Percentage Proof Spirit: A Matter of Proof
Many countries are involved in the subject of Proof Spirit, both technically and in everyday explanation of the strength of alcoholic beverages — notably spirits. In part this is ‘consumer protection’ . However, its value in this direction is doubtful as only a handful of drinkers understand the Proof system. There are three principal methods for indicating the strength of spirits, liqueurs, and sometimes of wines and aperitifs.
There is the British Proof system (some-times called Sykes after the inventor of the hydrometer employed to test spirits); the US Proof; and the simple Metric (called Gay Lussac after its originator in France). British Proof is also used in Canada, Australia, New Zealand, the Commonwealth generally, and in Ireland and South Africa. Under the British Proof system absolute alcohol is 175.25 (in practice, 175) and Proof Spirit 100. This makes British Proof Spirit 57% alcohol: we use round figures for convenience.
The USA Proof system is different from the British. In the US Proof scale, absolute (pure) alcohol is rated at 200 and Proof Spirit as 100; ie 100 US Proof indicates a mixture of 50% alcohol and 50% water. US Proof terms can be trans-lated into alcoholic strength simply by halving the Proof number: thus, 86 Proof (US) is 43% alcohol. The Gay Lussac system applies in most other areas, and indicates the volume of pure alcohol in a beverage. (The Ger mans differ slightly, using weight instead of volume.) Unfortunately, it is not simple to translate British Proof into American Proof or into Gay Lussac terms. However, the normal strength of spirits in Britain is 70 Proof, which, conveniently, is 80 Proof (US) or 40 Gay Lussac; that is, contains 40% alcohol by volume.
There are many variations, due to national preferences, but the table below shows roughly the average strengths of spirits in some countries: An important point to remember is, when examining the strength indicated on a bottle, to see in what system the strength is marked. Under both Proof systems — Gay Lussac is simplicity itself — it is essential to bear in mind that Proof Spirit is not pure spirit. Particularly in Britain, some wines fortified with spirit (latterly with Whisky) are labelled as ‘31% Proof Spirit’ — thus not attracting full spirit tax: they contain about 17% alcohol.
The word Proof comes from the days when scientific precision in assessing the strength of spirits had not yet evolved. The method then was to add a mixture of water and spirit to gunpowder, the mixture being made progressively stronger until a point when the gunpowder would not be neutralized by the water but would burn explosively when fire was applied. Spirit which would cause this conflagration was said to have been ‘proved’.