FIG. 212. ROMAN KILN FOUND AT HEDDERNHEIM, GERMANY.
In 1677 four Roman kilns were discovered in digging under St. Paul’s Cathedral for the foundation of Sir C. Wren’s building, at a depth of 26 feet. They were made of loam, which had been converted into brick by the action of the fires, and were full of coarse pots and dishes; they measured 5 feet each way. A drawing made at the time is preserved among the Sloane MSS. in the British Museum.[[3131]] In the kilns was found pottery of the kind typical of London and the neighbourhood. In 1898 two kilns, one of large size, with pottery bearing the name CASTVS FECIT, were found near Radlett in Herts,[[3132]] and another was excavated in 1895 by Mr. C. H. Read at Shoeburyness.[[3133]] In Norfolk a kiln of somewhat curious form was found in the Roman settlement of Caistor by Norwich; the shape is that of a shallow concave depression with partitions, and it contained vases placed ready for baking.[[3134]] Another found between Buxton and Brampton was recorded by Sir Thomas Browne,[[3135]] and a third at Weybourne.[[3136]] In the South of England kilns have been found in the New Forest, where there was a manufacture of local pottery[[3137]]; in Alice Holt Forest near Petersfield, Hants; at Shepton Mallet in Somerset; and a potter’s workshop at Milton Abbas, Dorset.[[3138]] The British Museum contains a model of a kiln unearthed at Worcester about forty years ago, on the site of the modern porcelain works. Finally, discoveries of kilns and pottery were made in 1819 at Colchester, and again in 1878, when five kilns, all of different forms, with local pottery, came to light.[[3139]]
To describe all these different types of furnaces in detail would of course be impossible, but much may be learnt from the very full, though now somewhat antiquated, descriptions of the Castor kilns given by Artis.[[3140]] It will be found more satisfactory to describe the generally-prevailing arrangements, noting the more important variations where they occur. It may further be laid down that the system was practically the same for terracotta figures and tiles as for pottery, and that in many cases both were made in the same furnace. But this was not invariably the case, and at Rheinzabern, for instance, the kilns for tiles were quadrangular, those for pottery circular.
The kilns were constructed partly of burnt, partly of unburnt brick, the interior, floor, and outside of the roofs being covered with a strong layer of cement. They consisted of two main portions, the fire-chamber with its adjuncts, and the vaulted chamber above, in which the objects to be baked are placed. The fire-chamber was usually circular, with a projection in front, the praefurnium[[3141]] which had either a vaulted roof, as at Castor and Heiligenberg, or a gabled roof formed of pairs of tiles, as at Rheinzabern. Through this the fuel was introduced, consisting chiefly, as charcoal remains show, of pine-wood. The fire-chamber was either divided up, as at Castor, by walls radiating from a central pillar which supported the roof, or by rows of pillars in a line with the entrance, as at Rheinzabern and Heiligenberg. Holes were bored in the roof to allow the heat to penetrate through, but the arrangement varies; at Heiligenberg each division of the furnace was vaulted, making grooves along which the holes were bored. The oven where the pots were placed has been destroyed in most cases, but we know that it consisted of a floor, a wall with entrances, and a vaulted dome. The pots were ranged partly on the floor, partly on terracotta stands over the holes, as at Rheinzabern and Heiligenberg[[3142]]; at Lezoux there are remains of holes in the walls for iron bars to support them. Special arrangements seem to have been made for baking the finer wares, in order to ensure the proper spread of heat, and to guard against their being blackened or otherwise injured. In the Romano-British Room of the British Museum is a lump of bowls of red ware from Lezoux, fused together in the baking and cast aside.[[3143]]
One of the kilns at Castor (Fig. [213]) is described by Artis as a circular hole 3 to 4 feet deep and 4 feet in diameter, walled round to a height of 2 feet; the praefurnium was about a foot in length. In the centre of the circular hole was an oval pedestal (with one end pointing to the furnace-mouth), on which and on the side wall the floor was supported, being formed of perforated angular bricks meeting in the centre. The vaulted dome was composed of bricks moulded for the purpose,[[3144]] and the sides of the kiln of curved bricks set edgeways in a thick slip of the same material. Brongniart[[3145]] compares the Castor kiln with that at Heiligenberg, near Strasburg, and others in the Rhine valley in which “Samian” ware was made.
FIG. 213. KILN FOUND AT CASTOR, NORTHANTS.
Another kiln found in 1844 Artis describes as having been “used for firing the common blue or slate-coloured pottery, and had been built on part of the site of one of the same kind, and within a yard and a half of one that had been constructed for firing pottery of a different description. The older exhausted kiln ... presented the appearance of very early work; the bricks had evidently been modelled with the hand, and not moulded, and the workmanship was altogether inferior to that of the others, which were also in a very mutilated state; but the character of the work, the bricks, the mouths of the furnaces, and the oval pedestals which supported the floors of the kilns, were still apparent.”
Artis was also of opinion that “the blue and slate-coloured vessels found here in such abundance were coloured by suffocating the fire of the kiln, at a time when its contents had acquired a degree of heat sufficient to ensure uniformity of colour.” Hence he denominated kilns in which this ware was baked, “smother kilns.” He further notes that the bricks of this kiln “were made of clay mixed with rye in the chaff, which being consumed by the fire [i.e. in the baking of the bricks] left cavities in the room of the grains, which might have been intended to modify expansion and contraction, as well as to assist the gradual distribution of the colouring vapour. The mouth of the furnace and top of the kiln were no doubt stopped; thus every part of the kiln was penetrated with the colouring exhalation.” From experiments made on the local clays he proved to his own satisfaction that the colour could not have been produced by any metallic oxide, inherent or applied from without; and this view was supported by the appearance of the clay wrappers of the dome of the kiln. But in view of recent researches, such as those of Blümner, it is doubtful whether Artis’ theories can now be upheld. As Mr. Haverfield has pointed out,[[3146]] the dark colour may be due to the chemical action of the carbonaceous vapour of the smothered kiln rather than to any “colouring exhalation.”
The process of packing the kiln in order to secure uniform heat in firing is thus described by the same writer: “The kilns were first carefully loose-packed with the articles to be fired, up to the height of the side walls. The circumference of the bulk was then gradually diminished, and finished in the shape of a dome. As this arrangement progressed, an attendant seems to have followed the packer, and thinly covered a layer of pots with coarse hay or grass. He then took some thin clay, the size of his hand, and laid it flat on the grass upon the vessels; he then placed more grass on the edge of the clay just laid on, and then more clay, and so on until he had completed the circle. By this time the packer would have raised another tier of pots, the plasterer following as before, hanging the grass over the top edge of the last layer of plaster, until he had reached the top, in which a small aperture was left, and the clay nipt round the edge; another coating would be laid on as before described. Gravel or loam was then thrown up against the side wall where the clay wrappers were commenced, probably to secure the bricks and the clay coating. In consequence of the care taken to place grass between the edges of the wrappers, they could be unpacked in the same-sized pieces as when laid on in a plastic state, and thus the danger in breaking the coat to obtain the contents of the kiln could be obviated.”