Low-Tech Communications Post-IHTF/ Invisible Ink! Part 2

Low-Tech Communications Post – IHTFCobalt(II)_chloriders

Invisible Ink! 

Part 2

By servantheart, Editor At Large

In Part 1 we talked about how to make ink that is red in color for writing “secret” messages. Today, let’s do “blue”! Here are chemicals that will produce a blue tone sympathetic (invisible or disappearing) “ink”:

  1. Cobalt(II) chloride; the name alone suggests the color, doesn’t it?! Vanishes upon cooling; mild heat makes it visible again.

Cobalt chloride is an inorganic compound; it is made up of cobalt and chlorine, and has the chemical formula CoCl2. In chemistry, it is most often referred to as hexahydrate with the chemical structure CoCl2·6H2O. It is commonly used in labs. Hexahydrate has a deep purple color; the anhydrous form has a sky blue tone. Blending these two compounds produces a mauve tone. When ignited, cobalt chloride produces a blue-green flame.

Available at Amazon and at www.grainger.com , among other places.

  1. Write with cobalt nitrate solution, then wet with a weak solution of oxalic acid; a blue color will then appear.


Cobalt(III) nitrate is an inorganic compound – chemical formula Co(NO3)3. You can get this from grainger, also.


  1. An aqueous solution of copper sulfate (cs mixed in water). An aquaeous solution uses water as a solvent. Make visible by sponging with a solution of iron chloride.


  1. Dissolve 15 grains of copper sulfate in one ounce of water. Sponge with a solution of 15 grains of ammonium hydrate in one ounce of water.

A hydrate is a compound formed by the addition of water to another molecule, thus “aquaeous”; as used in chemistry. Hydrates are inorganic salts containing water molecules that form a crystal with the host compound.

If you just want a short cut to writing with ink that disappears until wet, use the chemical bismuth nitrate. Let dry. Upon wetting, the writing becomes visible. The instructions do not tell us whether this is a one time thing, or if it will disappear when dry again, then reappear when wet again.

Or, just use silver nitrate in a weak solution (water), and allow to dry. When exposed to light, the writing will become visible again. DO NOT expose the paper to light until you are ready to read it.

So, what is the “right” way to use sympathetic ink? Well….

Write a letter that won’t interest anyone; leave just the right amount of space between the lines. Now use your sympathetic ink to write between the lines and watch it disappear! Or, write the REAL message on the back of the decoy letter. This is how it was historically done.

I wonder how well it would work to use two different hidden messages, two different chemical inks? One could be the decoy (false) message, perhaps “between the lines” that perhaps becomes visible only when wet, and the other “real” message becomes visible only when subjected to light? Most people aren’t going to be looking for two different “hidden” messages, using very different processes.

O.K., now we know how to write using sympathetic (disappearing or invisible) inks and red and blue. In part 3, we’ll learn how to write in the color black, and share more tricks and tips. Stay tuned….




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