Newly declassified documents that date back to World War I are going on display at the National Archives in Washington.
The 1918 documents detail Germany's secret ink formulas from World War I.
They went on display this week and will remain on view through the end of July.
One formula, written in French, is described in an Office of Naval Intelligence document. It describes the invisible ink's ingredients as compressed or powdered aspirin, mixed with water.
The materials were released in June by the archives in coordination with the National Security Agency as part of an open government initiative. They had been classified and kept secret for 95 years and were thought to be the oldest documents still classified by the United States.
National Archives: http://www.archives.gov