The third stage is fermentation. The "must" is carried from the press into open tanks or vats which hold from 500 to 5000 gallons or even more. The yeast cells which cause fermentation may be introduced naturally on the skins of the grapes; or in many modern wineries the "must" is sterilized to rid it of undesirable micro-organisms and a "starter" of "wine-yeast" is added to start the fermentation. Yeast organisms attack the sugar and must, breaking it up into alcohol and carbonic acid gas, the latter passing off as it is formed. When active fermentation ceases, the new wine is drawn from the pomace and is put into closed casks or tanks where it undergoes a secondary fermentation, much sediment settling at the bottom of the cask. To rid the new wine of this sediment, it must be drawn off into clean casks, an operation called "racking." The first racking usually takes place within a month or six weeks. A second racking is necessary at the end of the winter and a third is desirable in the summer or fall. The fourth stage is the aging of the wine. Before aging begins, however, the wine usually must be rendered perfectly clear and bright by "fining." The materials used in fining are isinglass, white of egg or gelatine. These, introduced into the wine, cause undissolved matters to precipitate. The wine is now ready for bottling or consumption. Most wines acquire a more desirable flavor through "aging," a slow oxidation in the bottles. _Champagne._ When champagne wines have gone through their first fermentation, they are racked off into casks to age until their quality can be ascertained, after which a blend of several different wines is made. This blend is called the "cuvee." The cuvee is bottled and a second fermentation starts. The bottles are now put in cool cellars, corded in horizontal layers with thin strips of wood between each layer of bottles. The champagne in this stage is said to be in "tirage." The carbonic acid gas generated at this second fermentation is confined in the bottles and absorbed by the wine. When the bottle is uncorked, the gas, seeking to escape, produces the sparkling effect desirable in sparkling wines. After the wine has been in tirage for one or two years, the bottles are placed in A-shaped racks, the neck of the bottle pointing downward so that the sediment formed during fermentation drops to the cork. To further the settling of the sediment, workmen turn or shake each bottle daily for a period of one to three months. The bottles are then taken to the finishing room, cork down and the wine is "disgorged." Disgorging is accomplished by freezing a small quantity of wine in the neck of the bottle containing the sediment, after which the cork is removed and with it the frozen sediment. The bottle is refilled, recorked, wired, capped, and the champagne is ready for shipment. _The vintage._