Because one of the services we offer is kiln repair, we get calls on a daily basis about various kiln mishaps. Some of these inquiries are challenges, which we do our very best to meet; others, however, are totally the type of issues that most kiln owners are able to address themselves, once they know what's going on. That's one of the great things about ceramics, right? You wind up doing or making or fixing all of these things that seem like they might be really difficult, until you actually do them! In the spirit of this sentiment, I've written a quick how-to for troubleshooting a fairly specific, but fairly common problem that we get calls about weekly -- your overfiring manual kiln.
Of course, there are many reasons as to why your manual kiln might be overfiring. You may need a new thermocouple, or you may have a burned-out element that is causing another element to work double-duty, resulting in a kiln hot-spot. However, the most common reason we have encountered is so simple, but so small you might not even think to check on it -- plus, it's a very quick at-home fix, so this certainly can't hurt to try as your first line of inquiry when attempting to fix your overfiring manual kiln by yourself.
Above, you see a pretty typical kiln sitter -- a Dawson LT-3, which is a very common model found on older kilns. Notice how the switch is in the 'down' position -- that's the position that switch will most likely be in for 99% of its existence. And that's fine! However, that is specifically what can lead to your kiln overfiring over time.
The reason for that is insidious gravity. Over time, that little metal plate -- the one that your kiln sitter switch latches on to when you set your sitter cone -- slides down in minuscule, imperceptible increments.
This means, of course, that when your switch is flipped into the 'UP' position -- that is, when a sitter cone is in place and you're firing your kiln -- that plate has actually migrated UP too far. The higher the plate, the more your sitter cone needs to melt to allow the latch to release the switch, turning your kiln off.
This fix is easy. Just using a teeny tiny Allen wrench, loosen the bold on the front of the switch, and with the switch in the UP position, slide the plate down very slightly, like a couple millimeters. Keep in mind the delicate nature of kiln calibration -- a very small distance can make a big difference! At this point in the fix, you can mark the front of the plate with a mechanical pencil to note the position it is in. Then, if you are able to do a test firing with cones -- say, pop a few 06's into a kiln full of bisque work, as bisque temperatures have a little more wiggle room in terms of workable firing temperatures -- you can compare your plate's position with the 'true' heat inside of the kiln. You may need to fiddle with the plate yet again, but once you have it marked and the calibration seems good, you will find this easier to fix in the future, and something you can keep an eye on over time.
If you need a new kiln sitter -- or any part! -- for your kiln, check out our website's kiln part section here. We should have what you need! And if we don't, just give us a call at 215-427-9665 and we can find it for you.