The oldest secret documents in the CIA's treasure-trove have been cracked open finally. Believed to be the only remaining classified WWI documents, the six files have revealed all manner of espionage tricks - including the recipe for German invisible ink.
Sealed and locked away in the national archives since WWI, the files date between 1917 and 1918, and outline the ways in which information was passed in secrecy. "When historical information is no longer sensitive, we take seriously our responsibility to share it with the American people" the CIA director Leon Panetta said, revealing that "recent advancements in technology made it possible to release them".
This was hardly schoolage lemon juice-quality invisible ink tomfoolery. In fact, the recipes - written in both English and French - called for all manner of ingredients, such as chloride of carbonade; alcohol; nitrate of potash (Potassium Nitrate) and acetic acid.
Naturally, each recipe calls for its own special developing ink. While the ingredients used to pen the secret messages were more complex than we used as kids, the techniques used to decode the messages were much the same. You probably remember borrowing your Mom's iron to reveal those secret words? Spies of the first world war also used irons, plus more complex procedures involving mixes of iodite of potassium with tartaric acid and water, or simply ink mixed with water and brushed over the paper with a paintbrush.
In a document dated October 30th 1917, and addressed to Mr Frank V Martenek, a run-through of the seven best recipes is listed by the assistant chemist A.M. Heinzelmann. He notes that the first, fourth and fifth formulas "will doubtless exert a very corrosive action on steel pens, and therefore if suitable in other respects would have to be used with a quill pen".
While some of these recipes proved hugely advantageous in passing cryptic notes on either side of WWI — and might still be used today — some were not so successful. A note left by a spy on one of the documents reads "tried — not successful." Presumably that was tried in the lab, and not in the field. [CIA via Yahoo News]
Declassified document 1:
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Declassified document 6: