For a more detailed Field Report please see the Bulletin of the NCEFG Vol. 2.
The Kiln at the end of Season 2 - Stanley IIb.
Stanley IIa
Stanley's superstructure suffered greatly over the winter so we decided on a complete rebuild, though we kept the original design of single-firemouth updraught kiln with raised ware chamber and discrete firebox.
We used the existing substructure so that the general shape of the new kiln, Stanley IIa, was similar (though not identical) to Stanley I.
Our experiences with Stanley I in 2003 dictated a different approach to the superstructure of our new kiln. Stanley I's ware chamber (Sonoma fieldstone coursed in a kaolinite rich clay and straw matrix) cracked and flaked badly after firing.
 We decided our new kiln would have a ware chamber made entirely of clay and straw briquettes leaving out the stone (except to support the firebox throat).
Instead of the kaolinite rich clay of 2003 we used a mixture containing mainly recycled shop clay. We kept the original granite pedestal support but angled it 45 degrees to the firebox in an attempt to encourage a cyclical flame path through the kiln.
 Stanley I had a mudstone floor (it split and delaminated during firing). Stanley II was to have a floor of radiating kiln bars made of the same clay mixture as the ware chamber walls.
Since our clay coated wicker firebox roof from 2003 did not work properly we decided to use stone instead. Research (Rhodes 1981: 127-8) suggested that we could use a large flat slab of sandstone as the firebox roof. We also used a smaller slab as a cap for the ware chamber.
Firing Stanley IIa
 The firing of Stanley IIa was a complete disaster and never reached above 600 °C. We used a type K thermocouple with an analog meter to take internal temperatures. It was inserted into a custom-built hole in the ware chamber wall.
We chose to fire to "cure" the kiln, the ware chamber empty of pottery, as we were unsure of the behavior of the floor struts, and the sandstone elements. This was a wise decision, for if there had been pottery in the ware chamber we would have lost it all. Within the first half hour of firing the first of the floor struts exploded, and by the end of the firing all but one were gone. The sandstone roof of the firebox quickly became too hot to touch. Four hours into the firing the kiln itself was too hot to touch and we began to worry about the temperature differential between it and the cold California evening. We worried that the walls of Stanley IIa were too thin.
 At about 6:45 PM the ware chamber began to spall badly on both the interior and the exterior, particularly around the thermocouple hole. At 10:30 PM the sandstone firebox lid cracked and collapsed inwards. We aborted the firing, raked out and cooled the coals and left the kiln, defeated.
Stanley IIb
 At first we thought that we would have to call off the 2004 season. But some serious reinforcements showed up resulting in a weekend crew of ten people. So we rebuilt once more: Stanley IIb.
We mixed together more clay, dirt, and straw, which we used to patch the cracks and spall holes on the interior and exterior of Stanley IIa's ware chamber walls. We were concerned that these might spall a second time, or break off during firing, but it was common practice in ancient times to patch kilns so we decided to go for it. We chose to forgo a thermocouple hole in the interest of structural integrity.
We delightedly discovered that the cantilevered stones of Stanley IIa, which braced the firebox throat where it connected to the ware chamber (our biggest floating, load-bearing area), had survived!
 We put our crew to work building an outer shell to better insulate the ware chamber. We constructed this shell around the outside of the already fired, solid walls of Stanley IIa using techniques learned from Stanley I: courses of fieldstone mortared together with a clay matrix. We built the courses up gradually using thick handfuls of mortar between the stone and the inner ware chamber to be as insulating as possible.
Our next major concern was the rebuild of the firebox roof. In order to get a fully functional kiln inside our time frame, we decided to use firebrick for the new firebox roof. This was a deviation of the NCEFG charter to use all natural materials. With the help of a skilled professional stonemason we ended up with a beautiful, load-bearing arch over our firebox that frankly put the rest of our kiln to shame.
As we were already straying from our charter, we used fragments of old kiln shelves to make up the ware chamber floor, and to lid its top.
After completion we left Stanley IIb to dry for eight days.
Materials
Clay
For the internal matrix of Stanley IIa we used recycled shop clay from a local school. This clay, Pueblo White, we mixed with a local yellow clay from Peacock Gap, in Marin County, California and some sandy dirt (for filler) and straw (as opener). We also used this mixture to manufacture the kiln bars for the ware chamber floor (these bars did not survive) and after the first disastrous firing, to repair the kiln.
For the external matrix of Stanley IIb we used a mixture of kaolinite clay (quarried from the Sierra Nevada mountain range in Amador County), local yellow clay (an upper sedimentary deposit from near-by Peacock Gap, Marin County), sandy dirt, and straw.
Stone
The only stones in Stanly IIa's ware chamber were the five large Sonoma fieldstone boulders cantilevered over the firebox throat.
For Stanley IIa we tried using sandstone (donated by Shamrock) as both the firebox roof and as a cap for the ware chamber. This did not work, and precipitated and interesting paper which can be read here.
We kept the original Stanley I granite pedestal support for both Stanley IIa and Stanley IIb.
For Stanley IIb we constructed an outer shell around the outside of the already fired, solid walls of Stanley IIa using courses of Sonoma fieldstone. So now there was stone in the ware chamber construction but it was shielded from direct contact with flame by the existing superstructure of Stanley IIa.
Fuel
We used two different types of very dry wood for fuel: kindling acquired from a woodshop; and fast-burning, high-heat eucalyptus logs chopped into three different sizes small (about 1.80m long, 0.20m wide), medium (about 1.80m long, 0.40m wide) and large (about 1.80m long, 0.60m wide).
Pottery
 We used five different types of clay, containing sand, talc, and/or grog tempering; red, orange, or white in color; low-fire or medium-fire in behavior. To make pottery we used an on site pottery studio with electric wheel, running water, and tools. Pots were constructed using the coil, slab, mold, and wheel methods. Most had at least a week to dry. In 2003 we felt many of our losses were due to insufficient drying time, so this year we tried to ensure our pots were bone dry.
Firing Stanley IIb: Bisque
 We used the same wood fuel in this second firing as we had in the past. Since this time around our kiln was fully loaded with pottery we used two types of temperature measuring devices.
First we used four banks of cones 906, 04, 02, 01, 1, 2 and 4) imbedded in clay logs distributed amongst the pottery in order to measure temperatures ranging from 995°C to 1160°C.
Second we used two type K thermocouples with attached analog meters, positioned to tilt down into the ware chamber from the top edge of the kiln. These gave us a general idea of the internal temperatures of the kiln as the firing progressed, but (as we learned later from the cones) were cooler than the actuality.
We'd learned our lesson the hard way with Stanley I so we took the initial temperature up very, very slowly. We kept track of our wood use by weight. After about five hours we'd reached 300°C, and used 28kg (approx. 60lbs) of wood.
An hour later, around 590°C, we heard our first pot "poof". We immediately stopped raising the temperature. We began to see water and steam escaping from cracks in the outer edges of the kiln. Eight hours into the firing we reached 710°C and declared our pottery officially ceramic. We began to see flames out of the top of the kiln (up to 250mm high) and the interior became a dark orange red color. We reached our highest thermocouple reading, 950°C, at 10:30 PM (11.5 hours in) after which the kiln stalled and all attempts to raise the temperature further proved ineffective.
Results of the Bisque Firing
This firing took just under 12 hours to complete. We used approximately 202kg (440lbs) of wood. The highest temperature measured by the thermocouple was 950°C. The highest temperature measured by the pyrometric cones was 2 (approximately 1160°C) but the kiln consistently melted at least 02 (approximately 1100°C) throughout the ware chamber.
 As a result of the high temperatures several of the low temperature clay pots nearest the firebox suffered warping. But we had no explosions or spalling of pots at all. We only lost 4 out of 90 pots as the result of firing damage.
We attribute this success to three factors: 1) we were careful to take the initial temperature up as slowly as possible; 2) the pots themselves were much dryer than in 2003; and 3) the skills of the potters constructing said pots had improved.
 The kiln itself survived this firing very well. The extra insulation shell around Stanley IIa's ware chamber appeared to solve the spalling problem. Only minor repairs were needed so we decided to push onwards and attempt a glaze firing.
Firing Stanley IIb: Glaze
The glaze firing went, if possible, even better than the bisque. We could not glaze and fire all the available pots. We were a little nervous because we had to reach a specific temperature (between cone 06 and cone 04) and hold it there for several hours. If we did not reach a high enough temperature, or hold it long enough, the glazes would not vitrify properly and opacify. If we went too far above cone 04 or held it too long the glaze would run straight off the pots. We also had to be much more careful with cooling the kiln, glazes are prone to cracking if cooled too fast.
 Nevertheless we sallied forth. Loading the kiln and keeping pots from touching one another took twice as long as when loading for bisque. Again we placed cone banks at various point in the kiln, taking care to make sure that three were visible if we looked in from the top of the ware chamber.
Again we used the same wood fuel as in previous firings. We'd lost one thermocouple during our bisque firing, so we relied on only one for this firing. We now knew it tended to run about 100 degrees below actual internal temperature, though we continued to faithfully recorded its reading.
Since the pots had already been fired once we could raise the temperature faster than with the bisque. Instead of taking 5 hours to reach 300°C we took only 3. Again we kept track of our wood use by weight. We noticed that at about hour five all visible pots appeared glassy. In just under 6 hours we reached our maximum of 925°C. We managed to keep the temperature between 700 and 925 for 2.5 hours, which we considered sufficient to achieve full vitrification. The pottery made very little noise, although as it began to cool we did hear a ping or two that indicating cracking.
Results of Glaze Firing
 This firing took just over 6 hours to complete. We used approximately 197kg (428lbs) of wood. The highest temperature measured by the thermocouple was 925°C. The highest temperature measured by the pyrometric cones was 04 (approximately 1060°C) and the lowest was 06 (995°C), which meant we hit out target glaze melting temperature throughout the kiln.
 Consequently the resulting glazes were quite spectacular, with true and vibrant colors. Several had cracks (probably due to too rapid cooling), and two of the thinner pots were cracked completely in half.
The kiln itself survived this firing even better than previously. Unfortunately our season was over and we had to abandon Stanley IIb to the winter elements once more.
The Ending
 First we must conclude that a solid inner well insulated with a thick outer shell of rock and clay makes for a great working kiln. This structure combined the sturdiness of solid clay with the high temperature insulation of kaolinte and rock.
Second, regarding the pottery, we attribute our high bisque ware survival rate to our extreme care in raising the temperature, and to the dryness of the pottery itself.
All in all we considered the glaze firing our greatest success: we not only reached and maintained to correct temperatures but managed to do so throughout the kiln. Yay us! For more scientific conclusions and analysis, as well as additional details, please see the 2004 Field Report in the Bulletin of the Northern California Experimental Firing Group Vol. 2.
Bibliography
Rhodes, D., 1981. Kilns: Design, Construction and Operation. Second edition. |
