Woodfire Tas 2011: the Nubeena firing page 2

SideStoke home | Nubeena firing 1 | Nubeena firing 3 | Zoe and Dave's nursery

The kiln
The kiln differed from the Oztrain kiln only slightly: the chimney had a greater cross sectional area, and an internal wall was introduced on the chimney side of the second sidestoke port. At no stage was the damper fully open so the increased chimney cross section is unlikely to have affected the firing. 
The internal wall was an unintended variation. Because of the limited time available for the firing it was decided to pack only bisqued pots, and since Zoe and Dave do not bisque their pots they had none available for the firing, except refires which had been left out in the rain and were deemed likely to explode. The wall was built to separate the refires from the rest of the kiln, and we rather pretentiously called the separated area a sutema

The firing was very much like our firings at Eungella, except that the wood was much drier than it ever gets at home.  The back of the kiln showed its usual reluctance to heat up as much as the area behind the firebox, and as usual was difficult to reduce.  Because of the absence of larger pots to block the flames at the top of the stack so as to force them down the flames tended to motor along near the roof of the chamber until forced down by the sutema wall.  Continuous side stoking through 2 ports relieved both the heat and reduction problem to some extent. The sidestoking started at the first port when the temperature behind the firebox was about  1000 C, and at the second port when the front temperature reached approximately 1130 C .The lengths and thickness of the available sidestoking wood encouraged us to do quite a lot of flamethrower stoking, which under some circumstances can yield short term temperature rises but on the whole seemed no more effective than ordinary stoking with shorter pieces of wood. Only towards the end of the up cycle, when stoking temporarily ceased at the main firebox, and flame throwers of thin, dry acacia sticks were used, did the temperature at the back rise quickly. If we use the sutema wall again we will move it to the firebox side of the second side stoking port to see if side stoking in the sutema chamber makes any difference.

Left: Arthur Rosser introducing bulrushes into the firebox for the reduced cooling phase of the firing. An oxy probe was used to test that reduction was maintained, but was not really necessary.  About one fertilizer bag of bulrushes was used.
 Reduced cooling with bulrushes as the reducing agent was uneventful, and of course quite quick, taking only 53 minutes to lower the temperature  from 1030 C  to 880C according to the pyrometer in the oxy probe (located about  65cm behind the firebox). Slightly lower numbers were reported from the probes located closer to the exit flues. All the numbers have to be regarded as approximate.
Slow oxidised cooling below 900C was assisted by a gas burner, and the gas supply- burner arrangement was untested prior to the firing. There did not seem to be enough pressure to hold the kiln above 800 for the desired 4 hours, and we had no way to increase the pressure.  The temperature drifted down to a lower temperature than we had planned, to a reported 770 C, before we sealed the kiln.
The temperature distribution was not what we would have asked for, being very hot at the top of the first  bung (cone 13)  and  about cone 10 at the second bung. The third bung was behind the sutema wall, and although the pyrometers were moderately optimistic about the final temperature there the well protected cones reported less than  cone 9. We regard cones 10-12 as acceptable and cone 13 as unnecessarily hot. If we use the sutema wall again we will move it to the firebox side of the second si

Some of the work from the firing

On the clay body we use at Eungella the pots looked very much like the pots fired at Eungella: good red flashing on bare body where the ash effects were minimal, dark brown or black (rather than the undesirable mid brown) at the next thickness of ashing, and greenish glaze where the ash effect was greatest. "Red shino" liner glaze - really a shino recipe using a proportion of iron bearing clay - was excellent, with a lustrous surface effect in most places. Shells used as wads produced good comet effects, and our high iron dark wad clay behaved normally.  Despite different woods and different reducing medium, which could easily have resulted in very different effects, the general resemblance to our usual work was unmistakable. We are more than ever convinced that the reduced cooling regime used is useful and transferable to different fuels.
Other clay bodies  produced a wide range of results, many of them very attactive.

Platter ruined by snotters
Many of the shelves used in the kiln had been purchased in used condition and precisely what they had been used for was not known. Usually this caused no trouble but one large silicon carbide shelf was placed across the top of the kiln just behind the firebox, and the temperature reached cone 13 at that position. The result was a liberal supply of snotters, probably because the binder for the silicon carbide crystals in the shelf had been affected by salt glazing. The platter shown was fired on edge, and the snotters disrupted the intended comet mark design.
More images of fired pots