When it comes to arcane historical terms for spirits and other alcohol, proof is one of the frustrating ones. In our modern vernacular, with the blessing of modern analytical chemistry as support, we simply think of “proof” as two times the alcohol by volume (ABV). But why is this measure even a thing? After all, if you could measure ABV, why double it? Then to add insult to injury, Proof Standards vary—in the U.K., the scale is different than it is in the U.S., as is the French standard. This all harkens to a time when brewers and distillers knew what alcohol was (sort of) and how to produce and concentrate it but not how to measure it. So, the three common methods of “proofing” your spirit were developed as a means of demonstrating that one brand or batch was “better” than another.
The actual term “proof” in connection with the alcohol content of liquors dates back to 16th-century England and refers to “a test, trial or demonstration.” The proof system is based on the selection of an arbitrary standard (called 100 proof) typical of the alcohol content of distilled liquors and the rating of the alcohol content of other beverages in terms of how much larger or smaller they are relative to this standard. The proof system was originally established for purposes of taxing liquors according to their alcohol content and varies from country to country. In 16th Century England, the original test involved shaking a mixture of water and alcohol, the faster the bubbles disappear the higher the content of alcohol. For example, a bottle of 80 proof Scotch when shaken will have small bubbles that will disappear after several seconds. Whereas a bottle of 180 proof vodka will have large bubbles that will disappear instantly. It’s the kind of subjective standard you really want the taxman using — NOT!
During the 17th and 18th Century, soldiers and sailors in the British Royal Navy, who were frequently paid both in silver (Sterling) and in rum would douse their gunpowder in rum to test the potency of the rum. If the wet gunpowder still ignited, it was “proof” the alcohol content was high enough, about 57% ABV. If it didn’t ignite, they knew the rum had been watered down and they were being cheated. Not a good idea since you probably had some angry—and armed—British soldiers on and your hands.
By the end of the 18th century, a Swiss German by the name of Johan Tralles developed an alcoholmeter or hydrometer that indicates the alcoholic strength of mixtures of alcohol and water. This device, also known as a proof and Tralles Hydrometer, density of the fluid. The more alcohol in the mixture, the less dense the fluid and hence the deeper the hydrometer will float. Modern brewers sometimes Tralles Hydrometers to measure their wort and then their beer to establish ABV. As the sugars in the wort are fermented and converted to alcohol, the resulting loss of specific gravity is assumed (not measured but inferred) to be a conversion of sugar to alcohol. A reading is taken before and after fermentation allows the approximate alcohol content is determined by subtracting the post fermentation reading from the pre-fermentation reading.
England had introduced specific gravity as the criterion for measuring proof or alcohol content around 1813 but since this was highly sensitive to temperature, it resulted in numerous problems with standardization. In the United States, the proof system was established around 1848 and was based directly on percent alcohol by volume rather than specific gravity, with 50% alcohol by volume being taken as typical of strong distilled liquors and as the 100-proof standard. The most scientific scale, however, was that used in France, which was established in 1824 by the famous French chemist, Joseph-Louis Gay-Lussac (figure 1), which took 100% alcohol by volume as 100 proof and 100% water by volume as 0 proof. Thus 100 proof on the American scale is 50 proof on the French scale and about 87.6 proof on the British scale. All in all, it is a good example of what happens when standards are set by politicians instead of scientists.
If all this seems a little vague and easily fudged to you, worry not. Today we determine true ABV with a gas chromatograph and simply report proof as a tradition.
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Michael Carver -- Historical Interpreter/Reenactor 