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Third firing of the earth kiln

 

A fortuitously dry spell in October allowed us, as we had hoped, to do a second firing of the wood kiln this year. The weather forecast was for a dry day, not too windy, and not too cold so we reckoned that, although it wopuld have been late in their season, our medieval potters would probably have taken advantage of such an opportunity too. The clocks had not yet gone back, so our lighting time of just before 7am was in the dark, with barely any breeze and once alight a 2-3 log fire, well towards the entrance of each firebox, eased the ware chamber temperature up to around 100ºC and, with some adjustments, kept it there fairly steadily. 

Small fires had been burned in the kiln the previous morning and the kiln packed while still warm from these. We had also built up the 'chimney' structure the day before, after packing the kiln, which meant it could be built a bit more carefully, and the joints plugged with clay cob, which we hoped would maximise its effectiveness. Did medieval kilns have this type of structure? As with the rest of the kiln superstructure there is little evidence one way or the other, but for a small kiln like ours it seems to help, and it is certainly the kind of thing that would have been entirely feasible for a medieval potter. 

Another difference from previous firings was that this time we had two pyrometers, rather than just one, on opposite sides of the chamber. Because of the way the pots ended up being packed in the kiln, one of the pyrometer probes projected into a large pot, tilted on its side, and so was well shielded from the direct flame path, while the other (inserted through a small hole in the spyhole bung) was close to the location of the cones and more exposed. The former tended to read 20-50ºC cooler than the latter throughout the firing, giving us some idea of the sort of internal temperature variation in the kiln.

Around 10 o'clock we began to raise the temperature, and over the next three hours or so ended up with a slightly faster temperature rise than we had intended - about 200ºC/h - though no harm seemed to come of it. It seems like the kiln having been fired recently was perhaps more responsive than previously. This was followed by a gentler climb towards our target temperature of 1020ºC, which we started to reach by about 7 o'clock. When the target pyrometric cone had dropped fully, and the next higher cone was just starting to bend we stopped stoking and begain to brick up the firebox mouths, and cover the flue to seal the kiln. The temperature falls fairly rapidly once stoking stops, and by the time the kiln was sealed the pyrometers were reading a little over 700ºC.

 

The day after the firing was dry to start with, but the forecast (and sky) threatened rain and the kiln was still too hot to open. It was also too hot to simply pull a tarpaulin over! Fortunately the wooden frames we built to provide the more substantial winter protection for the kiln last year were to hand, and did the job of holding the tarpaulin off the kiln sufficiently to stop the heat damaging it. The rain arrived, as predicted, and we ended up kept the kiln covered for the next couple of days, by which time it had well and truly cooled!

When we finally got a dry enough day to uncover things and open up the kiln, it revealed a fairly successful firing. There were a couple of breakages from damage when loading the greenware - which should be avoidable of course, but accidents do happen, and this time we did not have enough pieces to simply replace a chipped vessel with a better one, as our medieval forebears would no doubt have done. We needed everything we had to fill the kiln.  One pot had cracked where there was a stone in the pot, which had obviously gone undetected in the throwing process, and the base of one of the jugs which had been thrown in two pieces (base and body) had neatly become detached. This last problem one we've encountered before with vessels made this way, presumably the result of either mismatched shrinkage or poor bonding at the join, but it needs some more work to identify exactly why it happens. 

Aside from these issues, however, the majority of pieces were fired well. Firing to the slightly lower temperature resulted in less softening, and hence distortion, of the pots - though some still distorted a little where particular pressure points occurred. This is clearly something where greater attention to the way pots support others in the packing of the kiln would help. It is also, perhaps, a problem with a smaller kiln where the constrained space, especially for larger pieces, means that there aren't always many options when it comes to how to fit pots into the spaces available. 

 

The other improvement was that there was much less over-firing of the glaze. The glaze used in this firing was very simple: just a mixture of lead bisilicate with a small amount of china clay. This glaze has fluxed well creating a strong, thin, but glossy, glaze very reminiscent of the glaze seen on much medieval pottery. There were still occasional patches that were over-fired, presumably where a side of a pot was exposed to a more intense flame. Again packing may be important here.

Slight over-firing of the glaze on one face of a jug

 

Most pieces, even those with slight faults, would have been perfectly serviceable vessels however and very little would have gone on the 'wasters' heap from this firing. It is starting to feel like we are getting the measure of the kiln, the clays we are working with, and the interaction of the two. There is more experimenting to do, and more to learn, but for now this is a satisfying position to have reached.

Cooking pot anyone?