If you have a hankering to act the part of a WWI spy, no problem: You'll just need 5 drams of copper acetol arsenate, 3 ounces of acetone, a pint of amyl alcohol—oh, and you can't inhale the fumes as you're heating up the concoction. But assuming that's not a problem, voilà! You now have the ability to secretly open a sealed envelope. That recipe is one of the six oldest classified documents the CIA has, all of which the agency released yesterday, the Washington Post reports.
All six documents date from 1917 and 1918, before the CIA was even formed, and most of them deal with invisible ink for, as one document calls it, "secret writing." Another document recommends that you use a quill pen with your invisible ink; yet another recommends letting your solution absorb into your handkerchief. Even though most kids these days are fairly familiar with invisible ink, a Freedom of Information Act request to release the documents was rejected in 1999. But now, says CIA Director Leon Panetta, the information is "no longer sensitive." NPR has links to all six documents.