Show Me The Proof: How the Alcohol Proof System Came to Be

Assessing alcoholic strength is of profound importance to the tax man, since it provides the basis upon which duty is levied. As with other materials – precious metals, for example – the traditional method of measuring purity or strength was by subjecting the material to a trial, assay or ‘proof’. The word first appeared in England during the 1500s.

Early methods were crude and unreliable. Spirits were tested by soaking a pellet of gunpowder in them then attempting to ignite it. If the pellet burned, the liquid was termed ‘over proof’ (OP) and taxed at a higher rate; if it did not burn, the liquid was ‘under proof’ (UP). In fact spirits containing less than 57.15% alcohol by volume (ABV) will not allow the gunpowder to burn and this strength was termed 100 degrees proof. 100% alcohol, or pure alcohol, is 175 degrees proof, usually expressed as 75°OP.

A simpler method, and one that I use all the time, was to shake the liquid vigorously in the bottle and observe the bubbles (known as ‘beads’). If they dissipate rapidly, the liquid is under 50%ABV. Higher ABV does not increase the beading, however – white dog (new make spirit) and high strength white spirits will not hold their beads, and the beads on younger whiskies will not linger as long as those on aged whiskies. This is why the beading test was commonly refereed to in Scotland as a measure of quality.

Alcohol is less dense than water so weighs less; thus, chemists can measure the amount of alcohol present in a mixture by comparing its weight to than of the same quantity of water, at the same temperature and pressure. This is termed the ‘specific gravity’ (SG) of the solution.

In 1675 the English chemist, Robert Boyle, invented an instrument which used specific gravity to detect counterfeit coin. Although it could also be used to measure the SG of liquids, Boyle’s hydrometer was fragile and inaccurate, so not widely used for this. The first hydrometer to be designed specifically to determine the density of liquids was invented in 1730 by the Scottish instrument-maker, John Clarke. This was adopted by the Board of Excise in 1787, but it was not completely accurate and by adding other constituents to the alcohol, especially molasses, cunning distillers could achieve a lower reading and thus evade tax.

The first wholly reliable hydrometer was designed by Bartholomew Sikes, secretary of the Excise Board between 1774 and 1783, and adopted by the British government in July 1816. Soon after this (1824) the famous French chemist, Joseph-Louis Gay-Lussac, devised a scale based on %ABV which took 100% ABV as 100° proof and 100% water by volume as 0° proof. This was adopted by the most European governments.

In August 1790 the U.S. government adopted the Dykes hydrometer, replaced in 1842 by the Tralles hydrometer. These defined proof as twice the ABV. So 100° proof on the American scale is 50° proof on the French scale, and about 87.6° proof on the British scale.

In October 1935, Federal regulation set the minimum proof for whiskey at 80°/40% ABV. I believe it became mandatory to state the strength on the label in December 1960, either as proof or ABV. Previously this was optional.

All very confusing. By the 1970s many distillers in the U.K. preferred to state the %ABV, sometimes alongside degrees proof, and then on January 1, 1980, the former measurement replaced the latter throughout the European Union. Many American distillers still state U.S. proof alongside %ABV.

Changes in Minimum Proof Laws

In the U.K., strength was not regulated until September 1st, 1879, when an amendment to the Sale of Food & Drugs Act 1875 fixed the minimum strength at which whisky, rum, and brandy might be offered for sale at 75°proof (43%ABV), with no upper strength limit. There was no requirement to state the strength on the label and it would seem that the usual strength, at least for premium brands, was between 100° proof (57%ABV) and 88° proof (50%ABV).

So when in 1916 the Central Control Board (Liquor Traffic) was required by David Lloyd George, Secretary of State for War (and Prime Minister the following year), to make 75° proof the maximummaximum strength in the U.K. was relaxed – and will welcome any readers’ comments on this. As early as 1950, Marshall Robb remarks: “…ordinary bottled whisky and other spirits such as gin, rum and brandy are usually, but not always, sold at 70 degrees proof or 30 degrees UP, which is the same.” [Scotch Whisky]

Brand owners began to extend their product ranges with ‘cask strength’ bottlings during the 1980s – the earliest I know of is Glenfarclas 105^st century to 40%.

“High proof bourbon bottlings did not appear in the US until the late 1980s: Booker’s 1988, then Wild Turkey Rare Breed 1991.”