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Kilns and
Firing


The Bickley Project
Since 1981 the Bickley Ceramics Project has been exploring the ceramic technology of simple wood-burning updraught kilns in order to interpret the remains found by archaeologists. Ten updraught kilns have been built and more than 40 firings have been undertaken. The intention of the project is to recover the technology of low-temperature firing, characteristic of medieval ware, by the reconstruction and firing of traditional medieval and post-medieval kilns. For more information e-mail to:
oliver.kent@virgin.net
; or
dpdawson@somerset.gov.uk


kiln sites
Excavations of buildings on kiln sites have done little to disclose the functions of the few structures found. Nevertheless it is quite clear that pottery cannot be made by people working outside in the rain, pots cannot be stored where they will become wet and that kilns left  unprotected from the weather will soon disintergrate. In the British climate protection from the weather in terms of covered workspace for clay preparation, throwing, glazing, decorating, storage, drying and even firing is essential if any pottery is to be produced. . Medieval pottery work sites must have been extensively built-up and well-roofed (as, perhaps, the 14thC glassblowing site, left).

frost
Freezing weather is a serious matter, making clay unusable and putting at risk any damp pots in storage. A wet, freshly thrown jug is likely to be damaged when the temperature drops below freezing. Frost cracks open-up at angles in the sides of the pot and expansion forces out freezing water; with a rise in temperature the cracks close-up and disappear, only to re-appear after firing. Pots damaged by frost can sometimes, although rarely, be seen in museum collections. The real difficulty of making pottery on any scale in winter weather has led to the suggestion that potting must have been a seasonal activity. This is perhaps so, given the difficulty and expense of providing adequate heating, and the hard task of preparing clay in frozen ground. Yet the sheer amount of pottery produced by at least the major potteries suggests year-round production wherever possible.

Drying
How British medieval potters coped successfully with drying pottery is uncertain. Excavations at Limpsfield suggest that drying was accomplished there by placing pottery in a hot chamber adjacent to the kiln (Moorhouse 1981). An arrangement of the Limpsfield type seems ideally suited to a fast turnover, with the preheated pottery immediately replacing a fired load as soon as it has cooled sufficiently to clear the kiln. This will also yield a significant saving in fuel, a point underlining the importance of never letting the kiln cool down, but keeping it in continuous use. A high proportion of the fuel supply  can be used just to heat up the bulk of a kiln from cold. In the absence of any artificial drying arrangement the kiln itself must be used for drying, thus extending firing times. Ordinary shelf drying will work, eventually, although it is slow and entirely dependent on the weather.

kilns
medieval kiln construction: some problem areas

The typology of British medieval kilns is given in Musty (see below). The most typical 13th-14th century kiln type is probably Musty type 2a or 2b, an updraught, wood-firing kiln with opposed flues, probably open-topped. This will be assumed in the following discussions.

The ware chamber. There are two main types: (I.) ware chambers with flues or firing ports opening directly to a level floor on which pottery is stacked (best example, Laverstock kiln 6); and (II.) ware chambers with a central pedestal surrounded by channels connecting the opposed flues (best example, Kingston kiln 1.). The more efficient design of a chamber with a raised floor vented from below is a post-medieval innovation which, curiously, repeats a well-known Roman kiln type.

The most favourable interpretation of a pedestal chamber(II.) would connect the pedestal to the kiln walls with firebars, or special tiles, to extend the pedestal platform the full width of the kiln chamber. This would accommodate the maximum number of pots, and the flame channel would promote even heating by connecting both flues while directing flame along the walls and into the stack. If firebars were not used, then kiln props, or even pottery, could be placed in the kiln channels to support the ware placed above. The scarcity of kiln furniture in excavations may be explained by assuming that  re-usable pieces are valuable and normally salvaged when a kiln is backfilled or abandoned. Excavated kiln props and firebars may also be misdescribed. As an alternative, vessels to be fired might be stacked on the pedestal alone. With an overhanging pedestal the gap between walls and stack can be no more than two or three inches, and the stack might incorporate props and tile shelving. Both these interpretations have been tested experimentally and work reasonably well. There can be great variation in the ways in which individual potters pack a kiln and different 'traditions'of setting a stack, so it should not be assumed  that two kilns sharing the same chamber design were packed in the same way. 

The level floor chamber design (I.) at first seems less satisfactory, yet it frequently occurs in excavation. A single test firing was made in a one-metre diameter chamber of this kind with inverted vessels placed directly on the kiln floor exposed to flame from two wood/propane flame ports. Vessels directly exposed to the flame ports were over-fired, slightly distorted and would have been 'seconds'; however this could have been corrected by using tiles or prop baffles to break the flames - as apparently done in some medieval kilns. The kiln was densely packed and the firing was successful although glaze control could have been better. Like most simple updraught kilns the firing was uneven with about a cone temperature difference between the bottom and the top of the stack.

Kiln packing. A closely-packed, dense stack is generally to be sought, and this requires a high degree of uniformity in throwing pots of, especially, the same height. Shapes may be adjusted for close stacking, as in most Cheam pottery. The basic complication in good stacking comes from glazing: glazes run from one pot to another fusing them together, and glazed pots touching each other will stick together. There are several solutions. First,  apply the basic principle of separating vessels so they have minimum contact with each other. This can be done, almost automatically, by using basal thumbing  or recessed bases; If the pots are stacked together upside down, rim-to-base, often overlapping, minimum contact will be assured and runny glaze will pool harmlessly on a base instead of joining the vessels. Secondly,  make extensive use of 'spacers' or sherds to separate and level-up pots. And thirdly, limit the glazed areas on the vessels themselves, touching them with just a glaze 'bib' or keeping glaze well away from top and bottom rims. Pots with a minimum of glaze usually can be stacked together without too many 'joining-up' accidents.(see basal thumbing)

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Frost cracks in a Beverley-type jug

Cheam-type drinking
jugs illustrating an
interlocking stack