Barrels have been constructed the traditional way in Europe for centuries, and the method involves splitting the oak along the grain of the wood into strips, known as staves. The next step in the process of manufacturing oak barrels is the removal, or leaching of the harshest tannins from the wood. Once the staves have been removed, the tannins appear as black or gray residue left on the ground. This is achieved by allowing the split wood to dry outside, a process that can take between 10 months and three years. The wine that is stored in these oak barrels can be softer if the wood is allowed to season for longer, although it can also increase the cost of the barrels. It is a lot faster to dry the wood in a kiln than to leave it outside for as long as three years, and some cooperages, or barrel making facilities in the US adopt this method because it is faster. However, outdoor seasoning tends to soften the tannins a lot more.
The next step in the creation of oak barrels is to heat the staves over an open fire and then bend them into shape once they are pliable. Iron rings are then used to hold them together. The staves can be heated by steam instead of fire, although the wine then tends to be less complex and toasty. A cooper can typically make one oak barrel in a day using this traditional method.
The wood on the inside of oak barrels is also charred, or toasted to varying degrees, and a heavy toasting means that the wine has an added dimension; Burgundy wine typically uses these more heavily charred barrels. Wine in a barrel that has been heavily toasted can have a reduced coloring because of the increased amount of carbon, although it can reduce the amount of coconut note lactones. Barrels that have been more lightly toasted retains more of the tannins and oak flavoring. Wine that has a roasted taste to it tastes that way because of the furanic aldehydes in the wood becoming more concentrated; higher toasting also creates spicy and smoky notes in some wines as the toasting enhances the presence of the phenol eugonol and vanilla.