CIA recipe for invisible ink among newly released WWI-era documents

So you want to open sealed envelopes without getting caught?

Here’s the secret, according to one of the six oldest classified documents in possession of the Central Intelligence Agency:

“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.”

Nearly a century after it was written, the recipe was released Tuesday by the CIA as part of a cache of six World War I-era documents. The documents, which deal mostly with invisible ink, date from 1917 and 1918 — predating the agency itself by decades....