Kiln me softly
Nov. 20th, 2014 09:49 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
offcntrMy bisque firings are boringly predictable. Two to four hours on low, depending on how dry the pots are, first with the lid open, then propped ajar on a brick. Two hours on medium. Turn all three switches to high, close the lid, and in 6-8 hours, the kiln sitter will drop and shut things down. Just to be safe, I set the limit timer to 10 hours, to turn off the power in case the kiln sitter fails. Boring, predictable, 110 kwh, more or less.
So I was not pleased to find the kiln still firing, peeps glowing orange, with about a half hour left on the timer when I got up for breakfast the other morning. I wiggled the kiln sitter and it promptly dropped, leaving me with two possibilities. Either the cone holder jammed, giving me (potentially) over-fired pots, or one of more of the elements failed, giving me (possibly) under-fired pots. The kiln color told me I was at least in the neighborhood of cone 06, but that was all I could tell until things cooled down.
The next morning, kiln cooled and unloaded, it was time for diagnosis. The cone in the kiln sitter was bent, which was reasonably good news; at least the pots made temperature. My kiln is a stacking model, with three rings of brick. Each ring has a three-heat switch sending power to two elements. That's six ways things could have gone wrong. I need to narrow down my choices.
The first step is the match test. I turn all of the switches off, then pop the kiln sitter back on. Turn one switch to high (I start with the bottom ring), wait a minute, then with a pair of rubber-handled pliers, touch the tip of a wooden match to the bottom element. Pop! it lights. Blow it out, try the next one. It also lights. Turn off the bottom ring, turn on the middle one, try again. Five matches later, I learn that the top ring, lower element isn't heating like the rest. I resist my first impulse, which is to touch it with my finger to see if it's warm. Bad idea. Plus, electricity.
Instead, I go into the kitchen for my instant-read thermometer. It tells me the element is a little above room temperature, around 90°. By comparison, the top element shoots up past 110° and is still climbing. I shut the kiln down and let it cool.
A circuit breaker switch and four sheet metal screws later, I'm inside the electric box. All of the power leads are intact, and my circuit tester says the element isn't broken. What's going on?
I take a closer look at the switch housing. One of the power leads looks a little hot, the crimp-on connector corroded. Also, the bakelite of the switch looks a little, well, baked. I know this one!
High-temperature kiln wire is twisted from multiple strands, then insulated with woven fiberglass. A tube connector crimps it on to the end of the element, a spade or ring is crimped on to the other end, which is then screwed into the switch.
Over time, the individual strands of wire begin to oxidize, corrode. They make less contact with each other and with the crimped connector. Less contact means more resistance, heat, hence more oxidation. Eventually, the thing will melt through with a zap! and I need to clip the wire off, crimp on a new connector, and probably replace the burned out switch.
In this case, I caught it just before it went critical. I'll need to replace the wire--not enough slack to cut it short and re-crimp--and although the switch isn't actually broken, I don't like the look of that bakelite. Think I'd better swing by the ceramic supply and pick up a new one.
So I was not pleased to find the kiln still firing, peeps glowing orange, with about a half hour left on the timer when I got up for breakfast the other morning. I wiggled the kiln sitter and it promptly dropped, leaving me with two possibilities. Either the cone holder jammed, giving me (potentially) over-fired pots, or one of more of the elements failed, giving me (possibly) under-fired pots. The kiln color told me I was at least in the neighborhood of cone 06, but that was all I could tell until things cooled down.
The next morning, kiln cooled and unloaded, it was time for diagnosis. The cone in the kiln sitter was bent, which was reasonably good news; at least the pots made temperature. My kiln is a stacking model, with three rings of brick. Each ring has a three-heat switch sending power to two elements. That's six ways things could have gone wrong. I need to narrow down my choices.
The first step is the match test. I turn all of the switches off, then pop the kiln sitter back on. Turn one switch to high (I start with the bottom ring), wait a minute, then with a pair of rubber-handled pliers, touch the tip of a wooden match to the bottom element. Pop! it lights. Blow it out, try the next one. It also lights. Turn off the bottom ring, turn on the middle one, try again. Five matches later, I learn that the top ring, lower element isn't heating like the rest. I resist my first impulse, which is to touch it with my finger to see if it's warm. Bad idea. Plus, electricity.
Instead, I go into the kitchen for my instant-read thermometer. It tells me the element is a little above room temperature, around 90°. By comparison, the top element shoots up past 110° and is still climbing. I shut the kiln down and let it cool.
A circuit breaker switch and four sheet metal screws later, I'm inside the electric box. All of the power leads are intact, and my circuit tester says the element isn't broken. What's going on?
I take a closer look at the switch housing. One of the power leads looks a little hot, the crimp-on connector corroded. Also, the bakelite of the switch looks a little, well, baked. I know this one!
High-temperature kiln wire is twisted from multiple strands, then insulated with woven fiberglass. A tube connector crimps it on to the end of the element, a spade or ring is crimped on to the other end, which is then screwed into the switch.
Over time, the individual strands of wire begin to oxidize, corrode. They make less contact with each other and with the crimped connector. Less contact means more resistance, heat, hence more oxidation. Eventually, the thing will melt through with a zap! and I need to clip the wire off, crimp on a new connector, and probably replace the burned out switch.
In this case, I caught it just before it went critical. I'll need to replace the wire--not enough slack to cut it short and re-crimp--and although the switch isn't actually broken, I don't like the look of that bakelite. Think I'd better swing by the ceramic supply and pick up a new one.