http://www.foia.cia.gov/ wrote:The Central Intelligence Agency today declassified the United States Government's six oldest classified documents, dating from 1917 and 1918. These documents, which describe secret writing techniques and are housed at the National Archives, are believed to be the only remaining classified documents from the World War I era. Documents describing secret writing fall under the CIA's purview to declassify.
http://www.abovetopsecret.com/forum/thread691677/pg1 wrote:“Mix 5 drams copper acetol arsenate. 3 ounces acetone and add 1 pint amyl alcohol (fusil-oil). Heat in water bath — steam rising will dissolve the sealing material of its mucilage, wax or oil.” But there’s a warning for the intrepid spy: “Do not inhale fumes.”