From July 11-31, 2011, the National Archives is displaying newly declassified documents from 1918 detailing German secret ink formulas. The oldest newly declassified documents held by the National Archives, these materials were released April 19, 2011, by the National Archives National Declassification Center in coordination with the Central Intelligence Agency, as part of the President’s ongoing Open Government initiative.

The display, which is free and open to the public, is in the East Rotunda Gallery of the National Archives Building in Washington, DC.

For 95 years, the documents on display were national security classified material and unavailable to the public. Believed to have been the oldest documents still classified by the United States, they detail German secret ink formulas developed during World War I. One formula – written in French with translation – is described in this June 14, 1918 Office of Naval Intelligence document. The invisible ink’s ingredients – compressed or powdered aspirin mixed with “pure water” – and the method of causing it to appear are provided.

The documents were declassified as part of the work of the National Declassification Center, and can be seen in full at the National Archives at College Park, Maryland www.archives.gov/dc-metro/college-park.