Wooden casks, when new, confer a definite, vanilla taste on the wine they contain. This is desirable in some but not all cases. In Bordeaux and Burgundy, the properties of new oak casks are exploited, for both red and (to a lesser extent) white wines. But in other equally classic wine regions — the port country, Jerez, Champagne — new wood is avoided. Wood also "breathes", allowing a little air in, and can augment the wine's own tannins, helping its ageing ability.
What does wood do to wine? There are three aspects to consider: the size and shape of the cask, its age, and the wood it is made from. The wood first: oak is the preferred timber because of its strength and relative lightness, its cellular structure and its taste components. The structure of oak wood varies according to its source: winemakers identify close- (or tight) and coarse- (or loose) grained sorts. This in turn depends upon the forest from which the tree comes, both its environment and the way it is managed. Slow-growing forests produce the tightest-grained wood. This affects not only its air intake: such oak also has more aromatic phenol compounds (caused by the trees growing more in spring than summer).
Phenols are among the many substances found in oak, which can affect wine: so far, more than 60 have been identified, including 18 different phenols of which vanillin is the most important. The tannins in oak, ellagic tannins, are not the same as those that wine gains from grape skins and stalks. They add astringency, and reinforce the wine's structure.
Even more complex than, but related to, wood structure is the varying pattern of substances in oaks from different forests. Limousin oak, from west-central France, is fast-growing, has an open structure and more tannin (but fewer phenols) than Allier oak from further east, which has less tannin but more aromatics such as eugenol and lactone.
Oak from the Baltic lands, from Slovenia, Croatia, North America — all are used, and each has its qualities. Baltic oak was the staple in Bordeaux for many years, despite the proximity of the French forests, because it was easy to transport the wood in the returning wine-ships. It is also relatively neutral: a quality prized, even in Bordeaux, until this century.
How the wood is treated is equally important in determining the character of the finished cask. It must be seasoned by leaving the whole trunk to dry naturally: kiln-drying is less good. The wood should be split, not sawn, into staves. Only the best wood from each trunk should be chosen. The staves in turn need to be matured.