New oak is of course most heavily impregnated with flavouring compounds. As the cask is used, tannins and other substances are leached out by the wine, and tartrate crystals from the wine build up on the inside. Eventually the cask becomes intert: it is so coated with tartrate that it gives nothing to the wine. This is fine if you require a container, less of a help if you want the cask to contribute to the taste of the wine.
New wood breathes more than old: its pores are not clogged with solids from the wine, and thus the oxygen trickle through the wood is at its greatest. New oak contributes more tannin, but also more "softening" substances from the cellulose of the oak, and more of the characteristic vanilla flavour. Once used for a year, the oak contributes less tannin, but also allows less oxidation.
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