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

CIA recipe for invisible ink among newly released WWI-era documents - The Washington Post: "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.”"