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Open-topped kilns and loading. Loading and efficiently stacking a medieval kiln must have been exceptionally difficult unless the kiln structure was designed for top loading. Apart from the big late-medieval northern kilns having multiple flues and side-loading, and large multi-flued kilns using saggars, almost all excavated 12th to mid-14th century kilns were such that loading from an open top was the only practical and efficient way of placing pottery inside them. The speed and effectiveness with which an open-topped kiln can be stacked is probably its great attraction. Very warm kilns can be loaded - this cannot be done when loading via the flues or side entrance to an enclosed kiln - and the top closure of tiles, sherds or sealed clay is easily applied. So-called 'clay plates' probably from top-loading closures are often found at excavated kiln sites (left).
The number of flues or flame ports in a kiln will depend on the size of the firing chamber and the type of fuel used. The kiln should be built to avoid cold spots, which are likely to arise at the back of single flued kilns. Wood has a long flame path and two opposed flues are generally sufficient for firing chambers up to two metres in diameter. Coal has a short flame and several flues will be needed to give even heating. Coal firing kilns generally had a greater ware capacity than wood kilns.
The kiln atmosphere. The normal state of a wood firing kiln is neither fully oxidising nor fully reducing, but an intermittent combination of both created by cycles of stoking and burning. A neutral atmosphere tending towards reduction is to be expected unless steps are taken to make it otherwise. If the supply of oxygen is sufficiently restricted by controlling the amount of air entering a kiln, carbon monoxide will be produced and a reducing atmosphere created. The composition of the kiln atmosphere will depend mainly on the method of stoking and the control of the draught. Light and frequent stoking with a strong draught favours oxidation, and heavy charges of fuel in a weak draught create reducing conditions. The type of fuel may also matter. (For full discussion of the issues here see (1) Newell, and (2) Dawson and Kent below)
Porosity, hardness and reproduction 'medieval' jugs
With a few exceptions medieval earthenwares are not hard-fired and are slightly porous. Among the exceptions are South Hertfordshire cooking pots and Mill Green finewares. The makers of these splendid pots adopted a special firing regime enabling them to strengthen and thin their products, presumably to increase demand for them by improving their utility(see throwing). A more ordinary solution to the problem was simply to fire pottery to higher temperatures, which would be somewhere above 1000C, although this required suitable clay, kilns which could do it and better glaze control. There is evidence suggesting that this was the case with, for example, Cheam whiteware jugs and coarse border ware.
Recent work at the Campots pottery has produced an earthenware body which preserves the character and appearance of medieval ware and yet is strong and completely waterproof. It is based on the MillGreen/S.Herts body, and is ideal for displaying cut flowers but not for food or drink because of the external lead glaze. The galena glaze puts a limit on the top temperature, but must be used if the vessel is to have an authentic 13th-14th century look to it. There are few worse sights than a good medieval shape glazed with a bright lead or borax frit just to make the pot safe. The clay must be iron-rich, a common red earthenware, or a grey iron-bearing gault, and the pottery has to be fired in heavy reduction from about 400C to build-up a good ferrous oxide flux for the vitrifying elements in the body. From about 900C the body should be sufficiently tight and strengthened by partial vitrification; reduction can cease and an oxidised finish will give the surface a pleasing red-brown hue. The lead glaze will be a typical reduced green, or a clear yellow-brown when applied over a white-firing slip preventing the migration of iron from the pot body.
This experiment illustrates how major differences among pots can be brought about by very small changes in manufacture. In the present case, the difference between a weak vessel that will leak and a strong one which will not, and will resist thermal shock, is just a matter of starting reduction earlier and continuing it a bit later than in ordinary circumstances. A glaze allowed to melt before being reduced will will be different in colour from glaze reduced early on. Jugs stacked near the top and at the bottom in the same kiln firing will have a different appearance, and the addition of just a small amount of slip to the lead in a glaze mix will change the surface appreciably. Much variety in medieval pottery is as likely to be due to slight, often accidental, technical changes as it is to self-conscious attempts at innovation.
The essential reference to the typology of medieval kilns is:
Musty, J.G., 1974, 'Medieval Pottery Kilns'
Information on the construction and operation of medieval kilns:
Musty, Algar, Ewence, 1969, 'The medieval pottery kilns at Laverstock, near Salisbury, Wiltshire'
Mays and Scott, 1984, Pottery kilns at Chilvers Coton, Nuneaton
Coleman-Smith and Pearson, 1988, Excavations in the Donyatt Potteries
Newell, R.W., 1998-99, 'Reduction and oxidation in English medieval kiln practice'
Dawson and Kent, 1999, 'Reduction fired low temperature ceramics'
Other useful references concerning medieval kiln construction:
McCarthy and Brooks; Bryant; Manby; Orton
see bibliography for full details
next page: How to make a 'medieval' kiln
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