The Central Intelligence Agency on Tuesday declassified six World War I-era documents -- the oldest classified documents held by the spy agency -- containing the ingredients used to create invisible ink.
The formulas, which date from 1917 and 1918 and were the CIA's oldest classified documents, were apparently so sensitive that as late as 1976 the agency decided against releasing the information.
"These documents remained classified for nearly a century until recent advancements in technology made it possible to release them," CIA Director Leon Panetta said in a statement. CIA officials could not be reached to explain what processes had led to their declassification.
The declassified material, which the CIA said it believed were the last remaining secret documents from World War I, include a number of recipes for making secret ink. One consists of a combination of iron sulfate and potassium cyanide or rice starch mixed with ink and water. To make the ink visible, a combination of water, potassium iodate and tartaric acid must be applied.
Another memorandum dated June 14, 1918 -- and written in French -- reveals the formula used for German secret ink.
"The CIA recognizes the importance of opening these historical documents to the public," said Joseph Lambert, the agency's director of information management Services.
The CIA declassified and released over 1.1 million pages of documents in fiscal year 2010.
The documents will be available on CIA.gov and in the CIA Records Search Tool (CREST) at the National Archives in College Park, Maryland.
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