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Lossenham Pottery Project (2022) - The first kiln

 

 

The Lossenham Pottery Project, initiated by Russell Burden - artist in residence at Lossenham, ran over the summer of 2022. The project recruited a number of volunteers, who started by digging clay from a couple of sites on the farm and preparing it  for making tiles and pots. A session hosted at Aylesford Pottery provided opportunity for people to get started with tile making. Because of other commitments, not everyone was able to see the process through from digging their clay to the finished pieces, but a number of tiles were produced which made it to the final firing. In the meantime, Russell and I, with access to wheels and workshops of our own, were able to make sufficient in the way of pots, both ‘medieval’ and contemporary, to (we hoped!) fill the rest of a (yet to be built!) medieval type kiln.

 

The kiln itself was built of clay cob over two periods of a couple of days each, separated by few days to allow the first part of the structure to dry out enough to support the later stage of construction. The design was an updraft kiln with a single firebox (flue, in archaeological terminology) loosely based on evidence of such kilns from the Romano-British through to medieval period. Such a kiln consists of a roughly circular hole dug into the ground connecting, via a short straight trench, to another hollowed out area from which the fire, can be stoked. The trench is roofed over to form the firebox, and a perforated floor at is built inside the round hole, at about ground level, often supported on a central pier. Finally the kiln walls are built up from the ground level to form the ware chamber. 

 

At Lossenham the clay is generally very close to the surface, so digging out a hole for the base of the kiln and stoke pit is hard work, but leaves solid walls which require little further work. The floor of the firing chamber consisted of a central pier of layered clay and tile with a number of bars radiating outwards supported on the pier at the inner end, and on the edge of the kiln hole at the outer end. Kilns of the period probably used pre-fired clay bars for this purpose, but not being in a position to make these we opted for bars made by cutting a substantial clay chimney pot into lengthways sections.

 

From the ground level up the kiln was formed of clay cob - a mixture of (roughly) 10 parts clay to 2 parts sand and the same of straw, mixed with water and trodden into a stiff, but malleable and very sticky, consistency. The walls were built up with successive handfuls of cob moulded, scraped and beaten into a wall about four inches thick.

 

One of the challenges with reconstructing historic kilns is that virtually nothing is known about what the above ground parts looked like. Archaeological investigations usually just find  the remains of the below-ground elements. It seemed likely that the kiln would have had a wider firing chamber narrowing to an exit hole for the hot gases, creating a balance between getting enough air flow through the kiln and preventing too much heat being lost. So, with this general notion in mind we worked more by intuition than specification and ended up with a superstructure that was somewhere between conical and dome-shaped. During construction the dome was supported on a framework of green willow withies, which were removed once the cob had stiffened up enough to support itself - a suitably medieval construction aid!

Once complete, the kiln was left covered for a couple of weeks to firm up and dry out a little before the test firing - a short firing, over 3 hours or so, taking the temperature up to about 350°C. A few days later we packed the kiln with all the pots and tiles we and the other volunteers had been able to make. Packing was relatively straightforward as none of the pots were glazed, so it did not matter how and where they touched each other, but nonetheless a somewhat delicate three dimensional jigsaw puzzle with fragile greenware. 

 

The following morning, provisioned with a trailer load of small logs and kindling from the farm, we lit the kiln and began the slow initial stage of the firing. The challenge here was to keep the fire small, but within a narrow temperature range just below that at which any water remaining in the clay would turn too rapidly into steam and cause the pieces to fracture, and this was indeed what happened to a few of those most recently made, which we knew were not as dry as they should have been. Once past that point a slow temperature rise to about 500 °C was relatively straightforward and then, with increasing experimentation with airflow at both the entrance and exit to the kiln and through the firebox, which became increasingly choked with ash, we managed to achieve a temperature of about 850 °C (according to the pyrometer) and finally, about 12h after starting the firing, closed up the kiln and left it to cool. 

By mid-afternoon the following day, it was cool enough to open and extract the fired ware.  There were casualties, not unexpectedly, but most of these were steam fractures, particularly where pots had been insufficiently dry to begin with. However, the majority of pieces had fired well - some very well. Based on the quality of the well fired pots, and the variation of this in different parts of the kiln, it was clear that the temperatures measured at the pyrometer location were - unsurprisingly - not representative of those achieved at other locations in the kiln, and suggested that in places the temperature may have reached somewhere between 900 and 1000 °C. Underestimation of temperature may also have contributed to the steam fracture failures, if parts of the kiln were significantly hotter than indicated by the pyrometer using that as our guide in the critical early stages of the firing. 

A contemporary vessel, by Russell Burden, from the 2022 firing

 

Medieval style jug from the 2022 firing

 

The kiln survived the firing very well, with the least successful element being the kiln bars forming the firing chamber floor. Despite being substantial, and already fired, several had cracked - although for the most part, and fortunately, not collapsed into the bottom of the kiln. Aside from that, the kiln was in good shape, with the inner surfaces being fired into a hard ceramic to a depth of about  20 mm. There was no plan to reuse the kiln, but it was left exposed to the weather (including flooding of the stoke pit and fire chamber) and some above ground parts were still standing three years later.

 

As expected, there were many questions and much to be learned from the first year of the project, about the nature of the clay, the making techniques of the medieval potters, and the design and firing techniques of the kiln, but nonetheless the project clearly demonstrated the feasibility of the approach and the viability of the materials on site for contemporary and historic potters alike.

 

A fuller visual record of the 2022 (and 2024) project can be found here.