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Issue 71 - How to prove it

Whisky Magazine Issue 71
April 2008


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How to prove it

In the latest in the series on whisky terminology we look at the letter P – and the proof system.

When it comes to the alcoholic strength of whisky, it would seem straightforward enough.

Europeans have one way of measuring and the Americans are left seeing double.The European system of alcohol by volume results in a figure that is exactly half of the American proof system.

Easy, isn't it?

Or is it? Why, then,does a bottle of Isle of Arran 100 Proof have a strength of 57% alcohol by volume? It would seem that there is more to this than meets the eye.

Historically the word ‘proof' comes from the tests that were applied to a spirit in Britain to determine how strong it was for tax purposes.Over the centuries various crude methods have been used, the most common being to mix the liquid with gunpowder and to try and light it.

In theory the strength at which gunpowder could be ignited – when it was ‘proved'– would be 100 proof; anything above it was over proof and anything under was under proof.This was of course a load of old tosh – arbitrary at best and purely inaccurate at worst,because external temperature, atmospheric pressure conditions and the amount of spirit added all affect the outcome.

Over time the science became more sophisticated, and improved chemistry tests played their part.

Ethyl alcohol is lighter than water, and in 1818 a measure of pure alcohol was defined as weighing twelve thirteenths the weight of the same measure of pure distilled water when measured under the same pressure at a temperature of 51 fahrenheit – or a ratio of 0.9238 alco...

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