The kiln in 2003 - Stanley I.

The Site

The position of our kiln was dictated mostly by necessity. It needed to be out of sight of our sponsor's house; close to the road for materials and fire-engine access; and not too close to flammable trees and bushes. Some of these factors must have influenced all kiln location in the past.

The biggest decision at this point was which way to orient the fire mouth. Of course we hoped to get the prevailing wind to provide extra draught, and we were slightly lucky in our choice. The wind blew off the land, rather than off the ocean, and our gamble paid off. Whether we shall be so lucky next year remains to be seen.

Building the Kiln

Our first task for 2003 was to build the kiln, Stanley 1. Once we'd decided on the location and basic design of the kiln, we marked out some lines on the ground and began the hard work - digging.

We dug a cylindrical pit about 50cm deep and 60cm in diameter, connected to a slightly deeper rectangular firebox.

Beyond the firebox we dug a rectangular stoker's pit, about 1.4 by 1.6m. The stoker's pit took quite a long time to dig and was the least exciting element of the kiln. It provided a safe place for the stoker to work with easy access to the fire mouth and the fuel, and, importantly, room to move clear if anything went wrong.

After the foundations had been dug our helpers arrived and construction could begin. We built a pedestal from granite cobbles to support the ware chamber, and a floor of mudstone spars radiating out from this. Upon this the pots would sit.

The floor of our firebox sloped steeply upwards at the back.

To roof the firebox we constructed a framework of wicker, and covered this on both sides with a thick coating of clay and straw. Covering the underside of the firebox roof was quite tricky, and ultimately not massively successful. We might try prefabricating some roof elements for next year.

Once the pedestal and ware chamber floor were in place and the firebox roof was underway, we could start to build thewalls of the ware chamber.

The (almost) circular ware chamber was around 60cm in diameter. We coursed Sonoma fieldstones, and luted them together with clay. The inside and outside of this coursed stone wall were liberally covered with more clay for insulation and structural support, and then around the outside of the clay and stone wall we banked earth for further insulation. Five or six courses of stone completed the kiln's superstructure.

All this was achieved in about a day thanks to a hard-working group of friends preparing, treading, and mixing clay. This was quite labor-intensive (and messy) and would be a major task for a large kiln, especially if built without stone. The choice and preparation of the clay would also be increasingly important in a larger kiln.

At the end of that day we were able to stand back and admire our construction, and hope it dried in time for the firing. We still hadn't quite decided how we were going to cap the kiln.

In the end we tried to make a sort of wattle-and-daub vented lid for our kiln, based very loosely on the sort of thing found with the medieval kilns at Rattray, Aberdeensire, Scotland.

We fabricated this from a framework of wicker, covered on both sides by a thick layer of clay and straw, with a 25cm circular vent hole in the top. We let this dry out and then pre-fired it. It seemed to work fine. We hoped to be able to bond this to the top of the kiln after loading with more wet clay, and possibly then to close off the vent after firing in an attempt to produce a reduction atmosphere.  Unfortunately, since the kiln was not full the lid had nothing to rest on and so sagged more and more.  As the wickerwork burnt away the lid began to disintegrate and finally collapsed utterly.  We will have to wait until next season to test this method of capping properly.

The Pots

To make the pots, we utilized a small workshop facility pre-existing on site, in which we had access to an electric wheel, running water, and all necessary tools (including fire gloves).  Pots were constructed by volunteers using the pinch, coil, slab, and wheel methods. Most pots had at least three days to dry, a few slightly less. All of them could probably have used more drying time, with the exception of the fine ware pots, which were already extremely dry. At the last minute we added several fine bisque ware pots, which we coated with a lead glaze (red color) to assist in determining the variable atmosphere of the kiln.

Materials

We'd decided early in the project to try and make our kiln entirely from natural materials: no brick, no firebrick, no metalwork and no ceramic fiber. This was in keeping with the archaeological, experimental nature of the project. We were limited slightly by the geology of our chosen site however. Our section of the Bolinas Big Mesa contained no clay and no stone. This made for easy digging but left us lacking construction materials.

Stone

We had a virtually limitless source of stone from a nearby source. This Sonoma Fieldstone looked promising for construction but when tested on a fire it tended to explode. This made it less than ideal for use in a kiln. Following this setback we visited a stone-merchant. Rectangular granite New York cobbles seemed ideal for our pedestal and survived the 2003 season admirably. Less successful were our ware chamber floor spars, made of Red Mountain Ledge mudstone from Montana. Although ideally shaped, these cracked and delaminated during the first firing. For 2004 we may try and use sandstone for the floor spars which should withstand the firing temperatures. Once insulated with a thick layer of clay within the ware chamber wall, the Sonoma Fieldstone proved fine as a building material.

Clay

Our main clay source for building was a primary kaolinite-rich, white clay quarried dry from the foothills of the Sierra Nevada mountain range (near Middletown in Amador County, California). With this we produced two sorts of construction material. A mortar was composed of roughly 80% clay, 10% dirt, and 10% straw, mixed by feet on a tarpaulin, and used for most construction. In addition we made a finer insulating material. This insulation was composed of roughly 80% clay, 20% river sand, and no straw.

Fuel

We used three different types of very dry wood: kindling acquired from a woodshop; fast-burning, high-heat eucalyptuslogs (which made up the bulk of our fuel); and some slower-burning, low-heat elm logs (to control the temperature). We chopped these into lengths of around 30-50cm to easily fit our firebox, and three different thicknesses.

Firing the kiln

Firing day began rather foggy but cleared and became breezy, which probably assisted our firing.  We loaded the kiln and were slightly disappointed that our pots failed to fill it entirely. This caused problems eventually with our wattle-and-daub lid as it was unsupported.  Once the thermocouple had been connected we lit a small fire in the firebox and the firing began at around 11am. It seemed to take an awfully long time to raise the temperature. After an hour the thermocouple still hadn't moved. We thought it was broken, although we had checked it--twice. So we dug it out and played about with it, and finally stuck it directly into the firebox, which proved it wasn't broken and we were just paranoid. We put it back into its little hole and waited. Finally it began to move. In our eagerness, we probably took the temperature up too quickly during this initial stage. We could hear pots spalling and exploding at about 100°C.

After a while the sound of dying pots disappeared and we were able to stoke more confidently. 

The constant chopping of wood and stoking kept us fully occupied throughout the firing, and we were happy with our progress until at 400°C we encountered a major problem. 

Our unsupported experimental lid began to collapse and break apart. We removed it, as much as we were able, and used the larger pieces and other pottery shards to cover over the pots.

 

At 920°C we stopped stoking with medium wood and moved to our smallest, driest eucalyptus pieces.

We reached1000°C at around 6:30pm, a cause for great celebration in Northern California. 

At 1000°C lots of black smoke and great gouts of flame began to emanate from the top of the kiln. A visiting professional potter suggested that this might be due to the burning of carbon deposits on the interior walls of the kiln. Sure enough, when the temperature peaked half an hour later, the interior walls were completely clean of their previous black coating.  

Our firing peaked at about 1050°C, and the temperature then began to slip gradually. We stoked for another half hour or so to even out temperature within the kiln and then finished the firing.

During the first hour of cooling we heard the occasional 'ping' of pots cooling too quickly and probably cracking. We made no attempt to slow the cooling of the kiln by blocking the firebox or covering the ware chamber. We left the kiln to cool fully overnight and unloaded at about 11am the next morning - 24hours after firing began.

The State of the Kiln

Once we'd unloaded all the pots we were able to assess how well Stanley I had survived its first firing.

The ware chamber floor was our first worry. The mudstone floor spars had delaminated alarmingly and several had cracked. This had not caused any problems during the first firing as the spars were sufficiently well supported and keyed into the structure.

However we did not want to risk another firing on this floor.

The ware chamber wall had also cracked in many places.

There was no definite structural damage, but another firing might have caused large portions of the internal wall to flake off, exposing the Sonoma Fieldstone core.  This stone would then be exposed to the cool air and possibly explode.

Worst of all was the state of the firebox roof. The wicker support frame had burnt out (as expected) but this had caused segments of the firebox ceiling to collapse, leaving the whole structure greatly weakened.  This persuaded us, after some deliberation, not to fire the kiln again.

We had originally hoped for a glaze firing a few days later, and then intended to fire the damaged kiln just to see how it would cope with a second firing.  However the possibility of the firebox roof collapsing onto a firebox full of fuel burning at 1000°C was too dangerous for the stoker. We didn't want our first season to end with hospital visits.  Stanley I was gracefully retired after a single firing, having produced some lovely pots and taught us a great deal about kiln-building and firing.

The Pots

We were quite chuffed with our haul of pottery. We had a success rate of about 80%, and those that did survive were fairly hard fired and certainly ceramic. Some of the red clay vessels and the few glazed wares showed evidence of partial reduction but this seemed to be a localized phenomenon. We had little control of the atmospheric cycle within the kiln.

Most of our failures were caused by the pots being not entirely dry and our overeager stoking in the early stages of firing. Some were caused by bad building.  A number of Tofa's pots in particular were really rather beautiful, and everyone seemed fairly happy with their results.

In addition we were pleased to learn that the Orton Cone 01 in the centre of the kiln had melted, indicating a localized temperature of about 1110°C.

So: a successful season? We think so.

We built a kiln based loosely on archaeological models (see the Bulletin), made entirely from natural materials, and we fired pots in it to beyond 1000°C.The NCEFG learnt an awful lot very quickly about clay, rock, fuel, stoking, construction and firing, and learnt that we've a long way to go before we can call ourselves expert in any way. 

For next year we have a plethora of ideas to explore and areas to improve on, from construction to recording.

Perhaps most importantly we all enjoyed the season enormously. We're enthused about next season already. Potting and firing under the Californian sun - can you think of a better research project?