We recommend our large glass carboys as something to age or oak your spirits. They can also be used as a secondary fermenter for fermentations that need to sit for awhile. Large glass carboys are great for secondary fermenting because they never scratch or break down over time, plus you get to watch all the action! However, we do not recommend it as a primary fermenter as the residue can be hard to clean through the narrow opening. These carboys are made of glass, and you can use a #7 stopper or a small universal carboy bung.