This article was taken from the August 2011 issue of Wired magazine. Be the first to read Wired's articles in print before they're posted online, and get your hands on loads of additional content by subscribing online.
The Central Intelligence Agency didn't exist in the first world war, but the US government was even then offering official advice to its secret agents. The Freedom of Information Act means that documents recently unclassified by the agency have come to light, detailing the specific techniques it recommended that its operatives used for invisible photography and writing. But there's one overarching rule: "Suspect and examine every possible thing." For more information, see foia.cia.gov
▲ MAKE YOUR OWN INVISIBLE INK...
This better-than-lemon-juice technique has been familiar to generations of school "spies" since its use as a crucial tool in the first world war. Make a weak solution of starch, tinged with iodine. The blue writing soon fades...
▲▲ ...TO WRITE ON STIFF LINEN OR PAPER ...After it becomes dry, it's made visible by iodine fumes or a solution of potassium iodide, according to the advice. The writing turns blue, but washing the paper with a very weak solution of hydrosulphate of soda makes it disappear.
▲ ▲ ▲ PAY ATTENTION TO BLACK LINES "These can hide Morse code written with a clear solution of gum, or egg white beaten with water." To develop, heat the paper slightly and powder with dragon's blood." (No slaying required: it's a bright red resin used in printing.)
▲▲▲▲MAKE USE OF THE GERMAN FORMULA "Take one ounce [28g] of linseed oil, 20 ounces of liquid ammonia, 100 ounces of distilled water." Mix well. Use a quill to write in the free space between words written in pencil as a decoy.
To read, dip the letter in cold water.
▲▲▲▲▲ LEARN TO WRITE MICROSCOPICALLY "Take a quill and write with orange ink with very finely shaded letters across the face of a stamp. To read, interpose a ruby glass on the stamp." This makes the stamp invisible and the writing legible.
▲▲▲▲▲▲ YOU CAN ALWAYS USE YOUR BODY
You can print messages on the body using invisible ink. To make them appear, spray on a suitable re-agent. "To destroy the messages, the body should be scrubbed and then washed with lime or lemon juice.
▲▲▲▲▲▲▲ SUSPECT FRESH PAINTED METAL "Often, freshly painted surfaces are covering communications that have been scratched on the underlying metal surfaces." You can make these legible using turpentine paint stripper.
▲▲▲▲▲▲▲▲ REMEMBER TO BE ON YOUR GUARD
Look out for "writing under postage stamps, messages in medicine capsules and engraved messages on toenails, which later are made visible by powdered charcoal." Got that, 007?
This article was originally published by WIRED UK
