Cipher Ink – Medieval Invisible Ink

This was the documentation for the cipher ink entry I did for the South Oaken Regional Arts and Sciences Fair in the Middle Kingdom.  If you are a regular reader of my blog you may be surprised and how “thrown together,” this appears.  That is because it was thrown together.  I will be revising this documentation, fleshing it out and making it much nicer.

I promised I would put this up on my blog by Sunday night as I entered it at the fair.  So, here it is!


The purpose of writing, is for it to be read.  This is a simple enough concept, true, but sometimes you want certain people to not read what you are writing.  How then do you write something and have that group of people not read it?

You could write it in a code.  Or better yet, write it in a way that they don’t even know that anything has been written.  In other words use invisible ink.  Of course the people you do want reading it need to have a way to read it.

The use of invisible inks goes back hundreds if not thousands of years.  There are many recipes in period that tell us how to make or use invisible inks.  All of these methods are as valid today as they were then.  Just that today the methods hundreds of years old are very well known and frankly are not secret anymore.  Still the study of invisible, or cipher, inks is an interesting one and worth delving into.

Even in late period the secret of invisible inks was not a difficult one to procure.  The below recipe from “A Book of Secrets…” is the process that inspired how I made one of my invisible inks.

A Booke of Secrets showing diverse ways to make ink – 1596

To write without inke, that it may not be seen, vnlesse the paper be wet with water.

Take pouder of victriall, and put it into a cleane inkehorne, put a little cleane water to it, when the victriall is dissolued, write with it either vpon paper or parchment, and let it drie, and it cannot bee read: when you will read it, take halfe a pint of water, and put thereto an ounce of pouder of gaules, mix them well together, then straine them through a linnen cloath into a cleane pot, then draw the paper through the water, and the writing will be blacke, as if it had ben written with inke.[1]

“Pouder of victriall” is green vitriol which is more commonly known as copperas.  In modern chemical terms it is Ferrous Sulfate Heptahydrate or Fe(II) SO4 + 7 H2O.  It dissolves easily into water.  When you do so the water remains fairly clear assuming you do not put too much copperas into the solution.  One can imagine that writing with this solution would be invisible to the naked eye.

Not always.  Ferrous sulfate is a salt, meaning that it is made up of a salt ion negatively charged and another that is positively charged.  When put into water these ions separate from one another somewhat and interesting things can happen when the water evaporates.  In this case the metal is iron and the positively charged ion is sulfate which is composed of sulfur.  Sulfur is yellow in color.  If you put too much ferrous sulfate into the solution then it will dry yellow on the writing surface, not clear, rendering it visible to the naked eye.

It then is very important to put enough ferrous sulfate into the solution so that it will be strong enough to be readable later, but weak enough not to turn yellow on the writing surface.  Practice and experience lend a lot to ensuring you get this right.  I used ½ teaspoon to 4 fluid oz of water, or half a cup.

“Halfe a pint of water” is one cup of water.

“Pouder of gaules” is powdered oak galls.  Oak galls contain tannic and gallic acids in them.  One can soak or boil oak galls and extract the acids in them.  The extract can be colored anywhere from a lightly colored tea to a dark cup of coffee depending on the concentration of the extract in the water.  This is the normal action one would take in making regular iron gall ink and then you would throw in the copperas and voila! You have a nice black iron gall ink.

I used ½ oz of oak galls to 1 cup of water.

In making the invisible ink, you are merely changing when you introduce the gall extract and the copperas.  The chemical reaction is exactly the same, however belatedly you introduce the two together.

There are other ways of making invisible ink using these ingredients.  Also using camouflage over the invisible ink adds a layer of protection to being detected.  You can boil the copperas write with that solution and then write over it using a carbon ink. Then take a sponge soaked in gall extract and remove the carbon ink and show off the true message underneath in one motion.[2]

The other cipher ink I used was milk.  I used whole milk commercially purchased.  Raw milk, as was used in period, has many legal limitations for use and sale and it is illegal to transport across State lines thus making it problematic to procure.  Whole milk commercially purchased was the next best legal thing to do.

Milk dries with some sheen to it so it can be detected but only by close inspection under good lighting.  Once the milk dries and the message is received all one has to do is introduce the writing surface to heat and the milk will darken to various shades of brown.  Of course you have to use enough heat to change the milk’s color, without destroying the surface it is written upon. This can be very tricky but experience helps a lot with not making this mistake.

The study of invisible inks is a fun and diverting interest.  Today the medieval methods and techniques while still sound are not viable for nations or spies to keep their secret correspondences safe from prying eyes.  After all the technology and techniques are centuries old and well documented.


Figure 1

“If you desire that letters not seen may be read, and such as are seen may be hid.”

Let Vitriol soak in boiling water.  When it is dissolved, strain it so long till the water grows clear.  With that Liquor write upon paper.  When they are dry, they are not seen.  Moreover, grind burnt Straw with Vinegar, and what you will write in the spaces between the former lines, describe at large.  Then boil sour Galls in white Wine, wet a sponge in the Liquor.  And when you have need, wipe it upon the paper gently, and wet the letters so long until the native black color disappears.  But the former color, that was not seen, may be made apparent.  Now I will show in what Liquors paper must be soaked to make letters to be seen. As I said, dissolve Vitriol in water.  Then powder Galls finely, and soak them in water, let them stay there twenty-four hours.  Filter them through a Linen cloth, or something else, that may make the water clear, and make letters upon the paper that you desire to have concealed.  Send it to your friend absent.  When you would have them appear, dip them in the first Liquor, and the letters will presently be seen.

Giambattista della Porta  16th Book of Magicka Naturalis Chapter 1



[1] White, Edward. A Book of Secrets, showing diverse ways to make and prepare all sorts of ink and colors. Trans. W.P London. Adam Islip, 1596. Print.


[2] See Appendix Figure 1


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