A collection of different kinds of gunpowder dating back hundreds of years was given to the museum in 1995, but it's taken them this long to get to this particular bar.
Guy Fawkes was a Roman Catholic who tried to blow up parliament because the king at the time, James I, didn't like his religion.
Plot led to Bonfire Night
But the King found out about the gunpowder plot on 5 November before it could be completed and had Guy Fawkes killed. This is the story behind Bonfire Night.
The gunpowder was given to the library among other papers belonging to a 17th-century writer called John Evelyn.
His family made lots of money a very long time ago from making gunpowder.