SMALL PRINT:ATTENTION BUDDING spies, or perhaps more accurately, budding spies with a time machine. The CIA revealed its secret recipe for first World War invisible ink. Six of the oldest secret documents in US government archives were declassified recently, exposing some old-school espionage methods.

The chief method for exchanging secret messages, according to one document, involved soaking a handkerchief or detachable collar in a mixture of nitrate, soda and starch, drying the fabric and placing the material in water. The liquid that emerges is invisible ink.

Messages can then be revealed by spreading iodate of potassium onto the page. Upon releasing the documents, the CIA said that invisible ink had been rendered obsolete many years ago thanks to advances in digital encryption.

But invisible ink is still used elsewhere – in 2008, a British man was arrested after being found with a contact book of Al Qaeda-related telephone numbers written in invisible ink.