Revealed: Skin Messages, Invisible Ink and More Secret WWI Spycraft

[Feuer Frei!]

How to read the Hun’s letter without noticeably opening it: mix copper acetyl arsenic with three ounces acetone. Then add a pint of amyl alcohol. Heat the mixture in a hot water bath. The resulting fumes should break down the letter’s adhesive. Back in World War One, this was state of the art spycraft. Or maybe it wasn’t. On a document declassified on Tuesday by the CIA listing the nearly century-old formula for surreptitiously opening mail, an anonymous agent scribbled, “Tried — not successful.”
And maybe that’s why the document and five others, detailing espionage techniques used by the Office of Naval Intelligence in 1917 and 1918, haven’t seen the light of day since they were penned. Until Tuesday, they were the six oldest classified documents the government possessed.
CIA Director Leon Panetta explained that “recent advancements in technology made it possible” to release the documents after almost a century, without elaborating. (What, PDFs?) But they could also be an early example of the government keeping some of its national security blunders out of the public view. (Update: CIA spokeswoman Marie Harf adds, “In recent years, the chemistry of making secret ink and the lighting used to detect it has greatly improved.”)
Either way, they’re a rare glimpse at the U.S. intelligence community in the days before it ballooned to a global entity costing $80 billion every year. In a letter dated October 30, 1917, an “assistant chemist” at the Department of Commerce named A.M. Heinzelmann provides formulas for what appears to be invisible ink, warning that some of them “doubtless exert a very corrosive action on steel pens.” (Namely, 100 ccs of water when added to three grams of potassium bromide and the same amount of copper sulfate.)

Back in the first World War, even the human body came under suspicion for being a medium for secret messages. A pamphlet printed for Post Office inspectors advised counterintelligence agents to “develop a suitable reagent sprayed with an atomizer” to reveal the secret tattoos. To get rid of your own hidden tattoos, scrub down with citrus.
Two different documents from 1918, written in French, detail the Germans’ favorite methods for concocting invisible ink. Spoiler alert: compressed or powdered aspirin mixed with “pure water.”
Suffice it to say that spycraft in the first World War really relied on invisible ink. A different method to concoct it used five grams of “Iodite of Potassium,” mixed with 100 grams of water, two grams of tartaric acid, “Sulpharated soda,” “Ferro cyanite of potassium” and diluted ink.
Others are more traditional, like lemon juice mixed with potassium, a favorite of the Revolutionary War. Carefully put your parchment up to a heat source to read the hidden message.
All the more amazingly, intelligence bureaucrats believed for decades that these long outmoded tools of the spy trade needed to be kept out of the public purview. One of them carries a stamp from 1978 marking it “EXEMPT from automatic declassification.” The one with the invisible tattoos has a different non-disclose stamp from 1989. So figure we’ll be living in Martian colonies by the time we learn what the “secret weapon” used by special operations forces in Iraq during the surge really was. Especially if it didn’t exactly work as well as the hype suggested.

SOURCE