The tank method — also called the Charmat or autoclave method after inventor Eugène Charmat — has the second fermentation take place in a large stainless-steel pressure tank rather than in individual bottles. The base wine and liqueur de tirage are pumped into the tank, which is sealed. Fermentation produces carbon dioxide that pressurises the tank. Once sufficient pressure is reached, the lees are filtered out and the wine is chilled, dosed and bottled under pressurised conditions.
The main advantage is economy and speed: the process can be completed in weeks rather than years, and creates a lighter, fruitier style that preserves the grape's primary aromas. The method suits aromatic varieties such as Glera (Prosecco) or Moscato (Asti Spumante). The bubbles are often slightly larger and softer than in the traditional method.