Fig. 41. Open kiln showing saggers
Packing the kiln.
Fig. 42. Muffle kiln with glazed ware
In packing the kiln the ware is stacked as closely as possible for economical reasons, so that as much as possible shall be accomplished in one firing. In biscuit firing the ware can be placed so that it touches (fig. [40]); in glaze firing the pieces must be separated (fig. [42]) both from one another and from the bottom of the saggers or shelves on which they stand, since the melted glaze is apt to run. Glazed pieces are therefore generally placed on stilts made of burnt clay, and the marks of these stilts will often show on the bottoms of the vases.
That the chief features of modern and Athenian kilns were similar is clear from an examination of the ancient representations of kilns (cf. figs. [72-81] and [pp. 76 ff.]). How closely the ware was sometimes stacked is clearly seen in fig. [80]. The fuel used by the Greeks was probably wood and charcoal.
Almost all modern pottery is twice fired; once for the conversion of the clay into terracotta or biscuit, and the second time for the glaze. To glaze unbiscuited ware is a delicate business, and the risk of glazing a piece of raw clay is considered larger than the trouble of burning it a second time. It is done occasionally when very tough clay is used, for instance, in kitchen crooks and in stoneware; in that case it is best to apply the glaze when the clay is in leather-hard condition, for then the absorption is less. More than the two regular firings are often used for correcting mistakes in glazing, for additional coats of glaze, and for decorating the ware.