Shino tests

I'm teaching a glazing class at Claymakers, and we're focusing on American carbon-trap shino glazes fired to cone 6-7 in Claymakers' gas kiln. Carbon trapping happens thanks to soda ash dissolved in the glazes. As a glazed pot dries, soda migrates to the surface and crystallizes. These crystals can bind with soot that's produced by the reduction (low oxygen) atmosphere in the kiln, yielding smoky grays and blacks, on top of the whites and oranges typical of shinos. Wax resist swooshed onto the drying glaze prevents crystals from forming where the wax is, but can also encourage crystal formation along the edges of the waxed areas.

Here are some of my pots from our first firing; we'll fire again next week.

Malcolm Davis Shino with 6% Redart. The swoosh and dots are from wax resist.
The glaze was thick, and the pot was in a heavily reduced spot in the kiln.

Three salt rocks: Dresang Shino, Malcolm's Shino w/Redart and Carbon Trap Shino.
The swooshes, dots, and squiggles are all thanks to wax resist.

Cadogan teapot (a lidless teapot that fills from underneath, like a salt rock
with a spout); Malcolm's Shino w/Redart, wax resist. The pot was next to the jar in
the kiln, so well reduced, but the glaze was thinner--thus not as intensely black.

Reduction

I've been getting satisfying results with sprayed glazes, and the process has been helping me really appreciate my electric kiln--to the point that (hard to believe) I've been declining occasional access to soda and wood firings. But every once in a while, I glaze a pot for the gas kiln at Claymakers, and the results make me covet reduction firing again. Case in point: I really like the carbon-trap halos around the wax-resist dots on the shino-glazed bottle below. On one side of the bottle, the halos are black; on the other side, they're orange. (The bottle started out as a bottle demo, then turned into a darting demo. I'm not sure why I stuck a horn on it, but it does make a nice place to put a thumb.)

Cone geekitude

The kiln--filled to the gills with pots by Teresa, Rob, Lisa, and myself, in anticipation of this weekend's wood firing--is clicking away on the porch, slowly making its way toward that magic number, "cone 06." "Cone 06" is more or less ceramic-speak for ~1824^oF--in this case, a comfortable mid-range bisque firing.

Why the word "cone"? In theory, my kiln automatically measures temperatures with thermocouples, but in practice, thermocouples and computer controls are only so accurate. If I really want to know that the kiln is doing its job, I have to use pyrometric cones--slender little pyramids of specially formulated clay. The cones are designed to melt after a given amount of heat work.*

Industry conventions number cones from 022-01 and 1-42, although most studio potters don't fire any lower than 019 (~1249^oF) or much higher than cone 11 (~2361^oF). Cones were originally numbered from 1 to 20, until people started inventing cones that measured values lower than cone 1. Cone manufacturers could have taken the same bold initiative displayed by the physicists who said "to hell with -273.15^oC, for goodness' sake let's just call absolute zero zero Kelvin (after our pioneering colleague William Thomson, 1st Baron Kelvin)."** Alas, instead of resetting the scale, cone manufacturers chose to put zeros in front of the cone numbers and to count backwards from 01 to 022, so potters are stuck with this odd pseudo-negative labeling convention.

To get a sense of how well my kiln's computer is measuring heat work, I included a cone pack in this firing. The cone pack includes three pyrometric cones: 07, 06, and 05. If the kiln reaches cone 06, the 07 cone should be bent over; the 06 cone should be about halfway down; and the 05 cone should just be starting to bend. If I had put multiple cone packs into the kiln, I could observe differences in heat work in different parts of the kiln.

I had a cone pack in the previous bisque firing too, but I forgot that in cone-speak, 07 is a smaller value than 05. Order matters.

*Heat work is a function of not only temperature but also time. Consider the difference between a tray of brownies baked at 325^oF for 40 minutes and a batch baked at 275^oF for 90 minutes, or [oh dear] 325^oF for 5 hours. Time matters. Of course, the depth of the brownie batter also makes a difference, as does the density with which pots are packed into the kiln.

**That's a liberal paraphrase.

Before and after

We finally ran the kiln's "first firing" sequence, beginning at 7:30am Sunday morning and finishing at 1:30am Monday morning. Our porch's 121-year-old wood siding and 100-year-old wood flooring were protected from the heat by cement board, but because of all the scary warnings in the kiln manual, we had a fire extinguisher on hand and carefully monitored the temperatures around the kiln every hour or so with our new cool geek toy infrared laser thermometer. Because we are nerds, when we were done, we made a nifty little graph plotting interior and peak exterior kiln temperatures over time.

Below are "before" and "after" photos of our porch. As you can see, it did not ignite. All in all, the experience was similar to watching a rock warm up in the sun over 18 hours, but more thrilling because it was our rock. And because our rock reached an internal temperature of 2167^oF before its computer shut it down. It's not your average rock.

Because the first firing tempers the kiln elements, the only things I could put in the kiln were the stilts and shelves. The shelves had a coat of kiln wash on them that has now cured; they'll get two more coats. Next time, I'll actually put some pots in.

Wired up and ready to go

When I called the tech guy at L&L and told him what happened that fateful night, he said "that sure sucks" and sent us a free Brick Repair Kit. It took us a few tries, but we finally succeeded in patching the kiln floor. For all of you trying this at home, here are two helpful hints: (1) don't confuse "cement" with "grout," and (2) don't believe the instructions: wait way more than 24 hours for the cement to cure, and help it along with a heat gun.

Being married to an engineer who will spend upwards of two hours lovingly designing, cutting, smoothing, assembling, and mounting an elegant, weather-proof brass bracket outside the kitchen window to hold a cheapo $2 thermometer--the sort of engineer who defines "vacation" as visiting friends and relatives and repairing their broken doors and bathroom fixtures--has its advantages when it comes to making busted kiln floors look as good as new.

With the helpful upper-body strength of a few neighbors, we finally set the kiln walls atop the repaired floor last weekend. Our electrician came this morning to finish the wiring. He started two weeks ago, but was leery of mounting an outlet until we had completely assembled the kiln. His leericism* was well founded. In order to maximize the distance between the kiln (which will get mighty hot) and our combustible porch walls (protected by a free-standing non-combustible cement-board heat shield built by the aforementioned talented engineer), we ended up positioning the kiln too far from the wall for the cable to reach an outlet there. So as of this morning, the kiln plugs into the porch floor.

This weekend we'll assemble the vent system, and then it's just a matter of finding 16-19 hours when I can be constantly present for the initial test firing. For that, I will be vigilantly armed with my spiffy new infrared thermometer (a gift from my beloved engineer), so I can see how hot (or cool) various parts of the combustible porch get when the kiln is running.

*Not a real word, but it should be.

Kiln bottom free fall

I am the proud owner of a spankin' new L&L e23T-3 Easy Fire electric kiln. All 310 pounds of this baby plus 150 pounds of accompanying parts were unloaded on a wooden pallet into the street in front of our house this evening.

I expect, thirty years from now, after we have hundreds of firings behind us and the kiln is running better than I am, I will look back on this evening and laugh--ha ha--about how it wasn't until after someone started opening the lid that we noticed, for shipping, the kiln bottom was packed atop the kiln top, and the bottom began its free fall toward the pavement.

We might have had an easier time avoiding calamity had the UPS guy come between 4-6pm, as scheduled, rather than at 6:45, well after sunset (and, as I predicted to the friends who came over for dinner, exactly when the oven timer dinged to let us know the popovers were done).

Fortunately, kiln triage conducted on the front porch suggests the damage is nothing that a spankin' new L&L Brick Repair Kit can't fix (assuming the injured patient survives the critical first night).

Thirty years from now, we'll also have learned whether launching vital kiln parts onto the street brings good luck, like christening an airplane; and whether, having survived its harrowing maiden flight, the kiln will choose a quiet life (bisquing greenware, cone 6 glazing) over an exciting one (igniting the front porch and burning down the house).

Diatomaceous earth

Call me uninformed, but I had never heard of diatomaceous earth until S observed that my little hole-poked Klein Bottle reminded him of it. Diatomaceous earth, also known as diatomite or kieselgur, is composed of the fossilized remains of diatoms, which are a kind of algae. It's used as a potent organic insecticide and also as something to mix into your lemonade if you're in need of deworming or a good colon scrub. "The ladies love it because it grows longer, stronger fingernails!" declares one Nebraska enthusiast, even though he admits it's "not endorsed by the [pesky] FDA" or "cleared for actual human use."

In any case, there are enough images of diatomaceous earth out there on the internets to demonstrate that fossilized diatoms can be stunningly beautiful, tantalizingly symmetric, and impressively prolific in their variety.

When S showed me online pictures of diatomaceous earth, I finally had a plan for a vase I had thrown last weekend. The bulbous pot was about a foot tall, and the form annoyed me (dumpy, footless, no lift, and too evenly split between body and neck). After I made it, I contemplated squashing it, but then I remembered a friend's advice: if you can experiment on the surface, don't throw it out. So I saved the vase, and yesterday evening, the experiment began: Could I carve diatomaceous earth designs into the clay and still keep the pot functional (i.e. no hole cutting allowed)?

After I determined the answer ("go buy yourself some shellac and take a drawing class if you want to do this right"), I recycled the clay.

Stupidly, because I had forbidden myself from cutting holes, I forgot to experiment with drill bits before trashing the pot. My Kemper hole maker leaves messy clay goobers in its wake. Since I can't reach inside Klein Bottles to smooth out the goobers, I'm looking for a way simply to make cleaner cuts. Guess I have something to do with my next irritating form.

In other potting news, I'm just a few deep breaths and one phone call away from buying my very first electric kiln. Here's hoping I have enough oxygen in my brain to make that bold leap tomorrow.