A Brief Note on Proof

The 'proof' on a bottle frequently confuses people. This is hardly surprising. It is intended to. Before Mr Sikes, an excise-man, invented the hydrometer, 'proof' was tested in a fairly cavalier fashion. A small heap of gunpowder was doused with a sample of the spirit which was being tested. A match was then put to the damp heap and the if the result gave a steady flame the spirit was 'proved'. Too weak a mixture and the gunpowder only smouldered, too strong a one and you might get your silly head blown off. So much for quality control in the good old days.

Since Sikes and his hydrometer however we now have more scientific definitions and tests. According to officialdom:

Spirits shall be deemed to be proof if the volume of the ethyl alcohol contained therein made up to the volume of the spirits with distilled water has a weight equal to that of 12/13ths of a volume of distilled water equal to the volume of spirits, the volume of each liquid being computed as at 51% Fa.

In case you haven't quite grasped that, it means that 100° proof on the label means there's 57.06 per cent alcohol in it if measured by volume or 48.24 per cent alcohol if measured by weight. The British still use this ludicrous system of measurement but the Europeans and Americans use two different ones. By far the most sensible is the French system Gay-Lussac which simply expresses the proof figure in degrees, thus 40° on a French label means 40 per cent alcohol. The Americans, who also have a smaller gallon, double this figure, thus 80° on a bottle of US Bourbon means it's 40 per cent alcohol. The easiest way to convert British degrees-proof to percentage alcohol is to multiply by 4 and divide by 7. Thus the average bottle of spirits sold in British shops is marked 70° proof - or 40 per cent pure alcohol.

From the Appletree Press title: In Praise of Poteen.