Mid Range Wood Firing With Local Clay
The clay I currently use originates from the Coast Range of Northern California. It is primarily the result of decomposed basalt, sedimentary and trace amounts of metamorphic stones. As a result of its high iron, magnesium, and other organic impurities, its firing range is limited to cone 6 in its pure state. I first encountered this deposit coming from a background of using refractory, silica and alumina rich clays of the Virginia Piedmont. These clays easily withstood the intense temperature and reduction atmospheres of high temperature, extended firings. Switching to a less refractory material greatly influenced my choices in firing style. At first, I ruled out wood firing completely, as blends of this clay for a high temperature body limit the native clay content to 20%. I focused on a personally unexplored realm - cone 5 electric, taking advantage of this opportunity to pursue an interest in Japanese ‘Kohiki' / Korean 'Buncheong' ware.
Recently, I completed an experimental, mid range wood firing with this clay. Knowing that its temperature limitations were cone 6 in oxidation, I recognized that I would be not firing for melted ash surfaces. Instead, I wanted to take advantage of the clay's high iron content in the realm of a reduction cooled body of work. I took much inspiration from Japanese 'Nanban' ware, a relatively unresearched style of work produced in the southern island of Tanegashima and surrounding region. Nanban ware owes its qualities to the practices of potters in Southeast Asia, from whom the techniques and firing styles originated. As a general overview, the style of work features an iron rich clay fired to mid range temperatures in wood kilns with varying amounts of reduction and oxidation.
I first came across this style of work after speaking with several artists in Minnesota – Richard Bresnahan, and former apprentices JD Jorgenson and Samuel Johnson. Richard apprenticed with Takashi Nakazato in the Late 70s and has helped fuel an influx of techniques, views, and practices into the American ceramic spectrum via his studio and apprenticeship program in MN. After an afternoon at his studio at St. John's University during which he shared an abundance or technical information regarding the making and firing practices of potters in southern Japan, I felt inspired to investigate this realm of work within the context of my own place, materials, and setting.
All pieces are made from unblended clay from Comptche, CA. Many pieces were coated with a porcelain slip, using kaolin form the foothills of the Sierra Nevada Range. In addition to this white slip, I tested high iron slips made from ball milled sandstone and shale mixed with kaolin. These plastic slips display very fine crazing and crawling where thicker.
Work was bisque fired for safety of loading and handling. Pieces were nestled unglazed within one another and on shelves using rice hulls – an abundant agricultural byproduct in northern CA. Rice hulls are primarily silica, making them perfectly functional as a refractory layer between the work.
The kiln was fired slowly to cone 5 touching in the front and cone 3 bending in the back – a process that required 26 hours. During the end of the firing the back was side stoked with pine to introduce atmosphere and build temperature. At the peak of the firing I sealed all holes and reduction cooled the front to 1700 and the rear to 1650 – taking advantage of the opportunity to observe the effects of reduction in varying temperature ranges. Cooling took 4 hours.
The front of the kiln displayed a more oxidized atmosphere up top – likely coming from the work's close proximity to the door. The front bottom displayed good reduction from the ember bed. Work loaded In the side stoke area received the most color variation within individual pieces from ember contact. Overall, I was most impressed by the color variations and reduction atmosphere maintained in the middle of the kiln – lots of great dusty oranges, black, and maroon.
I was pleased to find subtle surface color effects occurring in areas shielded by rice hulls. Small iron rich stones fluxed beneath the hulls, creating small metallic bronze spots within the heavily reduced clay body. I look forward to finding how this coloration is influenced by the size of the stone impurities and atmosphere conditions of the kiln.
The section of the kiln directly in front of the exit flow was visibly hottest, as is indicated in the significant distortion and melting of the work in this area. While many of these pieces are flatted and bloated beyond traditional use, they display a wonderful record of the limitations of the clay.
In conclusion, I find great references in this body of work to the geology of the coast. Most of the iron rich, mafic rocks of the coast line were formed deep beneath the surface of the ocean in oxygen deprived environments. As a result, the iron is generated in a reduced, black state until pushed above the surface where it oxidizes over time, turning brighter in color. Scanning the coast line reveals a wonderful spectrum between reduced-oxidized iron that relates to the colors emerging from this firing.