Navy rum was supposed to be 54.6 ABV (alcohol by volume). In order to demonstrate to the crew that the rum was not diluted the purser would mix a small quantity of gunpowder with the rum and light it. If the gunpowder flashed, then the rum was overproof. If the gunpowder failed to ignite, then that meant it had been diluted. If the gunpowder burned with a steady blue flame, then the rum was at the correct strength and was considered “at proof.” Hence the use of the term proof when describing the strength of alcoholic spirits.