An oven used to bake or fire ceramics. Early pottery was simply baked in trenches with brushwood. Proper kilns were developed later with the fire walled in to conserve the heat and using various forms and types of air circulation. The most popular design in Britain by the 18th century was the bottle oven or kiln, with its characteristic chimney, fired by coal. Unglazed pieces were stacked up in the kiln for the biscuit firing, but once the glaze was applied the wares needed to be separated to avoid the melting glaze sticking them together. During the glost firing, kiln furniture, such as stilts, spurs, and firing rings, was used to separate the glazed pieces or raise them off the shelves. A muffle kiln was used to fire enamel colours on to pieces, protecting them from direct flames.