Anatomy of a bourbon barrel

 Inner side is burned/charred

Brewers have the following barrel choices for barrel aging:

>>> Bourbon barrels: For a long while there, American bourbon barrels were the hottest ticket for barrel aging (but as craft brewers are wont to do, they've moved on to explore other barrel possibilities). One reason was their easy availability. Another reason is their intense flavor characteristics.

By law, bourbon must be aged for two years in new American oak barrels, and the barrels can be used only once, which means bourbon distillers must rid themselves of thousands of almost-new casks every year. Soaked with the potent flavor of bourbon, these barrels have new roles to play in the aging of rum, tequila, sherry, and now beer.

Bourbon barrels are charred on the inside, according to the distiller's specifi- cations. They can be lightly charred or heavily charred. This char, along with the oak character, can permeate the beer, creating an incredible blend of vanilla, caramel, toffee, toast, or smoke aromas and flavors.

>>> Wine and sherry barrels: Wine and sherry barrels (and to a lesser extent, cognac barrels) are also used with great success for beer. Chardonnay wine barrels, for instance, which aren't charred, infuse the beer with warm oak toastiness and coconut and vanilla aromas and flavors.