To measure the volume of a cask, it may be assumed that the interior is approximately a portion of a spheroidal figure.
The small twigs, tied in bundles, are boiled for some time in water with broken biscuit or roasted grain; the resulting decoction is then poured into a cask with molasses or maple sugar and a little yeast, and left to ferment.
After five to eight years it may become an amontillado, and if it is left in cask and allowed to develop, it will, after it attains an age of nine to fourteen years, become an oloroso, and still later it may become a secco.
The sulphurous acid evolved destroys such micro-organisms as may be in the cask, and in addition, as it reduces the supply of oxygen, renders the wine less prone to acidulous fermentation.
The wines which remain for a long period in cask gradually lose alcohol and water by evaporation, and therefore become in time extremely concentrated as regards the solid and relatively non-volatile matters contained in them.
As a rule, the supply considerably exceeds the demand, and the stock in hand at the present time amounts to roughly four years' consumption of finished wine, but to this must be added the stock existing in cask, which is considerable.
The proteid matter combines with a part of the tannin in the wine, forming an insoluble tannate, and this gradually subsides to the bottom of the cask, dragging with it the mechanically suspended matters which are the main cause of the wine's turbidity.
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.