Down-draft Woodfired Salt Glaze Kiln
When I started contemplating building my own kiln, I knew I wanted to fire with wood, and salt glaze. In school, I never took an interest in kiln building. East Carolina University undergraduate studies found me using electric kilns to fire my work; the one of a kind covered containers I was making were painted with underglazes, then lead glazed. The oxidizing atmosphere of the electric kilns benefited these glaze types. In graduate school, at the University of Iowa, my kiln requirements were more physical than anything else. Due to the size of my architectonic sculptures, I had to beable to forklift them into the kiln. The gas kilns were large enough, and, although surface treatment was approached emulating various brick surfaces, how the kiln was fired wasn't of much concern. While at the University of Iowa ,I was exposed to woodkilns. One of my professors, Chuck Hindes, was (and still is) very much into woodfiring, but I was uninterested.
I became very interested in North Carolina's pottery heritage upon moving into the eastern Piedmont region of North Carolina (Seagrove, NC vicinity) . Woodfire and salt glaze was this region's tradition until modern times. Several potteries in the area had wood kilns, and still salt glazed. I had the good fortune to work at the Ben Owen Pottery, with Ben III, for nearly four years. We utilized a woodburning Groundhog kiln, with an attatched catenary chamber for salt glazing. There was also a woodburning Olsen "fastfire" kiln, and a gas-fired sprung arch salt glaze kiln.
When I began planning to design my own wood kiln for salt glazing, I knew what I didn't want : a kiln that was too large to fire without having a crew, or one that would take a lot of work to fill. Also, one that didn't require loading, laying on your side, like the Groundhog. I had been impressed with the ease of loading and firing the Fastfire kiln, so I decided on that type of plan. Never having designed and built a kiln from start to finish, I reaquainted myself with my shool textbooks, Fred Olsen's "The Kiln Book", and Daniel Rhodes' "Kilns". I knew there would need to be alterations to Olsen's design to accomodate salt glazing.
First order of buisness was to figure out the size of the ware chamber. I planned to use kiln shelves that measured 14" x 28", two deep, for a 28" square area of stacking. Also allowed was space between the shelves and the front and back walls. To the sides of the shelves would need to be space for the dumping of salt. Many people introduce the salt through the fireboxes. I decided against this in order to reduce more rapid deterioration of my metal fire grates than would be already brought about by extreme heat. Configured into this space was also the bag wall placement and the point of entry of the fire, from the firebox, into the chamber. Too, I figured in how high I wanted to stack my ware, and about how high the arch would be. The final interior measurement was to be approximately 58", side to side, and 38", front to back. The height , excluding the arch, would be 38".
Prior to beginning kiln construction I erected a skeletal shed and determined where the chimney of the kiln would be by hanging a plumb bob down to the ground. From that I could figure my kiln layout. In the initial layout of my kiln, I dug an outline trench to place treated lumber that would hold sand for leveling the kiln pad.
The first variance I decided on was to replace the chamber's interior insulating softbrick with hard firebrick. I was able to procure a couple pallets of new super duty brick, and a pallet of new medium duty, for a very good price. I laid out the supporting base for kiln and chimney, framed with treated 2" x 6" lumber, with salvaged refractory block, leveled in sand. On top of that I put a layer of medium duty hardbrick, followed by a layer of super duty. This top layer would be the floor of the fireboxes.
Next I laid up the walls of the fireboxes with the super duty brick. This kiln plan utilizes kiln shelves for the roof of the firebox, which also are the floor of the ware chamber. Olsen extended shelves to the edge of the firebox and built the chamber on top. I felt that if the shelves cracked and needed to be replaced, it would be better to design so they could be removed easily. Therefore, on the top header row, I indented the bricks, on the front and back, inward one inch. This would provide a ledge for the shelves to sit on inside the walls of the chamber.
