Prior to the introduction of the Sikes Hydrometer, in 1816, the ship 's Purser, or 'Pusser ', tested the strength of the daily issue by mixing neat rum, with a little water, to which was added a few gunpowder grains. When the mixture was heated through a burning glass, the gunpowder ignited, and the rum was served at that strength. When the method was tested by hydrometer, it was found to be 109% proof {54.5% ABV}. Consequently, the Pusser 's rum remained at 'Gunpowder ' strength, 109% Proof, until the daily issue was terminated, on 31st July 1970, known as Black Tot Day.