CIA: Learn how to make invisible ink, photos – from 1918

The CIA unearths some pretty interesting materials and this week it was touting the oldest once-classified documents in is collection.

The mostly cryptic documents, from 1917 and 1918 describe secret writing techniques are believed to be the only remaining classified documents from the World War I era. Any documents describing secret writing fall under the CIA’s purview to declassify, the agency noted.

The CIA website noted that one file outlines the chemicals and techniques necessary for developing certain types of secret writing ink and a method for opening sealed letters without detection. Another communication dated June 14, 1918 – written in French – discloses the formula the German’s used to produce invisible ink.

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Image courtesy of the CIA