Just for the record, I was right.
Someday, this will cease to surprise people. Since we have not yet reached that day, I will continue to remind people.
I was right.
When one talks about alcohol proof one is technically talking about the amount of ethanol (and by “ethanol” we [and by “we” I mean the scientific community] mean alcohol) in a beverage. In the US of A (yes, it’s different in different countries), alcohol proof is twice the percentage of alcohol by volume.
In other words, 100 proof whiskey contains 50% alcohol.
As we all agreed one cannot say “50% proof” unless one is talking about geometry. Which we most certainly were not.
So why the term “proof”? I’m glad you asked.
Turns out that in England back the 16th century, adult beverages were taxed at different rates depending on their alcohol content levels. More alky equals more taxy. Since the tax man (yes, they were all men back then) didn’t have advanced labs to carry around with them, they devised the gunpowder test.
Basically, they soaked a pellet of gunpowder in the liquid being tested. If it could still burn post-soak, it was considered above proof and therefore taxed at a higher rate. Hard to see how that would improve the taste of the alcohol.
In case you randomly find yourself on Jeopardy!, you should be aware that 57.15% alcohol was the level of proof because under that amount, soaked gunpowder wouldn’t burn. So BackInTheDayintheUK, 57.15% was 100 proof.
By the 18th century, there were more complicated tests that don’t interest me very much and didn’t really change the system.
Here in the US, the idea of calculating 57.15% (or even thinking about calculating 57.15%) gave people migraines, so when the proof system was established (1848 in case you were wondering) we skipped all the complex specific gravity stuff and went with 50% alcohol equals 100 proof.
These days, people don’t care about proof very much (unless you’re a college student in which case saying you’re drinking something 60 proof is much cooler than saying you’re drinking something 30% alcohol).