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My Adventures Firing an Electric Kiln to Cone 10
If you are purchasing or connecting an electric kiln for the first time, this blog is a must read. We all know the wiring to a kiln is important, but everyone tries to get away with the electrical system in place. It won't work. Every electric kiln has a metal plate on it that outlines the exact electrical requirements. Even the smallest variance will prevent your kiln from working properly. Here is how I learned about this first hand. -Dawn
Mar/09/2005 15:30 Originally Posted on:
After firing a huge gas downdraft kiln for the past 5 years, the poor old thing was condemned and the school purchased two manufactured kilns: a gas updraft and a computerized electric. Even with years of firing experience, starting a new kiln for the first time is nerve wracking, but hopefully educational, so I am blogging the experience here. The gas updraft experience can be found in a separate post “Updraft Gas Kiln to Cone 10: A Firing Blog" which will start March 17th-ish.
As in any classrrom situation, time and money are scarce. I was not able to create a large selection of glazes specifically for the new kilns. We had the following leftovers from last quarter: Bright Green (recipe is posted in the Glaze Forum: Bright Green Cone 10 Glazes), B6 (a pale blue), Shaner’s Shino and Colemanite.
The kiln was loaded on a mild, dry day. Instructions for loading were followed carefully including more space than I would normally leave between the pieces and the shelves. (In classroom work you tend to pack it tight.)
Once loaded, the computer took over. I simply punched in Cone 10, standard heat increases w/ a 30 min hold at top temperature and 100 degree/hr cooling time.
That seemed too easy, so I checked it every 5 minutes for a while. Press “Check Current.” The computer tells me it is in preheat and the current temperature of 51 degrees. After pressing the check button for the 10th time in an hour, the computer somehow portrays annoyance at me, so I leave it alone. After three hours of slow heat increases during pre-heat with the lid braced open an inch to allow steam to escape, I lower the lid. The temperature increases 360 degrees in the next hour, exactly as programmed. I almost faint.
Although I had grown attached to our grand old downdraft kiln, it is really hard to pine for a kiln that required constant adjustment and attention and took 24 hours to fire. It was an excellent experience that taught me a lot. But holy smokes computers are cool!
When firing a kiln, it should never be left alone. I know this, but I did it anyway.... Sometime in the middle of the night, the kiln shut off. The computer tells me it was due to power problems. Not a complete power outage, but the kiln could not heat at the rate programmed and never reached temperature. Based on the cones I had in the kiln, it passed cone 06, but got no where near cone 9. Based on the appearance of the glaze on the work, I am guessing that we did not even reach cone 1. I tried to keep the cursing and breaking things to a minimum, then gathered my thoughts. At least with such a low temperature shut off, there should be no problem with just trying again. I started by checking into the power problem. I teach at Pioneer Craft House. Many of the buidings are 100 years old and I think the power is too. Sure enough, the breaker in the kiln room is 120, but the power to that breaker runs through a breaker in the next buiding over which is a 50. In addition, the electric heater in the other buiding is on the same power line. I suspect the heater drew enough power from the line to cause a "brown out" while not tripping the breakers. Its a working theory, anyway. We shut off the heater, lights and everything switch we could find in the other building. I looked at the preset heating plan for cone 10 on the computer. It includes heating at 150-160 degrees per hour from 1112 degrees to 2129 degrees and then 108 degrees an hour thereafter until cone 10 at 2345 degrees. I know that people increase their temperature at that rate all the time. That is why the cone to temperature charts all have a column of numbers for 108 degree temperature increases. However, I have never had that rapid an increase at the end of a fire. It is very difficult and takes a lot of fuel or power to increase that quickly at high temperatures. I always use 50-60 degrees an hour after 1600 and 27 degrees per hour at the end of the firing. My theory is that a slower increase like that will require lower power needs and, therefore, may be possible with our power problems. So I programmed a custom firing like this: STEP 1: 500 degrees per hour to 1022 degrees. (Hey, there is no water in this kiln after being fired past cone 06. I might as well get a quick start. Plus, at the start I am watching the kiln, if a breaker blows I am there to fix it. STEP 2: 180 degrees per hour to 1830 degrees STEP 3: 55 degrees an hour to 2230 degrees STEP 4: 27 degrees an hour to 2284 (cone 10) then hold for 1 hour. STEP 5: Slow cool, not allowing teperature decrease of more than 150 degrees per hour to 1000 degrees, then off. Should take 17 hours to fire this program. I would like to say I sat next to the kiln and watched it the whole way, but I didnt. Keep your fingers crossed. More tomorrow....
When I arrived to check the kiln, the power was off. Grumble Grumble Grumble. The breakers had been tripped. But once the power was restored, I regained my love for the computer system on this kiln. It dutifully reported that they kiln had reached 1560 degrees after 11.7 hours but because it could not continue to rise at 180 degrees per hour as programmed, it shut off with a "fire too long" error. The breakers had nothing to do with it. Note, I could have had this type of information for yesterday's shut off, but I pressed a button on the computer deleting the info before I understood what it was saying. Hey, at least I learn from my mistakes, occasionally. Consulting the manual I found that low voltage is the most common problem to cause this error and, sure enough, our friend the electric heater in the other building had been turned back on. An excellent custodian came to my rescue. He completely disabled the heater so it could not be turned on. We then started checking the actual power measurements going to the kiln. Calculations revealed the kiln is receiving about 11,000 watts. The kiln specifically lists that it requires 14,400 watts. So, no wonder we are having problems. Lesson learned (for the 100th time): professionals such as electritricians, ventalation specialists, etc. are not potters and haven't a clue about pottery equiptment needs. Because the kiln was still at 1000 degrees, I hated to see it go to waste. So, just for fun, I set the kiln at a very slow temperature increase. 75 degres an hour to 1800 then 50 degrees and hour until the last 2 hours then 27 degrees an hour to cone 10. I then set the delay (again, I love this computer system) so that the kiln would not start until everyone had left the school. Other than a few security lights, nothing will be drawing power from the kiln. Maybe we can squeek by. More tomorrow....
Well, 20 hours later, the kiln is still climbing slowly but surely. I fiddled with it a bit, but nothing major. It will finish tonight, but the school is closed for the weekend. Results on Tuesday....
No luck. The kiln simply does not have the power it needs to fire. A work order has been requested to rewire the kiln, but who knows how long that will take in a public school....
I learned one major lesson in this: non potters cannot believe how difficult it is to create pottery. I have been told repeatedly throughout this process that "11,000 watts should be enough" and "why would a kiln need so much power?" People seem to think I am firing salt dough in my oven.
If you are installing an electric kiln for high fire, discuss the kiln with the electrician. Make him look at the specs on the kiln and get assurances that the wiring will meet or exceed those needs. (Just like any electrical appliance the amps, volts and resulting watts required are attached directly to the kiln on a metal plate.) In addition, the manual on this kiln notes that if the wiring is longer than 50 feet, it must be a larger gauge. The note is so burried in the manual, that I am sure no one would have noticed it if we were not searching so strenuously for answers.
At least we never got over 1500 degrees so no harm was done to the work or the glazes in the kiln. I will move them over to the new gas fire kiln as we light that one for the first time. This saga, hopefully, has a happy glaze ending under that post, "Updraft Gas Kiln to Cone 10: A Firing Blog".