London, April 20 (IANS) While many readers of detective novels today may be at ease to tell how to prepare invisible ink, the US intelligence agency CIA has waited until now to disclose its 'secret writing' practices, revealed Wednesday in some of the oldest secret documents ever declassified in the US.

 
 The revelations are about tips given during World War I (1914-1918) to the would-be spies to help them learn how to open envelopes without being detected and how to prepare invisible ink.
 
 The six files, dating back to World War I, detail the methods used by spies, generals and diplomats to make invisible ink to send secret correspondence between allies, Daily Mail reports.
 
 CIA said that despite the old-fashioned methods, it has only now been able to release the documents since the technology is at last 'obsolete'.
 
 The documents have only now been released because recent advances in the chemistry of secret ink and the lighting methods used to detect it have made the secrets obsolete, said a CIA spokesman.
 
 "These documents remained classified for nearly a century until recent advancements in technology made it possible to release them," said CIA Director Leon Panette in a statement.
 
 "When historical information is no longer sensitive, we take seriously our responsibility to share it with the American people," he added.
 
 But a request to release the files, some of the oldest in the CIA's care, was rejected in 2002.
 
 The documents also throw light upon chemical methods used by agents to open sealed envelopes without being caught, complete with a safety caveat for would-be sleuths: 'Do not inhale fumes'.
 
 The papers, some of which are handwritten, offer a tantalising glimpse into a secretive war-time world. They were originally kept by the Office of Naval Intelligence, many decades before the CIA itself was founded.
 
 One, dating from June 1918, is written in French, and describes the Germans' secret ink formula, showing they had cracked the enemy's code.
 
 Another describes how to carry invisible ink in your clothes. And spies weren't just taught to make invisible ink - they were told how to interpret it, too.