BAG No, not a container for taking home the goodies from the January Sales! But, part of a bottle oven. A small firebrick chimney found directly above the firemouth on the inside of a potter's biscuit of glost bottle oven. Separates the combustion space from the stacked ware space. Prevents hots spots among the pots nearest the fire. The photo shows three bags. In a 'usual' large bottle oven there would be eight bags.