American Oak Casks or White Oak
For the most part, bourbon is aged in American oak casks, though at times French Oak or other oak may be used.
There are 10 different types of American Oak trees each with an unpronounceable name some are better than others and then there is the best. The one called Alba is considered the best.
The question you must be asking yourself now is why, well the holding structure of oak for one. This is because upon tree death and formation of the heartwood there forms tyloses a bubble-like structure in the wood which prevents leakage by the state ends or top and bottom of the barrel.
The number of bubbles found in oak makes it uniquely suited for storing.
Another feature found in white oak is the multiseriate medullary rays, these are the tree arteries used to transfer water to the top of trees they can’t leak water. There is so much overlapping in the tree that it makes the barrels waterproof. They are so dense that if liquid trying to exit the barrel would try to beat a straight line it would have to cut through 5 layers.
Another important factor is the flexibility of the wood most wood after seasoning, seasoning is leaving the barrels out in the open for 36 to 48 months most wood becomes brittle oak does not. Thus enabling the cooperage to work with the wood to create barrels.
 Singleton V.L. 1974, Some aspects of the wooden containers as a factor in wine maturation. Chemistry of winemaking
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