From the September 1968 magazine.

About Alcoholism - Alcoholism Information, Research and Treatment

Proof of the Pudding

Proofing spirits dates from the early days of distilling when the strength of liquor was measured by pouring some on a small pile of gunpowder and lighting it.

A bright blue flame meant that the liquor was too strong. A steady blue flame, which was found to come from spirits containing about 50% alcohol, indicated that the potion was about right.

It was described as "proved 100% perfect," an expression that eventually became 100 proof.

-- New York Sunday News