Stanley IIc: "We Could Just Keep FIring, Amazing!" ~ DW

The Northern California Experimental Firing Group's (NCEFG) main kiln, Stanley, spent yet another winter swimming. After extensive cleaning, participants decided there was no major damage, although the substructure under the firebox throat where it connected to the flame chamber did cause some concern. Still we decided to proceed with the 5th season using the same kiln. Stanley IIc is a single-firemouth updraught design with raised ware chamber and discrete firebox.

Stanley IIc's ware chamber was made of a solid clay interior (fired to ceramic during in the first firing of Stanley IIa in 2004) surrounded by a thick outer shell of coursed stone in a clay-and-straw matrix. Outside of this was a buttress of built up dirt, stone, former pots, and clay all the way to the top of the chamber. We kept the drilled thermocouple port on the east side of the ware chamber.

The firebox was made of firebrick mortared together and covered with the same clay and straw matrix used to mortar together the rocks of the ware chamber. We also continued to use the fragments of old kiln shelves to make up the floor and lid of the ware chamber. (For more details on the construction materials used to build Stanley please read the field notes from 2005 or the Bulletin Vol. 3.)

Thus, before being put back into action in 2007, Stanley IIc's ware chamber had been fired eight times and patched five times, its firebox had been fired seven times, and lower underground portions had been fired eleven times over five years. For more details, diagrams and construction information please see 2004 and 2005 field notes, and Bulletins 2 and 3.

Repairs

As in previous years we had to perform extensive patch repairs to interior cracks in the ware chamber walls, which developed during the 2006 firing and propagated during the winter. We had to clean and weed the kiln, which was covered in a thick growth of insidious cape weed. To our surprise repairs from the previous year to the shoulder area of the flame chamber appear to be still in place.

This zone is one of our biggest problems areas as the act of stoking causes blocks of wood to crash into this highly friable joint, which is also supports the main weight baring area in suspension. We decided to leave of repairing this area until after the first firing. We did build up and clean the stoker's pit.

The new wood was mostly oak, but there was also some pine and elm in the mix. These we divided into small, medium, and log (tree branch) size. The eucalyptus was, as in 2003, 2004, and 2005, well-aged, and divided into three rather arbitrary sizes: small, medium, and large.

We continued to use the weight method of tracking wood use. After cutting and sorting by size, we divided the fuel into piles of 20lbs each. This year we labeled the piles with descriptive tags, which were passed to the recorder every time a pile was delivered to the stoker's pit. This meant we were keeping track of wood amounts with both tags and field worksheets. This method proved its worth right away, as final calculations after the first firing showed the worksheet off by 20 lbs according to the tag record.

Pottery

Fuel

We decided to try using a different kind of fuel this year in addition to our standard eucalyptus of the past. The new wood was mostly oak, but there was also a little pine and some elm in the mix. It was divided into small, medium, and log (tree branch) form. The eucalyptus was, as in 2003, 2004, and 2005, well-aged, and divided into three rather arbitrary sizes: small, medium, and large.

We continued to use the weight method of tracking wood use. After cutting and sorting by size, we divided the fuel into piles of 20lbs each. We labeled the piles with descriptive tags, which were passed to the recorder every time a pile was delivered to the stoker's pit. This meant we were keeping track of wood amounts with both tags and in the field worksheets.

Firing Stanley IIc: Single Fire

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The day of our second firing in 2006 was beautiful and sunny with a brisk wind: absolutely ideal firing conditions. We had about 100 pieces of pottery, with sample tiles and loom weights all grouped together and counted as one item. This filled the kiln with 8 inches to spare. We added some pre-fired bits from last year to fill it completely and commenced firing.

As in previous years, we used the same two types of temperature measuring devices: type K thermocouples and Orton pyrometric cones. We used 4 banks of large-size Orton pyrometric cones in rows of 5 each. Each bank did not contain exactly the same range of cones. We had 4 sets of thermocouples, 2 with analog meters, 2 with digital data logger attachments. They were much less problematic than in previous years and we got good clean records throughout the nearly 11 hours of firing time.
 

Results of the Single Firing

The single firing took over 11 hours to complete. We used 124kg (about 273lbs) of wood, considerably less than in previous years, almost exclusively eucalyptus. The highest temperature measured, by Thermocouple A at the thermocouple port 6" in, was 930?C. We did not melt any of the Orton cones.spalling.

All 46 of the pots made it though as bisque perfectly, there were no losses to cracks or spalls. However, we were too cautious in our temperatures and 8 pots had glazes that did not mature.

The kiln itself was experiencing integrity problems in the firebox throat where it connected to the flame chamber. Despite previous repairs the wall underneath the shoulder area was still crumbling away. We conducted more substantial repairs, using bricks and shards of old pottery to shore up this extremely friable area. We also built up the floor shelf support in the flame chamber near the thermocouple port, and the central floor support pedestal (minerals in the granite were breaking down).

The mixture used to repair Stanley IIc in 2007 was comprised of shop clay (both white and orange with sand, talc, and grog tempering), mixed with equal parts fine grain river sand and fresh grass clippings. The grass made a particularly good organic opener: it dried at the same rate as the clay, was less resistant to shrinkage, and was small enough to fit inside the cracks that need repair. Unfortunately, as time goes by it also decomposed and the mixture smelled rather badly. We found that the best approach was to continually patch every fifteen minutes over several hours, filling in even the tiniest cracks. This was not the most time efficient method but seemed to work. (See Bulletin 4 for an in depth discussion of kiln repairs.)

Firing Stanley IIc: Glaze

Because we had underfired some of the glazes in the previous firing, we decided on a glaze firing of these pots combined with some already bisqued pottery from previous years.

We reconfigured the floor of the kiln with an eye towards better heat flow

We had good cross-the-board cone coverage, with 08 down everywhere, and 05-04 everywhere but the very top, all without going to 02. As always with glazes, instead of pushing our kiln to new temperature heights, we had to think in terms of reaching a specific temperature (between cone 06 and cone 04, approximately 995-1060°C) and holding it there for several hours.

Results of Glaze Firing

Firing was short, sweet, and generally quite successful. We used only 95kg of wood (209lbs), again much less than in previous years and all of it eucalyptus. One of the skill areas we have experienced the greatest improvement in over the years is our wood use, every year we use less. The highest temperature measured by a thermocouple was 1100°C (near the firebox), reached about 6 hours in. We had good cross-the-board cone coverage except for the bank at the top, with 05 down everywhere but the very top.

The resulting glazed pots were successful with only a few of the more sensitive glazes bubbling, and some persistent underfiring problems at the top of the ware chamber victims of the up-draught kiln's temperature differential. The firing was pronounced a resounding success and all parties returned home clutching their respective loot.