Something to consider when reading old bar guides. Not all proofs are created equal.
Proof as a measure of alcohol content comes from the British Navy. While sailing there are a number of factors that need to be managed on a long voyage. Among them the need for fresh water, vitamin C and Morale. Water kept in casks for a long time would spoil, growing algae or bacteria. Being at sea, fresh fruit was often hard to come by. And getting doused on rum was far easier than trying to keep the crew entertained in any other fashion.
At some point an enterprising seaman combined the three items together and struck upon a single solution to all three issues. The citric acid of lime juice and the alcohol content of the rum kept the water from growing anything. The addition of rum and lime made water an attractive proposition for sailors to stay hydrated and the lime juice kept everyone from getting scurvy. Once this caught on the number of limes being consumed by the Navy led to all brits being called Limeys.
The one downside to this proposition was greed. Like cheap bar owners since the dawn of time, someone wanted to save money on rum by watering the drinks. They were already watered anywhere from 2:1 to 6:1, but there is a point at which what you’re drinking isn’t really a cocktail anymore and the sailors would grumble. Eventually even the accusation that the Grog might be watered was enough to start the officers worrying.
A wily purser hit upon a method to assure the crew that they were getting the full cask strength. Gunpowder doused with rum will not ignite if there is less than 57% alcohol in the liquid. So at the start of every voyage the purser on british ships would take a sample from the rum casks and *Prove* to the crew that the rum was sufficiently strong.
So in the British Navy 100 proof was equal to 57.14%. Anyone familiar with American spirits might find this a bit odd. That is largely because American distillers use a different formula for proof simply doubling the ABV. So in the UK 100% alcohol was 175 proof and in the US it was 200 proof.
A lot of this is moot since the US hasn’t had a Grog Ration since 1862 , and the UK hasn’t had a Grog Ration since the 1970’s. In addition the UK switched from the proof measurement to ABV in the 1980’s making the distinction about 30+ years out of date.
This all becomes important when you’re reading prohibition era or post prohibition bar guides. A lot of those guides were written in countries other than the US because cocktail culture in Britain didn’t dry up in 1920. As a quick look at the Vesper will tell you if Bond is calling for a 100 proof vodka in his drink is that a 50% ABV US measure vodka or a russian import using the UK scale at 57% ABV?
Is that vintage bottle of gordon’s gin the original 84 proof at 48% or the modern reissue 80 proof at 40% ABV?