Kilns and Kiln Maintenance

Well that was a fun event last weekend! I went to an electric Kiln Maintenance workshop at The Union Project.

The kiln we used for the hands-on exercises was apparently a Skutt KM1231-PK. (My kilns are not by Skutt. I got that model-name from the diagram on the page that came with the new elements we installed and then went online to confirm that!)

It is a big, electric, ceramic-studio kiln (almost 10 cu ft inside: Skutt calls it “Our Tallest Kiln”!). Such big kilns (any brand) heat up and hold the temperature reasonably evenly throughout their entire interior: good enough for most ceramics but not always close enough for the tiny safe range between sintered and melted that we have with metal clays! We tend to use much smaller kilns (often on the order of 0.20 to 0.25 cubic feet, so smaller by a factor of 40 to 50!) to help resolve that issue. Skutt does have a few great little ones, but they still require 240V circuits. Paragon makes ones that run on regular household 120V, so I went with those.

I got my Caldera first, in the winter of 2008-09 (before starting this blog) and set it up at home. I moved my workspace into a studio in 2010; that’s also around the time I got the SC-2 that I now use there. Both are great for firing metal clays! Because the Caldera can go up to 2350°F, I can also use it for mid-fire ceramics; the 2000°F limit on the SC-2 pretty much restricts me to low-fire ceramics in that one. But the front-loading door on the SC-2 makes it much easier to load and unload hot things like metals with enamels or pottery to then be raku-fired.

The little SC-2 did have some issues right at the start, resulting in my learning how to install replacement digital controllers provided under warranty (which turned out, in the end, to not have been the source of the problem!). It has continued to work great ever since that got sorted out. And the Caldera has been great too, despite having been pushed a bit harder by some of the more complex (for metal clay) firings I’ve set it to do.

Which one is better? I’m happy with both! It depends on what you want to do. I feel so lucky that I am able to have two and that they are different! I can pick the right one for certain tasks, when that matters. And when it doesn’t, I can just fire whichever one is located in the most convenient place. If I’m done making pieces in my studio and want to go home, I can take them home and fire there. If I have a lot that have been sitting around for a while, I can fire them in my studio kiln while I start making my next batch.

But while they’ve worked well for ages already, I know that eventually some part(s) or other(s) are going to need to be tended to. So when I saw this being offered I figured it was worth a shot.

The class was interesting: it was aimed at potters, and it began with some discussion of basic firing processes before going on to how various ramp speeds and hold temperatures affect particular glazes. I do some pottery, mostly mid- and high-fire at places where they do all the firing for me. But I’ve always done a bit of low- and mid-fire on my own and (since covid) have been exploring and making more small pieces, so hearing all that was a delightful bonus.

Then we went to work on one of the studio kilns! We basically dismantled the whole thing! We replaced elements. And a thermocouple. And saw how to replace relays (though that didn’t actually need to be done, so it just involved loosening and retightening a few screws). We didn’t have time to replace a couple firebricks along the top, in part because of one small technical glitch (a mis-placed tool), so that topic ended up being another presentation. But by that time I at least was getting kind of tired, felt I’d already learned a lot, so I appreciated getting that information simply as discussion. And there were lots of good handouts too. While I don’t know when I may have to apply all the knowledge I gained, I’m glad to feel prepared when the time comes!

Here are a few fun thoughts resulting from the day:

Question: How many people (students in addition to the instructor) does it take to remove the lid of a big Skutt ceramics kiln tucked into a rather tight corner?! (At this point in my life, I was just glad that half of the class, all much younger, stepped up to help!)

Answer: A lot more than the one person it takes for a little Caldera! Its lid is not connected, so you just pick the little thing up by its handles. Easy! (Removing and replacing that Skutt lid would have been far and away the hardest task of that whole day, and I’m very happy I won’t have to deal with that on mine.)

I usually run this firebrick-construction Caldera kiln on my covered & enclosed back porch. I usually plug it straight into the GFCI outlet right there. I took this photo when I was documenting the need to use an electric-company-supplied surge protector power strip during several weeks when we were urged to use them anywhere we could while they were making repairs. (I should take a better photo, but when not in use that kiln lives just inside the house, by the door you don’t see to the left there and, mid-winter, I’m not hauling it back out just for this post.)

The uppermost three “layers” are the kiln. Counting from the top here, the first and third are the two interchangeable and reversible base and cover layers. The second one down, between those two, is the “functional” middle section with three complete rounds of elements inside and the blue control section you can see towards the back. In this configuration, it can reach 2350°F. (I believe that is the maximum that any firebrick kiln can reach on 120V current. (I’ve seen some quoted as 2400, but I think that’s just a rounding-error / marketing-promotion for what is really more like 2350…) If you are aware of real exceptions, please let me know!)

The lowermost (hollow inside) layer shown here is simply serving as a sort of stand to let air circulate under the kiln. In reality, it is a “bead shelf” that can be moved up one level (so the base is really at the bottom and the “middle” level with the elements is just above this one). Doing that adds another 0.15 cu ft to the firing chamber (for a total of 0.40 cu ft) but drops the maximum temperature to just under 2000°F.

The reason to move the bead shelf up, besides getting room for a third metal clay shelf or a tall (but now low-fire) ceramic piece, is because it has a little hinged opening (on the back as I have it set up here, so not visible in this photo) that you can open to insert and remove glasswork and some other media (certain kinds of enameling, miniature ceramics you want to treat as raku, etc.).

My mind did wander a few times in this weekend’s class as I thought: how many of my entire little metal clay kilns would fit (unusably, but just thinking for fun) inside the Skutt? Well more than I own! I could stack two SC-2s over to one side, and then tuck various arrangements of components from two Calderas in around them. (I confirmed that by looking up the specs…) I could continue to fit more, perhaps even all the pieces that make up a third Caldera, but only if I took it all apart, which is clearly going way too far with that silly but fun thought exercise.

So I’ll just end with a couple photos peeking into my (original blue color) SC-2. The SC part means it was designed specifically for use with Silver Clays. The 2 means it’s the kiln in Paragon’s SC line that is sized to hold up to two shelves with silver clay (though these photos both show it with just one).

The SC kilns are a “muffle” style, not firebrick. (I believe that 2000°F is the maximum temperature for any muffle kiln, regardless of voltage: if you can document that I’m wrong, please let me know!) The elements are stashed behind that inner white material (the muffle), and when those finally fail, you end up having to replace the entire muffle…

The first SC-2 photo shows it at the end of a silver firing cycle. Many pieces are on the shelf; a few others are supported by vermiculite in a small crucible.

(I often leave my annealing pan just sitting on that same cabinet top; it’s not serving any purpose here since I let everything cool before opening the kiln door.)

This last photo shows my SC-2 at the beginning of a “base metal” firing. Those metals (e.g., copper, bronze, steel) need to be fired in a reduction atmosphere. With metal clays, we can easily achieve that even in an electric kiln by completely burying the pieces to be fired in a box or bowl of activated carbon. (I love some reduction glazes, so I sure wish I could do that for them…)

Enough about kilns for today. I hope you found something useful in these ramblings!