The secondary fermentation proceeds slowly and the carbonic acid formed is allowed to escape by way of the bung-hole, which in order to prevent undue access of air is kept lightly covered or is fitted with a water seal, which permits gas to pass out of the cask, but prevents any return flow of air.
During this secondary fermentation the wine gradually throws down a deposit which forms a coherent crust, known as argol or lees.
As regards the latter, indeed, it is nowadays held that it is at its best within a very short period of the vintage, and that when the characteristic slight " prickling " taste due to carbonic acid derived from the secondary fermentation has disappeared, the wine has lost its attraction for the modern palate.
The secondary fermentation proper is generally finished at the end of about six weeks to two months, and the first.
Much of the characteristic flavour of Madeira is due to this practice, which hastens the mellowing of the wine and also tends to check secondary fermentation inasmuch as it is, in effect, a mild kind of pasteurization.
