The unfinished kylix in the collection of the Metropolitan Museum may shed light on this problem (fig. [47]). It is not so fragmentary as the pieces in the other museums, being complete except for portions of the rim. The foot is very roughly turned (fig. [48]), very different from the average kylix foot, as if it had not been worth while to spend much time on this product. The decoration itself is also quite cursory. This suggests that the piece was merely a “test,” such as potters use often nowadays for making trials of their clay body, or their glaze, or their kiln. The kylix is, as a matter of fact, too soft fired, and the glaze has turned reddish in parts. May we be permitted the guess that this was a trial to test out a new kiln? It is only a possibility and there are many others. The important point is that the evidence of the unfinished fragments does not make it necessary to assume more than one glaze firing.
The probability, therefore, is that Athenian pottery is once fired,[25] all ornamentation—both glaze and accessory colors[26]—being applied while the vase was in leather-hard condition; for in the case of the accessory colors also there would have been no advantage in an additional firing.
Injuries in the firing.
The action of the fire on the potter’s products was apparently as much an open question in Greek times as it is now. Practical experience must have gone a long way then as today; but full control could not be achieved. In forming an estimate of what proportion of the pottery was spoiled in the kiln we must remember that in our museums we are apt to encounter the survival of the fittest—what the potter considered worth preserving, what the Greek client deemed adequate to his need, and what the modern museum curator considers good enough for exhibition. But even in this selection we meet with a number of kiln mishaps, which apparently were so common that they were hardly noticed. When our eyes have become trained to observe such things, we shall note that in any collection of Greek vases there are many cases of warping and sagging, especially in the overhanging lips of the hydriai and amphorai.[27] There are many cracks and dents,[28] many faults in the glaze. A very conspicuous fault is the change of the clay from a pink to a grayish color.[29] Archaeologists often explain this as due to over-firing.[30] The real reason is not that the temperature has been too high, but that the clay has been subjected in the kiln or in the funeral pyre, to fumes the carbon of which has been absorbed by the clay. In other words, there was either reduction and the red ferric oxide in the clay has been changed to black ferrous oxide (cf. [pp. 30 f.]), or the clay has absorbed the black carbon physically. When controlled, this change is very useful to the potter for obtaining certain effects. Thus bucchero pottery is simply red clay fired under completely reducing conditions; and in the Vasiliki mottled ware some carbonaceous pigment like tar was probably placed on the spots which were intended to be black, whereupon the carbon would be absorbed by the clay and the iron reduced.[31]
The commonest injury to the glaze in the fire is its change into a brilliant red instead of the intended black. This can be observed on many vases, sometimes as a large spot (cf. fig. [49]), other times as a less clearly defined variegation.[32] The cause was irregularity of fire, a jet of air passing through the kiln coming in contact with parts of the vases. In other words, there was an excess of oxygen (or the reverse of reduction) which turned the black ferrous oxide of the glaze into red ferric oxide.[33]
Fig. 49. Black-glazed amphora with large red spot on one side
Met. Mus. Acc. No. G.R.607
Such red spots caused by jets of air coming in contact with the vases must not be confused with the very similar red spots which are due to the wearing off of the black glaze and the exposure underneath it of the ochre-tinted clay (cf. [p. 58]). Examination with a magnifying glass will show the difference: in one case the red is part of and level with the black glaze, in the other it is on a layer beneath the black glaze; in the former case the red will not come off when rubbed, in the latter it will.