As a drinker, I’m mildly interested in the alcoholic strength of the potion before me. I might select a drink because of its ABV or alcohol by volume, a comparative scale that allows me to judge the relative strength of one drink against another. My mood, the time of day and the amount I have already consumed will be inputs to determining which alcoholic beverage of which strength I will choose. The ABV is helpful as a guide of relatively but does not help in the other major determinant, taste. Oh, and of course, price!
There are only two certainties, they say, in life – death and taxes – and it was taxes that were behind the institution of a proof system for alcohol. The word proof means a test or trial or demonstration and from around the 16th century the tax authorities were interested in establishing the amount of alcohol in a particular drink, the greater the strength, the higher the tax rate. The test was fairly rudimentary but ingenious in a way, involving a pellet of gunpowder. This was soaked in the liquor in question and the wet gunpowder was ignited. If it went off, then the alcohol was rated as above proof and attracted a higher tax rate. If the gunpowder failed to go off, then it was taxed at the lower rate.
The science behind this is that the tipping point for a spirit like rum to allow the gunpowder to burn was an ABV of 57.1% ethanol by volume and this was used to represent 100% proof. This rather quaint and possibly dangerous way of proofing alcohol persisted until the late 18th century but more rigorous tests were being conducted. In the 1740s the Customs & Excise and many London distillers used an instrument devised to measure spirit strength called Clark’s hydrometer. This was improved upon by Bartholomew Sikes and was adopted as the standard following the enactment of the Hydrometer Act in 1818.
The measure for 100% proof was 12/13ths the specific gravity of pure distilled water at the same temperature. As luck would have it, it was the equivalent of 57.15% ABV.So, to convert proof into ABV, all you had to do was to multiply it by 1.75. Thus alcohol with a proof of 100% will have an ABV of 175% and a standard gin with a 40% ABV will have a proof of 70%. This method of proofing remained in place in Britain until 1st January 1980.
The Americans, however, chose to use a different system. In around 1848, they developed a measuring system that based on the percentage of alcohol present in the drink by volume rather than worrying about more complex matters like specific gravity. Under this measuring scale, a liquor with 100% proof would have 50% alcohol by volume.
The French, naturally, took a different route, adopting a scale developed by a famous chemist, Joseph-Louis Gay-Lussac in 1824. This system gave a spirit which was 100% alcohol by volume a notation of 100 and pure water a notation of 0. It m ay be unfashionable to say, but this seemed perfectly rational. But for the drinker at the time a liquor marked as 100% on the American scale was only 50% proof on the French scale and about 87.6% on the British scale. Enough to give the drinker a headache!
Whilst the American system is still in use, the EU, including Britain who adopted it from 1st January 1980, deploy the ABV system which is based on Gay-Lussac’s scale.