once I find out where I stashed the cards, I'll post some "very after" pictures. I am willing to bet that the un-revealed cards will still be all but invisible.

This project started with me looking for a recipe and reading the line in Le Menagier about the ink. My first thought was, "That's weird." And then I spent a few weeks trying to think of why some old dude would be telling his young wife how to make spy ink. I went back and re-read the line and then had to do some research on what sal ammoniac was and what other invisible inks were available and by then, I was hooked. I had to try it.