Kiln

So. Last week I fired the latest batch o’ Robots and cephalopots.

I used the new clear on the Robots. I’d seen it on some test tiles, and it seemed like a good time to go for a larger-scale experiment[2]. It works… okay. It’s not as actually *clear* as the old one, but it didn’t run or craze. It pinholed slightly in the general studio firing that also happened that week, but that one was underfired slightly, while my kiln actually went a tad over temperature, so no Robots were harmed.

It did some wacky stuff over the purple underglaze. It looks like it reacted with something or other in the formula, and the combo came out looking like a variegated glaze, full of movement and pink specks. I think it may to do with the gerstley borate. Gerstley always gives you a surprise[1]. Here’s a mug with the new clear, and a bowl with the old clear for comparison:

As a side effect of the kiln being overfired, the cephalopots came out glossy. (See overfired mug VS just-right-fired pate dish below.) See those funny triangle jiggies? That’s a cone pack. Cones were invented because temperature-monitoring inside of kilns was not (and often still isn’t) very exact, and depending on your glaze, as little as five or ten degrees difference between firings can make a big difference in the finished look of the same formula. Cones are commercially manufactured doodads which are formulated to melt at specific temperatures. So if the temperature readout on your kiln says you’re at 1190 degrees, but you flip open the peephole and see the cone for 1222 degrees is bending, you know the readout’s lying to you. Or if you don’t have a temp readout, or you know the thermocouples need to be recalibrated, and no portable pyrometer is around (still the most common firing setup), you know when the kiln has hit temp and when you can shut it off.

Your typical glaze firing has three cones in it: the first is the guard cone. When it starts to bend, you know it’s time to start checking the kiln every ten or fifteen minutes, because you’re almost done and you don’t want to mess up your shutoff time. The middle one is the cone for whatever temperature you fire to. In my case, that’s “cone 6”, or 1222-1243-ish Celsius. The last cone is there so that if you overfire, you have some idea by how much. In this case, the firing hit somewhere around 1235-1240. That’s a very ripe cone six or a mild cone seven.

I really like the overfired pots that came out. They weren’t so hot they warped or bubbled or did any of the other nasty things overfired wares can do. But there is a huge difference in the texture and look of the glaze. It’s a little hard to see in the pic, but the overfired pots have a lot of movement and subtle colour changes on their surface. They even have some areas that look like the flashing you get in soda firings, where a few splashes of buttery matte held on. I may end up doing them all like this in future… I don’t know yet.

[1] Not always such a pleasant one, mind…

[2] Less because I was feeling brave and scientifically experimental, but more because there was a jeezly big bucket of it, and not enough of the old clear to make dipping the blissfully mindless job I like it to be.

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