A few days ago I was enjoying a fine glass of whisky with a few friends and we got to talking about alcoholic ‘proof’ and exactly what this term means.
Back in the 18th and 19th centuries the taxman needed some method to calculate how alcoholic a particular liquid was in order to tax it correctly. This was carried out by subjecting the liquid to a trial, assay or proof.
The most common proof of a spirit in early times was the so-called gunpowder test. Spirits were mixed with a small amount of gunpowder and an open flame was then placed in contact with the surface of the liquid. If the powder ‘flashes’ then the spirit is termed overproof, and if it doesn’t then it is underproof. This has since been found to occur at 57.15% ethanol, defined as 100 degrees proof.
The gunpowder test was a very unreliable, and completely arbitrary test, but provied a quick way of comparing the relative strengths of different spirits. The proof was the official method of measuring spirits all the way through to 1980, when percentage alcohol by volume took over to become the official method by which spirits were taxed and sold (note that in the US a proof scale is still used, but it is defined so that 100 degrees proof equals exactly 50% ABV, so divide american proof by two to obtain ABV)
Over the centuries more complex methods of measuring proof became available, including the Sikes hydrometer (pictured)
I have spoken before about how much I love old 18th and 19th century lab equipment and this is no different! Spirits are poured into the metal bulb, and since ethanol has a different specific gravity to water the proof scale on the right is pushed up and down in proportion to the strength of the spirit.
Sikes hydrometers are still used in certain applications today and are really beautiful things.
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