Coopers are also great cognac alchemists, transforming oak into real aromatic gold. First they air-dry their oak staves for several years, then plane and trim them to form the famous barrel staves. The staves are then assembled, bent, heated and hooped to form a cask.
Coopers select wood from different oak sources, sorts it according to porosity and thus its ability to oxidize the eaux-de-vie during aging, then toast it to a greater or lesser degree and for more or less long periods in order to develop new aromatic palettes for the cellar masters.
Coopers also keep track of barrels, repairing them when necessary and selling off used barrels. They may also extend their expertise from making standard barrels to very large capacity casks (foudres).
Conical trunk vats, often made to measure, can contain from 1,000 to 50,000 liters and are particularly used for blending cuts.