I'll do the honors.
Back in the days before prohibition when bourbon was starting to take off as a
nationally marketed product there were many people passing off sub-par
products as straight kentucky bourbon. Bourbon producers got the government
to step in and begin overseeing the production of bourbon. So, if a bourbon
distiller wanted their product to be stamped with approval by the government,
they had to follow a set of guidelines for the government to "bond" it. To be
"bottled in bond" a bourbon has to follow the requirements for being a bourbon
as well as being bottled at 100 proof and aged at least 4 years. The "bond"
came from the label the government put on each bottle. Also, the standard way
to check if whiskey could be bottled in bond, before specific gravity testers,
was to soak some gunpowder in it. When you lit it, if they whiskey burnt off
then the gunpowder burned, it was at least 100 proof. If the gunpowder did
not burn, the whiskey was impure, i.e. less than 100 proof.
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