[Clayart] Clayart Digest, Vol 83, Issue 34
villagelady10 at gmail.com
Sat Oct 29 17:08:32 UTC 2022
Vince and Mel make such valuable points here. I would not have thought of creating a flue channel with a false floor (the contiguous layer of shelves on 4 1/2” bricks); brilliant!
Springing from Mel’s emphasis on the balance of the influences in a kiln on the flow of fuel/air comes this idea: If a fan were placed at the top of the flue (blowing out) and the kiln stacked with video devices all over it, then the kiln closed normally,smoke could be introduced through the burner ports (each separately, then all together) to capture the movement through the kiln.
Even though without heat the air movement might be different, I think such an exercise could be very instructive - both about a specific kiln (e.g. revealing stagnant corners unlikely to get much reduction) and for students to learn what is happening convectively in a kiln (including in non-fuel-burning ones, as those have air movement as well).
> On Oct 29, 2022, at 8:03 AM, clayart-request at lists.clayartworld.com wrote:
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> Today's Topics:
> 1. reduction and balance (mel jacobson)
> 2. Re: Clayart Digest, Vol 83, Issue 32 (vpitelka at dtccom.net)
> 3. Olympic DD-12 kiln (Wayne Rives)
> 4. Microcristalline glazes (Edouard Bastarache)
> Message: 1
> Date: Fri, 28 Oct 2022 14:10:25 +0200
> From: mel jacobson <melpots at mail.com>
> To: clay art <clayart at lists.clayartworld.com>
> Subject: [Clayart] reduction and balance
> <trinity-8f5f2550-d42e-4f8a-8ccb-b44a7f56c499-1666959025929 at 3c-app-mailcom-lxa12>
> Content-Type: text/plain; charset=UTF-8
> last note from a clayarter on kilns.
> Thank you for that great description.
> I would like to add that if a kiln is
> built with perfect balance, said a different
> way..."all the elements, stack, flue, gas pressure
> and the stacking of shelves is done right...the kiln
> fires time and again perfectly."
> I remember years back when I said that firing a kiln was
> like a dance...gas pressure, perfect flue, a stack the perfect
> size and length and a damper that controlled it all. If you are
> stepping on your partners feet, something is wrong.
> Heat dams are always a problem. Those are created by the potter.
> Tight shelves, shelves in front of the flue, too many pots in the
> firing closes everything down.
> I have always used a flame gauge. How much back pressure is coming from
> the kiln. You can close the damper, increase gas pressure and achieve
> reduction. If the kiln inside is clear, and you can see the back wall,
> you more than likely have no reduction. The flame coming from your
> favorite peep hole will tell you much. I suggest for modest reduction
> about three to four inches of soft flame from the peep. But again, it all
> depends on how your kiln is configured.
> I have been hired often to make a kiln work. It is amazing what I find.
> Back pressure of 20 inches, (like a jet engine screaming)
> so many pots in the kiln there is no room
> for the heat to get up the stack. And the best is the 20 hour firing in
> 30 cube kiln. And the craziest is the stack of old brick with holes all over
> for a stack. And my final thought was always..."they would hit you in the
> head with a 2x4 if you threw a beer can in the garbage, but spew out millions
> of BTU's of heat into the atmosphere and never be concerned. "
> website: www.melpots.com
> Message: 2
> Date: Fri, 28 Oct 2022 09:00:01 -0400
> From: <vpitelka at dtccom.net>
> To: "'Clayart international pottery discussion forum'"
> <clayart at lists.clayartworld.com>
> Subject: Re: [Clayart] Clayart Digest, Vol 83, Issue 32
> Message-ID: <email@example.com>
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> 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, bu
> t 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 sid
> e 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.
> - Vince
> Vince Pitelka
> Potter, Writer, Teacher
> Chapel Hill, NC
> vpitelka at dtccom.net
> -----Original Message-----
> 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.
> Message: 3
> Date: Fri, 28 Oct 2022 16:43:16 -0400
> From: Wayne Rives <wrives43 at gmail.com>
> To: clayart at lists.clayartworld.com
> Subject: [Clayart] Olympic DD-12 kiln
> Message-ID: <F622A310-6B35-4244-B341-F5464B3918A5 at gmail.com>
> Content-Type: text/plain; charset=utf-8
> David, et al,
> This kiln has 6 Venturi burners positioned under the kiln floor, three on each side, and uses propane.
> I have followed Olympic?s suggestions starting the kiln with the burner shutters open 1/2 inch, closing them completely when cone 011 goes down.
> I then use the damper to initiate reduction, closing it to the degree that causes some smoke and flame from the upper and lower observation ports, which are quite small.
> I follow Mel?s recommendations concerning firing down, raising the temperature, and then firing down a second time. I always get good reduction in the middle area of the kiln, occasionally in the upper areas, and rarely on the bottom shelf.
> The bottom shelf is elevated 3 inches per Olympic?s recommendation. I have reduced the flu opening to approximately 35 square inches per the recommendation in one of my kiln books.
> Thanks to Vince?s input I see that I have been doing the wrong thing by closing the shutters on the burners. I will correct that issue forthwith! Thank you Vince. This is my first kiln with Venturi burners. My others had power burners.
> I will also try setting the kiln a little tighter than I have before to see if that helps as well. And thank you David for the kind words about my painting displayed on the Artspace web site.
> Sent from my iPad
> Message: 4
> Date: Fri, 28 Oct 2022 22:49:24 -0400
> From: "Edouard Bastarache" <edouardb at colba.net>
> To: "Clayart" <clayart at lists.clayartworld.com>
> Subject: [Clayart] Microcristalline glazes
> Message-ID: <000001d8eb41$0e9d48a0$2bd7d9e0$@net>
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