In theory, alcohol burns sufficiently at a 50 percent content or 100 proof, though it can produce a weak flame with a lower proof. This number is derived from an early method used to proof alcohol.
Before technology simplified the proofing process, the percentage of alcohol in liquor was determined by mixing it with gunpowder and then setting a lighted match to it. A blue flame indicated that liquor was 50 percent alcohol, which was considered the ideal level. Proof is simply twice the percentage of alcohol by volume, so 50 percent alcohol is considered 100 proof. Lower percentages can produce a weaker, yellow-tinted flame.