Washington: The secret of ‘invisible ink’, which was used by spies, generals and diplomats to send secret correspondence between allies during the First World War is now out.

The six de-classified CIA documents, dating back to WWI, painstakingly detail the recipes of the "invisible ink".

"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," 'The Washington Post' quoted one of the documents as saying.

But there's a warning for the intrepid spy: "Do not inhale fumes."

One document lists chemicals and techniques to create invisible ink for what is charmingly called "secret writing". Another document from June 1918 and written in French provides the formula the Germans used for invisible writing during WWI.

Another describes how to carry invisible ink in one's clothes. Spies were instructed to soak their handkerchief or starched collar in a mixture of nitrate, soda and starch before drying the fabric.

The instructions continue: "The article thus treated is later on again put in water and a solution obtained, which can be used for invisible ink. The best means for developing are iodite of potassium."

And spies weren't just taught to make invisible ink -- they were told how to interpret it, too. Instructions include "examine through powerful beams of light directed on surface at different angles' and 'run a warm iron over the surface".

They documents were originally kept by the Office of Naval Intelligence, decades before the CIA itself was founded.