FIRING THE VASES
Production of Temperature.
In the fire the great miracle takes place and the dry clay, most friable and perishable of materials, becomes one of the most durable. This is accomplished by the softening of the feldspar grains which cement the whole together and thus form a dense mass. To produce the temperature at which this phenomenon takes place two things are necessary, fuel and draught, the former supplying the carbon, the latter the oxygen. The liberation of the carbon in the fuel and its union with the oxygen of the air develop combustion, during which heat is generated. Combustion can be complete or incomplete. It is complete when there is an excess of air and the carbon can combine with two molecules of oxygen to form carbon dioxide (CO₂). This condition is called oxidation. It is incomplete when there is not enough air and the carbon can get only one molecule of oxygen, forming carbon monoxide (CO). This condition is called reduction. Carbon monoxide, being very hungry for oxygen, will try to extract it from whatever source it can. If ferric oxide (Fe₂O₃) is present in the clay—which is the case in red clay—the carbon monoxide will take one molecule of oxygen from it and convert it into ferrous oxide (CO + Fe₂O₃ = CO₂ + 2FeO). The important feature in this process is that ferric oxide is red and will make the clay burn red; but ferrous oxide (FeO) is black and will give the clay a blackish color. If no ferric oxide is present in the clay, that is, if the clay is not a red clay, then reduction has no effect on the color of the clay and can be freely used. In Europe potters regularly burn under reducing conditions, while in America the general practice is to burn under oxidizing conditions. When the draught in the kiln is faulty, partial reduction will often occur, and many faults in the burning, especially in the glazes, are attributable to this cause.
Fig. 39. Open kiln
We shall see presently how important is a knowledge of these chemical changes during the process of firing when we come to consider the defects on Greek vases caused by injuries in the firing (cf. [pp. 44 ff.]).
Types of kilns.
Fig. 40. Muffle kiln with biscuit ware
There are two chief types of kiln construction in use today: (1) the open kiln, in which the flame passes through the kiln chamber (fig. [39]); (2) the muffle kiln, in which the flame passes around the chamber and not through it (fig. [40]). In the open kiln the ware either comes in direct contact with the flame, or is stacked in saggers, i.e., boxes made of fire clay fitting one on top of the other (fig. [41]). The muffle kiln is, so to speak, one large sagger, and the ware is stacked on shelves. The draught in the kiln can be either an up draught or a down draught; in either case the air supply, as well as the fuel supply, must be under control, so that the combustion shall be as desired. The draught can be regulated by means of dampers, the fuel by attention to the burners. An arched top is an advantage, for it imparts greater strength—an important item considering the strain to which the kiln is subjected—and it facilitates the circulation of the heat. Coal, wood, gas, or kerosene oil can be used for fuel. Of these, oil and gas are now the most popular; coal and wood are rapidly coming into disuse on account of the greater labor they entail.
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.
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
Fig. 44. Design on red-figured krater
(a) Preliminary sketch (b) Completed painting
Furtwängler u. Reichhold, Griechische Vasenmalerei, I, pl. VII
Number of firings.
Was Athenian pottery once or twice fired?[14] That is, was it decorated in leather-hard or in biscuit condition? This has been one of the most debated questions in Greek ceramics. Archaeologists often assume offhand a number of firings,[15] but without stating any evidence or squarely facing the problems involved. Briefly, the arguments for and against are as follows. As is well known, a large proportion of red-figured vases of good period show a preliminary, colorless sketch for the design traced with a blunt[16] instrument directly on the clay (cf. figs. [43-44]). The smooth grooves of this sketch show beyond doubt that the sketch was made while the clay was in leather-hard condition, that is, before firing. If the vase had been fired, even at a low temperature, the sketch would have had to be scratched in with a sharp tool, and would have left a ragged, not a smooth line.[17] Now it is not a natural procedure for an artist to make a rough sketch for his design, and then to leave his vase to be fired before completing his work. Furthermore, an examination of the incised lines on the black-figured vases—which clearly go over the black glaze—shows also that these lines must have been made while the clay was still leather hard. The ragged edge of the glaze along the incisions has sometimes been thought to indicate that they were made after firing. But just this effect is produced by cutting through dry glaze on unfired clay; and it would have been very difficult to attain the required delicacy, swing, and smoothness by incision into hard, fired clay. Any one who will try the experiment will soon become convinced of this.[18] So that, for the black-figured period at least, this evidence points to a once-fired pottery.
On the other hand, it might be urged that if we assume that the decoration was executed in leather-hard condition, the vase painters whom we see depicted on Greek vases should be handling their pots with considerable care, and that this is hardly conveyed in the representations. On the Boston fragment, for instance, the painter is holding a kylix by its slender foot without any apparent fear of breaking it (fig. [67]). And whoever painted the scene knew what he was doing, for he was in the act of decorating such a kylix himself. However, if the clay used by the Athenians was of a tough variety,[19] this would, I have been told by potters, be a perfectly possible procedure; and experiments made with imported Athenian clay[20] bore this out to an astonishing degree. Vases made of this clay could be handled quite freely in leather-hard condition. So that if the Athenian potter of the fifth century used similar clay to that of his present-day descendant, his handling of these pots on the vase paintings would be perfectly justified in the leather-hard state.
There is, moreover, evidence which seems to settle this question beyond dispute. On a number of the Athenian vases there are dents such as can only have occurred while the vase was still in a leather-hard state. The mark of the object contact with which caused the dent is invariably over the black glaze (cf. fig. [45]), showing clearly that the glaze must have been applied in leather-hard condition.[21] In some cases we find still adhering in the dents a little burnt clay, apparently from another vase contact with which caused the accident. Here it is probable that the accident was caused not while the vase was leather hard, but when red hot in the kiln, at least in those instances, as in the black-figured amphora in the Metropolitan Museum[22] (fig. [46]), where the glaze shows a rough fracture due to the separation of the two pieces which had stuck together; for this fractured edge would have become fused and smooth upon subsequent firing.[23]
Fig. 45. Detail of hydria showing dent with mark over black glaze
Met. Mus. Acc. No. 17.230.15
The fragments of unfinished vases which have been found from time to time[24] have been used as evidence to prove several glaze fires; for they show fired vases at a definite stage in the glazing, the outlines and inner markings painted, but the background not yet filled in. Must we, then, suppose that it was the regular practice—at least in the later red-figured period to which all the unfinished pieces belong—to fire before and after the painting of the background? It is difficult to see what would be gained by the process. If the pottery were fired before any decoration was applied, there would be the advantage of safer handling of the ware; but to have an extra firing with the decoration more than half completed gives no apparent gain and there is the distinct disadvantage of the extra expense of firing.
Fig. 46. Detail of amphora showing dent with clay from other body still adhering
Met. Mus. Acc. No. G.R.530
Fig. 47. Unfinished kylix
Met. Mus. Acc. No. 11.212.9
Fig. 48. Foot of unfinished kylix
Met. Mus. Acc. No. 11.212.9
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.