Column stills behave like a series of single pot stills, formed in a long vertical tube. The tube is filled with either porous packing or bubble plates. The rising vapor, which is low in alcohol, starts to condense in the cooler, higher level of the column. The temperature of each successively higher stage is slightly lower than the previous stage, so the vapor in equilibrium with the liquid at each stage is progressively more enriched with alcohol. Whereas a single pot still charged with wine might yield a vapor enriched to 40-50% alcohol, a column still can achieve a vapor alcohol content of 96%. A continuous still can, as its name suggests, sustain a constant process of distillation. This, along with the higher concentration of alcohol in the final distillate, is its main advantage over a pot still, which can only work in batches. Continuous stills are charged with pre-heated feed liquor at some point in the column. Heat (usually in the form of steam) is supplied to the base of the column. Stripped (alcohol-free) liquid is drawn off at the base, while almost pure alcohol is condensed after migrating to the top of the column. Column stills are frequently used in the production of grain whisky.