Firing the Kiln
Busy today firing the pottery kiln, so will not be able to post a plethora of articles at multiple websites.
My wife is the Ceramic Artiste [here]; I am the Kiln Master. The kiln, which I built, is about 5 feet by 4 feet and 6 feet tall. The loadable space is 14.5 cubic feet. The kiln walls are 8 inches of K-26 refractory bricks with a 1 inch outer layer of refractory ceramic fiber board. It has a “Wisconsin flat roof” design, with the roof bricks held up by stainless steel rods wired with titanium high temp wire to steel angle irons supported by the exterior steel frame. It is a downdraft kiln, employing a venturi chimney damper with a maximum aperature of 44 square inches. The kiln is fired with propane under pressure using two venturi burners from Ward Burner Systems [here].
Today’s firing is Cone 6, 1222 degrees C or 2232 degrees F. That’s hot enough to melt almost anything, although a little below the melting point of steel. It will soften steel, however, and cause it to bend easily, which is why the steel frame is exterior to all the insulating bricks.
I follow a kiln schedule of my own devising. Since the pottery is already bisqued, I can run the temp up a little faster than when firing greenware. Generally I shoot for an increase of 250 to 300 degrees F per hour, but slow down to about 100 degrees per hour for the last two hours. Between 1150 and 1350 degrees F the rate of temperature rise naturally slows as the silica in the clay goes through some isomeric transformations. To get the temp to rise at the proper rate I need to monitor the kiln all day and increase the gas pressure every hour or so. I start at about 1 psi and finish at 5 or 6 psi, split between the two burners. I could reduce the gas needed by manipulating the damper but the Artiste prefers the bright glaze colors that come from oxidative firings. If the damper is employed to boost the temps, there is a danger of reductive (oxygen poor) conditions which can mute the colors. Some people prefer reduction firing; we don’t.
The Artiste also prefers soda firing, which requires the introduction of sodium bicarbonate in saturated solution late in the firing. The sodium reacts with the silica to alter glazes and to form it’s own glaze on unglazed ware. Soda firing is an alternative to salt firing, which was originally discovered in 14th century Germany. It is quite the process to spray a mist of soda through spray holes into a kiln when the temps are over 1200 degrees C.
Thankfully (IMHO) today’s firing is not a soda firing. The firing process is much simpler, and no spray holes need to be opened at extreme temperatures with flames licking out due to the propane being blown in under pressure. Today’s non-soda firing did, however, require a complete interior scraping of the kiln, kiln shelves, and supports to (hopefully) eliminate any residual soda. That took all day yesterday.
That’s it for now. Back to the kiln for monitoring and pressure adjustment. As the temp builds and the pressure increases, the burners roar. The noise can be almost deafening. Luckily we are in a rural farming neighborhood and all the neighbors make noise with tractors, guns, and other machinery, and the houses are far apart, so nobody is particularly disturbed. They are curious, however, and inquiries generally result in the gifting of mugs and bowls (the freebies are seconds, but everybody is happy about it, giftees and Artiste alike).
PS — the Artiste departed to babysit the grandson, so it’s just me, the dog, and the kiln today. The Kiln Master is an artiste in his own right, IMHO, though not generally recognized as such.
Have kiln, will fire.
Once you obtain or build yourself a kiln, the possibilities for making various objects are almost limitless. There are dozens of books out there on how to do it and they all have something to offer, but my favorite turned out to be The Art of Firing by Nils Lou [here].
Up to 1350 F and the interior is starting to glow orange. Eventually it will get white hot and so bright that a welder’s mask will be required to see the indicator cones.
by bear bait
My experiences are all “fire in the hole!” and then a big boom… although I have set some real raku slash burns. Too bad we didn’t have some pots in the slash piles.
I recall a 7 mile road we built for the BLM. During a pre-final inspection the BLM road inspector found a stump about 60″ that was 6″ into the toe of the fill. Had to go. So OK. So me and the outfit’s owner’s son found the key to the powder house, got a couple of bags of fertilizer and a box of stumping, some caps, wire, and the hell box. I spudded under the stump in two places, put a half stick in each, and stood on the stump and blew the powder holes under it. Then we figured it was a stick to the inch and we had a few extra, so they went in too. And two bags of ammonium nitrate soaked in diesel. A couple of sticks with electric caps, wired them into the mainline, and unrolled the wire. Hmmmm. Not as much wire as I would have wanted. No problem. We just drove the water truck up to where we could be under it when we fired the shot…
“Fire in the hole!” Hang fire. A second time: “Fire in the hole!” And twisted the handle. Holy Crap!!! I saw about a third of the stump clear the ridge to the right by a hundred feet. It seemed like an eternity, but knowing better we stayed on our bellies under the water wagon to wait out flying things that would sooner or later come to earth. And then Blam!, a chunk of sandstone hit the top of the tank, which was rounded and didn’t suffer any real noticeable damage. And then there were the footfalls of other debris hitting the ground. Finally, we determined it was safe enough to go look at our handy work. We had a direction charge under the stump and wanted to blow it away from the road. That we had done. Now there was no stump in the toe of the road, and instead, a 3 foot deep hole. No stump. Not even a visible root. Gone! We got a couple of fire shovels and a McLeod and sort of raked, shoveled, and smoothed out the area. Strategically moved some of the erosion control straw in the fill, and viola’!, we had lost the stump and saved the road. My compatriot’s father got a bill from the owner of the water wagon for the damages. But we sold the road a day later, and that meant we could log at last. The old man thought the damage bill was a hoot, and like he was wont to do, never paid it. Might not have paid for the rent either.
Two days later my friend went to Viet Nam to work for US-AID, and was there over two years, including the deadly Tet offensive year. I ended up logging in Alaska, and my closest encounter with the military was at SeaTac, waiting for a plane south, in late fall, having not had a haircut nor having shaved since the 4th of July hiatus. Black Bear wool mackinaw, watch cap, stagged off pants, hickory shirt, and navy blue collared, V neck sweatshirt, and romeos. My uniform for the rest of my life. Some Army guys going overseas thought I might be a hippie, and made disparaging remarks. I moved to another area of the waiting area. “Fire in the kiln.” A much more constructive use of rapid oxidation.
My kiln is fueled by propane, but there are purists who fire with wood (the Anagama method). Wood-fire kilns are no joke; they achieve cone 10 (2450 degrees F) and higher.
Which should give you an idea how hot forest fires can get. Pile up a couple of hundred tons per acre of course woody debris and set it on fire, and you can achieve cone 10 temps quite easily.
Forest fires in heavy fuels can melt the soil, cook it good and deep, and glaze it like pottery. That treatment effectively sterilizes the soil and renders it impervious to rain and snow melt. The precipitation runs off rather than percolating, causing erosion and failing to recharge ground water. A domino effect of death and destruction to terrestrial and aquatic life follows.
The Rattle Fire last summer was a good example of a heavy fuel burn that reached cone 10 and cooked and glazed the soil.
The “greens” love to burn old-growth because it not only kills the trees, it renders the site incapable of growing any forest for hundreds, perhaps thousands of years.
I don’t know why people call radical pro-holocausters “green.” It is an extreme misnomer, an oxymoron, because the morons are completely opposed to vegetable life forms, especially forests.
Ahhhh, such memories!! In high school, I was very much into ceramics. My senior year I had 2 classes and was a teacher’s assistant in both. Each class had a different teacher and they certainly had some hefty clashes. She had the tenure and he was new that year. She was a traditional kind of teacher and he was progressive. Of course, I always got caught in the middle. I was pretty darn good at throwing pots on the wheel. We had 2 high-fire kilns and a small electric kiln. Us students were very limited in dealing with the kilns, for safety reasons.
I’m thinking of getting back into clay, now that I’m semi-retired due to disability. I have an idea to throw some pieces and warp them into picture frames to pair up with my photos. The place we got our school clay from is just down the road from me.