Firing.
Different wares and different glazes require different temperatures. Thus porcelain and stoneware are fired to much higher temperatures than ordinary pottery, and salt and alkaline glazes need a higher fire to mature than the ordinary lead glazes. When the required temperature has been reached—which nowadays is determined either by means of a pyrometer or with the help of pyrometric cones which melt at a given temperature and which are watched through a spy-hole (cf. fig. [40] where the cones are set up opposite the spy-hole in the door, and fig. [42] where the cones have melted)—the fire is gradually extinguished and the kiln left to cool slowly. Twelve hours for the firing and twelve for cooling is a rough estimate for the firing of an ordinary kiln. It is important not to hurry the process of cooling, as a too rapidly cooling fire may crack the ware or affect the glaze injuriously.
The Greeks fired their pottery at a considerably lower temperature than potters do today. It seems to have been about 960° centigrade (corresponding to about cone 010) since any increase over this temperature causes a change in the color of the clay together with an additional contraction. Mr. Tonks has made the ingenious suggestion that, 950° and 1065° being the melting points of silver and gold respectively, the Greeks may have used these metals in the same way as the modern potters use cones, to regulate the heat of the kiln.[13]
When the kiln is finally opened comes the exciting moment of seeing what the fire has done with one’s products. In taking out the contents of the kiln, gloves and sticks are often useful for handling ware that is still too hot to touch. Invariably there will be surprises—what one has expected to be a great success often turns out a failure, and what one thought little of may become a rare thing of beauty. In the biscuit firing the adventures of the pot are comparatively few; it may crack or sag or warp, but as a rule the expected shape is maintained. But in the glaze firing so many elements enter in that even an experienced potter can never be sure of the result. The color may turn out a different shade from that desired; the glaze may unexpectedly be matt (dull) or too glossy; it may blister or peel or crack; it may be too thin or too thick. Such defects are almost invariably due to faulty composition of the clay or the glaze or to the conditions of firing. They can often be remedied by further glazings and firings; but quite often a pot on which much time and labor have been bestowed is hopelessly ruined. A good potter, however, will soon learn to bear such mishaps philosophically; and it is certainly true that one often learns much more from failures than from successes. Moreover, the element of uncertainty lends spice to the craft.
A careful consideration of the modern processes of firing pottery described above will help us to settle the problems connected with the firing of Greek vases—for the action of fire on clay remains the same even though the kilns in use by the Greeks were probably somewhat more primitive than now.
The chief problems which confront us in Athenian pottery are (1) the number of firings, (2) the interpretation of defects on Greek vases as injuries in the firing.
Fig. 43. Detail of amphora showing preliminary sketch
Met. Mus. Acc. No. 06.1021.114
