[Clayart] Clayart Digest, Vol 83, Issue 32
vpitelka at dtccom.net
vpitelka at dtccom.net
Fri Oct 28 13:00:01 UTC 2022
Karen Shuler's message reminded me of my first experiences achieving reduction. I learned ceramics at Humboldt State College up in the Redwood Empire in Northern California. All we had was electric kilns and a row of Alpine updrafts, and through most of the program, the Alpines were fired by the kiln techs. In our senior, we each got to fill and fire an Alpine with one other student, and that was the extent of our experience with the gas kilns. After graduating I worked as a mechanic and welder for the City of Arcata, and several years later my wife Linda, son Morgan, and I moved to Blue Lake, five miles inland from Arcata, where the college is located. I bought Daniel Rhodes's kiln book and Fred Olson's kiln book, and set up a studio and built my first kiln. The kiln was a 35-cubic-foot hardbrick downdraft with a barrel arch and four venturi burners entering from the sides, with the flames deflecting off the bagwalls like the one Karen described. There was no flue channel, but I got good results after fine tuning the bagwalls and the firing protocol.
That kiln served me well for a few years, but then I decided to quit the mechanic's job with the City of Arcata and be a potter full time. We built a large pottery studio at the back of our property and I built a 100-cubic-foot downdraft car kiln with six venturi burners - three on either side aimed at bagwalls. On such a deep kiln front-to-back, it could be a challenge achieving even reduction throughout. I had a sense of that, so I incorporated a flue channel. I had met Hank Murrow up in Eugene, and he turned me onto kiln posts for floating set, manufactured by Ferro Refractories. Hank still uses similar stilts in his doorless fiber kiln. The posts are silicon carbide and were expensive, but with the setup I designed, the bottom, middle, and top rows of shelved stayed in place all the time along with the posts, while the other shelves (14x28 sil-car) could be slid in at whatever level was needed. The first layer of shelves was supported on a row of hardbricks down either side of the kiln car, 4 1/2" off the surface of the car. The space beneath the shelves served as the flue channel. It sealed fairly tightly against the flue opening leading to the chimney when the car was rolled all the way into the kiln. There were spaces between the support bricks and the shelves, increasing in width moving farther towards the front of the kiln. The chimney was outside the building and was tall, providing plenty of draft, and of course the draft was greatest at the back of the kiln at the flue opening. The draft would naturally be less moving towards the front, and thus the progressively larger openings between the shelves and the support bricks.
This system worked fantastic. I fired thousands of pots in that kiln, and as long as I was on the ball and remembered to initiate a partial reduction at the right time, I would have absolutely consistent reduction throughout the kiln. If you wish, you can see the kiln at my website. Go to "Gallery" and then "Railroad Stoneware," and scroll down to the studio images and technical information. One image shows a load of bisqueware in the kiln car, and you'll be able to see how the bricks supporting the bottom shelves and the shelves themselves are spaced apart a bit, wider towards the front of the kiln. Please feel free to email me if you have any questions about this kiln.
Potter, Writer, Teacher
Chapel Hill, NC
vpitelka at dtccom.net
From: Clayart <clayart-bounces at lists.clayartworld.com> On Behalf Of Village Lady
Sent: Friday, October 28, 2022 3:03 AM
To: clayart at lists.clayartworld.com
Subject: Re: [Clayart] Clayart Digest, Vol 83, Issue 32
Ps. I should have added my name: Karen Shuler
> On Oct 28, 2022, at 2:59 AM, Village Lady <villagelady10 at gmail.com> wrote:
>> Re achieving reduction of glazes throughout a kiln: The most satisfying for me were achieved consistently in a 30 cu ft sprung arch downdraft kiln I built in about 1982. The earlier kilns I built were all cross draft and worked well but I needed a larger kiln. The most helpful parts of the design of the later kiln - as far as achieving both even reduction and even temperature - were the bagwalls and the flue channel (which is in the floor).
>> There were four naturally aspirated venturi burners fueled with LPG at about 11”wc. The air intake on the burners was, as has been said already, set to produce a clean oxidizing flame, then never changed. The gas pressure was set with a regulator and left where it was. The gas volume was adjusted through the firing with a valve on each burner.
>> Two burners were aimed into each side of the kiln, with the flames hitting a bagwall about 4 or 5 bricks high but perforated, with the arrangement finetuned over many firings. The flames were thus mostly forced up to the arch and then drawn down to the channel in the floor, which is a brick wide and a brick tall and runs the full distance from the front wall of the kiln to the back, where it exits to the flue. Across the channel are laid bricks (some half-bricks), their placement also finetuned over multiple firings to produce evenness in the results.
> The horizontal damper in the vertical flue was adjusted during the firing and was the only control for reduction. The amount of reduction was monitored using a carbon monoxide analyzer, which gives a reading in inverse relationship to the oxygen present. Using the analyzer was cumbersome but extremely helpful, especially during the first firings to know what was happening at various places in the kiln so that the arrangement of the bagwalls and flue channel cover bricks could be finetuned.
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