I’m currently studying for a spirits exam and during the next few weeks I’m going to be sharing some fun facts that I’ve discovered in the process.
You’ve probably noticed in my postings that I’ll often give the percent alcohol by volume (ABV) of the beverage in question. In the U.S., the “proof” measurement is used as well. The proof is twice the percentage of ABV at 60°F – i.e. 80 proof=40% ABV.
Alcohol content is easy for distillers to measure these days with hydrometers and other modern equipment, but back in the old days, gunpowder was the analytical tool of choice. Equal parts spirit and gunpowder were mixed and set alight. If the gunpowder didn’t burn or just sparked, the spirit was too weak and if it burned too brightly, the spirit was too strong. If the mixture burned evenly, and with a blue flame, it was considered to be “100% pure” or “100 proof”. Turns out that a blue flame will appear at a 50/50 ratio.
In Britain, the proof to ABV ratio is 4:7. In the 18th century payment to British sailors included a ration of rum. These sailors also used the gunpowder trick, making sure the liquid would ignite and that it hadn’t been watered down – these thirsty fellows didn’t want any spirits that were “under proof”. Rum that passed a sailor’s test was later found to contain 57.15% ABV, which is quite close to a 4:7 ratio of alcohol to total liquid. The definition then became (4÷7) × 175 = 100 degrees proof spirit. 100% alcohol had (7÷7) × 175 = 175 degrees proof spirit, while alcohol with 50% ABV had (3.5÷7) × 175 = 87.5 degrees proof spirit. To convert percentage of alcohol to degrees proof, multiply the percentage by 1.75 (and watch out for scurvy).