Curators 'find' Guy Fawkes' gunpowder

Gunpowder destined for use in Guy Fawkes' plot to blow up the Houses of Parliament may have been found in a box in the basement of the British Library.

The solid bar of powder was wrapped in paper inside a shoebox as part of a collection from the archive of the 17th-century diarist John Evelyn and his family, which the library acquired in 1995.

Inside the box was an old envelope, on which was written: "Gunpowder. Large package is supposed to be Guy Fawkes' gunpowder."

Evelyn was born into a wealthy family of Surrey landowners whose fortunes were founded in gunpowder manufacture. The box contained assorted gunpowder samples, most dating from the 19th century, but some – like the solid bar – may be much older. Curators at the museum have spent the past seven years working through the enormous Evelyn archive and only made the find last week.

Ironically, the box and the envelope with the reference to the Guy Fawkes plot of 1605 bear the words "Westminster Bond" and an illustration of the Houses of Parliament. Historians have no idea what happened to the barrels of gunpowder found with Fawkes, but the best guess is that they were sold or refined.

A spokesman for the British Library said: "We know our collections hold some explosive material but, until this discovery, we never realised that we were quite literally sitting on a powder keg." The library will hand the gunpowder over to the Royal Armouries for analysis.