Black-on White Trench Kiln Firing – Bob Casias

This year’s conference, held in Tijeras, New Mexico, enjoyed excellent firing conditions and the trench kiln firing produced quite a number of well fired pieces with distinct black-on-white designs produced. Approximately 45 pieces from various artist were fired. This volume of pottery required that length of the kiln be expanded to about 2.5 meters. This was a large kiln and was attended by three potters, Mike Savage, Bob Casias and Cherylene Caver. The pre-fire was ignited early in the morning and by noon the coal bed was well established and ready for loading.

The coals produced in the pre-fire allow for additional drying and warming of pots to try and avoid heat shock and spalling. There was only one casualty in the loading of pots that resulted from a damp sandstone, used for kiln furniture, exploding into small pieces and breaking a seed jar that had been placed on it. Ample juniper spanners and additional fuel were gathered in nearby forests as well as wood that was donated by local residents.

Cover sherds serve the purpose of protecting the green ware from the intense heat wave produced in the secondary fire but more importantly, to create a microenvironment within the kiln that maintains a neutral atmosphere for the organic paint to reduce within and produce the signature black coloring.

The size of spanners is important. If they are too small in diameter they burn out and the secondary fire collapses early and smothers the setting, not allowing a clearing of the free carbon in the atmosphere. If this happens the pots stay covered in black smudgy carbon but if the fire stays elevated a free flow of heat ventilates throughout the setting clearing this carbon coating.

I use split juniper for the firing of my pottery, as this wood burns hot and cleaner than more resinous woods like pinon and ponderosa pine. I have discussed the fuel model with my friend and colleague, Rod Swenson, and I have come to agree with him that the volume of woody mass used for this firing, could not have been sustainable, nor practical, in ancient times. Given the scarcity of wood overall in the Pueblo II-III environment, it seems much more likely that smaller diameter wood, piled densely around the setting, could produce enough sustained heat rise to fire the carbonaceous clay to a state that when thumped, pings like a bell. Experiments will be conducted in the coming year, which will embrace not only the small fuel size theory, but also the absence of covering soil to quench the fire, as a means to stop oxidation, as performed in this firing at Tijeras.

The peak temperatures, which you hope are around 950 C, are reached within the first 30 minutes of firing. After about 90 minutes the secondary firing is covered with soil and pots are set to heat sink, with a reduced oxygen atmosphere, until excavated later. This model of cover soil also is a point of departure from what ancient people may have done to achieve the reduction needed for white color in the montmorillonite clay to come forward. Again, Rod Swenson has demonstrated that the ash cover of smaller diameter, more condensed, wood piles serves the purpose of keeping the pots isolated from a full oxidized atmosphere. This exercise too seems outside what ancient people would have done given the scarcity of cover soil, effort to excavate and save for reuse, and ability to achieve black-on-white results without covering the setting. This theory too will be tested for reporting at next year’s South West Kiln Conference.

Carefully removing the soil from the setting reveals that the cover sherds are all of a very light coloring and little carbon was left on the surface of the sherds, nor the pottery.

This firing, and this model, produce the classic black-on-white pottery of the ancient southwest potters. The replication of source materials such as the carbonaceous and montmorillonite clays, coupled with the use of rocky mountain bee plant paint for design application, allow for good look a like replications.

A good time was had by all!