The Proof is in the Poof

Today’s topic looks into the concept of proof.

While spirits in the United States are labeled as Alcohol by Volume (ABV), the notion of proof, which is defined as twice the ABV (40% ABV is 80 proof), is deeply ingrained in the industry. Our taxes are paid based on “Proof Gallons” and many prefer to refer to their spirits by proof rather than ABV. By where does this term come from and why is 100 proof equal to 50% ABV?

I’ve heard many theories over the years, typically involving fire, so we found it a good opportunity to play with matches.

The Blue Flame

In younger and more foolhardy days I was told that 100 proof (50% ABV) is the minimum alcohol strength that you can set booze on fire. So we started by putting the flaming shot theory to the test.

We slowly added water to a high proof spirit, measuring the proof as we diluted it. At first the spirit eagerly ignited and provided a nice blue flame. As we added water the rapidity of the ignition reduced. At approximately 115 proof the spirit proved unwilling to ignite.

Our initial experiment was performed on a late afternoon of a nice fall day that at 68 F. We tried heating the spirit and at 90 F to simulate a warm summer afternoon and found the spirit would ignite at only 70 proof. When we cooled the spirit to 61 F we needed to go up to 123 proof to obtain the blue flame.

While 100 proof is the threshold for flammability at a very specific temperature just a a couple degrees change in temperature will dramatically change the minimum ABV required for lighting a spirit. It seems implausible that a simple test of flammability was ever used as a proof of a spirits strength.

The Gunpowder Plot

Another common story claims that gunpowder can still ignite when dampened with spirits 100 proof or higher. Sailors were paid in rum and to prove that their rum was not watered down, they would test their allotment with some gunpowder. Of course, this sounded like a chance for even more excitement so we gave it a go.

We started by igniting a small amount of black powder to understand how it normally reacts. You can see the results above on left. We then put a drop of spirit onto the black powder and let it soak in.

We may not have the best experimental technique, but I have to imagine 17th century sailors were not the most fastidious even when it came to rum ration testing. We found that there was a wide range of spirit proof for which we could get ignition of the damp powder. Above right we show an energetic burn at 125 proof. This burn would slowly lose energy as we used lower proof spirit. We found the range of partial ignition to be 105-125 proof and could not ignite as we got below 105 proof. Our experiments were done at about 70 F.

A Bit of History & Science

Now that we are done playing with fire, it’s time to explore a bit of history and science:

Historically, a simple flame test was never used as “proof” of the strength of a spirit because of the variability of temperature on flammability. The temperature at which the vapors over a liquid will ignite is called the Flash Point. At 62 F booze would need to be 192 proof before it would catch fire. 100 proof spirits have a flash point of 75 F and by the time you get to 97 F on a hot Caribbean day even a heavily diluted 40 proof spirit can ignite.

The gunpowder test was in fact used as an official test of the strength of alcohol for tax and trade in the 16th and 17th century before being replaced by density measurements during the 18th century. However the traditional British definition of 100 proof was 57.15% ABV. It was only in 1980 that England adopted the modern notion of 100 proof being 50% ABV. This means that if the traditional gunpowder test should show a flare at 114 proof when we use modern definitions. Surprisingly this matches our crude experimental result.

The reason the gunpowder test works is not based on the flammability of the spirit, but rather the solubility of Potassium nitrate which is oxidizer in gunpowder. Potassium nitrate readily dissolves in water which renders wet gunpowder useless. However, Potassium nitrate is 200 times less soluble in ethanol than it is in water. So as the ABV of the spirit increases, less Potassium nitrate will be dissolved into the booze and at sufficiently high ABV the dampened gunpowder will still ignite and give proof of the strength of the spirit.