CIA reveals invisible ink formula

**Jenna** · 04-19-11, 08:29 PM

The document listing formulas used to make German secret ink is written in French and dated June 14, 1918.
A related document in English tells spies how to expose the German correspondence, starting with the instruction, "examine through powerful beam of light directed on surface at different angles."
The instructions continue with spy novel-type techniques, including dusting a little powdered charcoal over the paper's surface.
"Run a hot iron over the surface being careful not to scorch the paper," the instructions say. "Wet with water."
The directions resemble a mix of the scientific and simple, calling for chemicals such as hydrogen sulphide and ammonia, with instructions to "rinse with water and dry in the air."
A few other steps include wetting the paper with iron sulphate and a "little solution of potassium ferrocyanide," all of which, according to the document, collectively expose a host of other substances like iron and copper used to conceal writing, thus revealing the words and making the pages readable.
The "secret writing" document instructs spies how to disguise their correspondence.
The directions call for a nitrate and starch solution, which, according to the document, "may be carried for example in handkerchiefs or starched collars."
"A tumbler of water is boiled together with a table spoon of starch, allow to cool, and add ten gramms (sic) of nitrite of soda," the instructions say.
One of the CIA documents outlines detailed instructions on how to open a sealed letter without detection, largely by using chemicals including acetone and amyl alcohol.
"Heat in water bath -- Steam rising will dissolve the sealing material of its mucilage, wax, or oil," the instructions read. "Do not inhale." Mucilage is a type of glue.

More here: Spy agency reveals invisible ink formula - CNN.com