Over proof has its origins in seafaring. When sailors were still being paid with rum, they had to check that the rum was not "too watered down". To do this, the "gunner" soaked his black powder in the rum and lit this mixture. If it did not burn, the rum was too weak and thus not accepted as payment. If it did burn, it was tested and accepted and "over proof" at over 50% by volume.
High Spirits Distribution by Langatun
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