Since the last test kiln firing in October some maintenance / changes have been made to the kiln in an effort to speed up the firing and to use less wood
The kiln was packed and then pre-heated overnight with logs from a nearby bonfire as it was quite damp . Packing was fairly dense - most pots were small to medium size with a few pots in the firebox. Pots were also packed into the tunnel and the chimney. I had decided to try without using any side stoking. This meant that the shelves could be closer together, leaving room for another half stack in front and slightly more room in the firebox. Orton cones used were - 015(805°C), 05(1030°C), 04(1060°C), 03(1100°C), 7(1240°C), 8(1250°C), 9(1260°C) and 10(1285°C). Hearth and firebox are extended by dry laying bricks and adding the fire bars.
Firing started at 9.00 on Friday. we had no pyrometer until 12.30, which was not a problem. In discussions at the end of the firing, the consensus was that the pyrometer is good for showing whether the temperature is moving up or down, but can be misleading - there were a couple of occasions when following the pyro readings might have led to cone fall being missed - at 800°C and at 1030°C.
We started stoking into the lower firebox with small bundles of cedar and continued with larger cedar/oak to 550°C. I think that we lost time here and should have opened up the top firebox door for larger wood at 500°C.
After 12 hours the pyro was reading 750°C but cone 015 was by this time flat, so reading was possibly 100° out, at 805°+.
After 17 hours the pyro reading was 880°C, but cones 05, 04 & 03 at the front were all flat, (1100°C), so we were late in starting reduction. I think that this has resulted in some underdeveloped body colour - greys rather than buff in some of the clays. There was a large difference in temperature between the front and back of the top shelves.
Reduction cycle starting for an hour - 20 mins. reduction then 20 mins. oxidation. Should have continued for longer with this. Reduction achieved with slight overload of wood and by using the active damper. As part of the work on the kiln we had closed the gap above the damper by raising the level on top of the tunnel by adding insulation, and as a result of this reduction was apparent with the damper at 2.
After 24 hours the set of low cones at the rear are softening. The temperature difference between the 2 sets of cones stayed at about 3 hours. I think that this was in part because of the way I packed the kiln. The rear shelf was about 20mm higher than the shelf in front. This allowed the flame path to miss the rear shelf completely. We opened the top bung (pieupieu?) in order to draw the flame over this shelf, which it did, but I don’t think that this necessarily increased the heat and may have acted as a vent.
Stoking at this point is: cedar only into the top door, oak in a train at the right side only of the bottom hearth - 1 piece fed in in a continuous line, so that the hardwood is underneath the quicker burning cedar and is warmed up and burning by the time it actually is inside the firebox. The metal poker with a square end is used periodically to clear an air passage underneath the wood, scraping along the floor of the firebox so as not to disturb the embers.
After 32 hours front set of cones 7, 8 & 9 are down, pyro reading 1100°C, so temperature readings are disregarded from this point on.
After 37 hours the front set of cones are all flat, rear 7 and 8 down, 9 softening.
Reducing 20 mins on, 20 mins off for 2 hours.
Continued to 42 hours, but no further movement to rear cone 10. (Probably because a strong wind was reducing the draft in the kiln, resulting in lots of ash building up but little flames or rise in heat - TT).
Ended firing by loading the firebox with wood and charcoal and then clamping up the kiln in order to encourage carbon trapping. This was apparent in a very few of the pots. This was an experiment. If the test kiln is to behave like the large anagamas it should be crash cooled to 1100°C as this is what would happen in the larger kilns.
Results from the kiln were in the main really pleasing, apart from the top back shelf which is definitely underfired.
Possible solutions:- Pack so that the rear shelf is lower than the front top shelf. Might be even better if there are large /tall pieces on the rear shelf with the top shelf lower than the top of the tunnel so that the flame path has to pass over the top of the rear shelf. If not side stoking then the half shelf could go at the rear of the kiln.
Ochre colour possibly from oak fly ash, some dry surfaces - might be good to react with exterior glazes (use saucers to catch drips) Wadding used was high alumina salt wadding, which came away more cleanly than clay/fireclay/sawdust used last time (see wadding section for recipes).
I think that the new insulation at the back of the kiln definitely helped speed up the firing. Time could also be saved in having more efficient changeovers, and perhaps in changing the shift patterns, also in being more vigilant in checking the cones. I had intended to try to be more efficient in wood use - the weather especially towards the end of the firing was very boisterous, and the wind made it difficult to get a heavy reduction, pulling heat through the kiln, so I think with different weather conditions it would be possible to make better use of the wood. There was a good amount of ash even with the faster firing time (42 hours).
The cool spot at the top rear of the kiln can either be used for what it is, and pots with 1260°C glazes would go well here or, as previously suggested, could have tall pots on a shelf lower than the top of the tunnel opening. The front of the tunnel was a good place, the new insulation helps here, but the bottom of the chimney visibly cooled after dark on the first evening and this too is a cooler spot that can be used as such.
I would put more 9, 10 and 11 Orton cones in the kiln to check top temperatures throughout. Pots on the floor of the kiln up to half way did well and had good ash build up (to the level of the top of the tunnel).