Slightly Off Center

One of my favorite Ann Reed songs from back in my radio days was a fast-paced ditty listing all the things going on in the songwriter's life. I don't remember the lyrics in detail, but the hook has stayed with me: "More complications to a simple life!"

It me.

So the plan was, I'd finish Holiday Market, take a few days for Christmas. Pack and ship presents to family in the midwest. Spend a week relaxing and reading and eating cookies, then head back into the studio at the point when my hands couldn't stay away from the clay any longer.

I was going to take it easy, limit myself to 50 lbs. throwing a day, spend three weeks on wet work, throwing and trimming, instead of the two that I raced through before the Holidays, then a long week glazing. Fire on the first of February.

It mostly worked? I mean, I also had to recycle 400 lbs. of clay, before it got too dry, but that means I had 400 lbs. more clay, right? And the extra three special orders that popped up on email were fine, I was gonna throw pasta bowls and dessert plates and little tiny toddler mugs... well, one outta three ain't bad.

But I finally got everything done, trimmed the last three dozen soup bowls last Friday morning, so I could run errands in the afternoon, buy bread... and Great Harvest Bakery was down to nine mugs (out of the 45 I took them in December). So I came home and used up the last of my recycled clay making two dozen mugs to carry them over to my next firing in April. It's fine, right?

Then the kiln blew a circuit breaker while firing Friday afternoon. I caught it before we'd lost too much heat, reset, restarted and it continued to completion. But the following firing blew the breaker twice, and that was the firing that had the ball clay I had to calcine to mix glaze because I ran out last month and only made two buckets, which weren't enough for this glaze run so I needed to mix more. And I got an email from someone wanting to pick up some stew mugs for her daughter-in-law's birthday and could she get six? (Including one in a pattern I'd run out of at Holiday Market.)

I've been getting very little sleep lately.

But things are slowly getting better. I went over the trouble-shooting flow chart at the Skutt Kilns website, and talked to one of their support techs, and we both concluded that it wasn't the kiln (whew!) but might well be the breaker, which is frustrating, because it's only been... oh lord, five years? since I got my kiln and had to rewire the studio with a heavier-duty breaker. Still, it's an easy fix, requiring only two trips for parts (Jerry's didn't have 80 amp breakers, only 70 and 90, so I had to drive down to Garfield Street to get the one I needed. Had it back together by lunchtime, left Denise to keep an eye on it while I went and glazed pots, staying until almost 8 pm to catch up on forty tall mugs.

The firing went fine, as did today's follow-up, so I guess it was the breaker. And I took the van to the studio, mixed up a 10,000-gram batch of glaze this morning, and pulled out stew mugs while the glaze was slaking, just in time for my customer. I didn't get that much glazing done--32 stew mugs and the last batter bowl--but I'm hoping once tomorrow's Club Mud meeting is over, the complications will slack off a bit and I can just paint pots for a few days.

Pictures tomorrow, I promise.

Back at the studio this morning, helping rewire another kiln. This one went wonky in March, and Laura replaced the relays, but it still was confused. Temperature reading was bouncing all over the place when it was sitting at room temp, not even firing. One of the old relays had been fried, so she thought we might try replacing the wiring, in case any of it was compromised. This was a much easier fix than last week's. We swapped out four red (control) wires leading from the relays to the motherboard, and I also noticed that one of the power leads from the formerly fried unit also looked a little toasted, so we replaced that as well. Once we put the box back on the kiln and plugged it in, it went through a warm-up cycle, announced it was a stable 65° F., and idled, just like it ought to.

The hardest part of the project was getting the kiln settled and level again. They'd traded the Left and Right kilns, because the left outlet needed replacing about the same time the Right kiln failed, so it was very wobbly on its base, rocking back and forth. We finally had to unlatch the bottom slab from the kiln, lift the body off and get everything straight and aligned (incidentally cleaning out some big rust flakes, which had been part of the problem). Lifted the top bits back on, snapped the latches in place, and programmed a quick test firing. Half hour later, it was already over 225° and climbing nicely, so we canceled the program and declared it fixed. The whole job took less than two hours.

Unlike my last kiln repair project. When last we left this tiny trouble-maker, Johnny and I had replaced the elements, but couldn't trim them to length, nor did the connectors fit over them. Laura, replacing the relays, couldn't figure out the wiring arrangement, as the new units were a different style, and had to wait until Tuesday to call the manufacturer, as, unlike potters, they don't work through Memorial Day.

I wound up buying a Dremel and cutting discs, as none of my wire-cutters would make a dent. (Club Mud had a Dremel, but it didn't work; motor spun, chuck did not. I eventually fixed that as well, with a $6.50 part from the internet.) I also took all the connectors home and reamed them out with a drill bit. Eventually, everything was trimmed, assembled, crimped, and ready to plug back into the control box.

Which was still sitting open on the table. The new relays were wired in, but couldn't be attached to the mounting plate, because of stupid bad design. Skutt designed them to attach to the plate, not with sheet metal screws, which could be just screwed in and out from the front, but with little bolts, that required somebody to hold the nuts in place in the quarter-inch clearance between the back of the mounting plate and the box. So we needed to unbolt the mounting plate, but I didn't have my socket set with me, and the mounting nuts were too tightly spaced for any kind of wrench or pliers.

I ended up taking the whole unit to work on at home. Managed to get the mounting plate unbolted, but had to disconnect the power cord and remove its hardware before I could get it free. Once the relays were bolted down, the whole works went together mostly smoothly (one of the power lead screws wanted to cross-thread, but eventually surrendered). And I could finally put the thing back together, three days after we started our one-day repair. Test-fired it last Friday night, and it was fine. All four electric kilns are now officially (knock wood) working again.

tl;dr: I'm really tired of kiln repairs right now.

With a week of temperatures in the high 80s, it made sense to wait an extra half-day to unload the kiln. Thursday morning was much cooler than Wednesday night, even if the kiln was still potholder-hot on top. I'd tinkered a little with the fuel and air, this firing, hoping to have less over-reduction on top. The result was slightly better--fewer pots to re-fire--but a lot that were, if not oxidized, not terribly reduced either. Aggressively neutral? I've decided not to care so much.

Here's a sample of the results. A pair of pie plates, the one on the right a little paler than I'd like (most noticeable on the rim, though if the image included iron or rutile, you'd see the bleaching).

One of two full ware boards of tall mugs; hoping this means I can stay ahead of demand for a bit. Also a rather nice cephalo-cream pitcher.

A pair of pasta bowls. They haven't been selling, necessarily, but they're easy to throw and fun to paint. And they help fill up the kiln.

An aerial view of a really nice batch of dessert plates.

And of course, the return of Spider Plate! Now in dinner size! (And thank the kiln gods, it matches the first one.)

We loaded the big kiln again on Tuesday, and I succumbed to an odd impulse--I did a slow video pan, showing all the pots stacked up waiting, and the big, empty space where they were going. Popped the raw footage up on Instagram, but spent a little time polishing it today. Found the perfect musical accompaniment: Lou and Peter Berryman's "So Many Pies," from their first album, No Relation.



We started loading around 9:30 am, finished by 3 pm. Contrast how the shelves and kiln looked at the start, with how they looked afterwards.

Just for fun, I calculated just how many pots were loaded in the kiln this time. I came up with 310, total. And of that huge expanse of pots from the beginning of the process, this is all that was left.

Heard a quiet boom! from the bisque, just over 700°. On unloading, found this: three stacked pie plates, reduced to wreckage. This after a slow warm-up, and a hold for three hours at 200° F with the vent fan going.

It is so hard to thoroughly dry pots in the fall.

This kiln is giving me stress.

I don't realize it, while I'm making pots, bisquing pots, glazing them. It starts when I'm loading the kiln, trying to guess where the bad spots will be this time, deciding which pots will be sacrificial, probably un-sellable, but needed to fill the empty spots, because without the thermal mass, things will be even more unpredictable. Trying to be sure that special orders are near the top, near the back, even if it means they'll be over-reduced and I'll have to refire them in the electric kiln. Thinking longingly of my days back at the Craft Center, where the glaze was always the perfect cream-with-speckles, not dead-white or overly brown. I realize that I'm beginning to dread firing.

I spent a good bit of time trying to knock the bugger back into shape, and it seemed to have helped: it fired evenly, with a minimal adjustment of the damper. Still some oxidation in the bottom front, but most of the space was filled with work for Empty Bowls. (Who knows when that will happen again?) Top and back had some pretty dark pots, enough for nearly a full (electric) kiln's refiring, including five commissioned bowls. But other than that, it was a pretty good firing. 66 units of gas, which is dead average. Still took nearly two hours to drop the final cone, but I think I may have an answer for that, and one that may address the over-reduction.

But damn, I wish I didn't get so anxious every time I light the burners.

My last two firings were a struggle. The kiln didn't want to heat evenly, and when I did finally get the cones to drop more-or-less together, I had horrible amounts of oxidation. Around the door, along the sides, pretty much everywhere but the very top. So this time, I decided to try a different tactic. Rather than adjusting the airflow via the chimney and damper, I'd cut it off at the source.

A venturi gas burner uses the pressure of the natural gas flowing through it to draw in air through the back of the burner into the mixer, where it (duh!) mixes with the gas, then ignites at the tip of the burner, shooting flame into the kiln. This is primary air, and you can adjust it by turning a rotating shutter at the back of the burner. Secondary air comes in around the burner (or through leaks in the kiln door or walls), drawn by the draft of the chimney. With the big gas kiln, we normally set the primary air early in the firing and leave it, doing any later adjustments with secondary air and draft. But I'd been using less and less gas the last few firings, and it occurred to me that I might have in fact had too little gas in the burners for the amount of air I'd entrained. So this time, when I set the shutters at the start of the firing, opened them only two turns, rather than my usual two-and-a-half.

It wound up being a struggle. The top still got ahead of the bottom in temperature, and I still had to mess with the damper to even it out. The firing took about three hours longer than usual, and about 5 units more gas. But for the first time in ages, I had a glaze firing that wasn't a mess of oxidation.

It was a mess of over-reduction, instead.

The pots were much browner than I like. Some were still passable; others, you could barely make out the pattern. Fortunately, I have a solution.

A few months back, Mason Stains stopped making my preferred black, and the replacement tended to blister when it went on too thick. I eventually solved the problem by changing the ratio of stain to Gerstley Borate (1:2, rather than 1:1), but in the meantime, I had pots with bubbles and blisters. I tried grinding them down, and dabbing on a little Gerstley, and refiring, but while the bubbles would be fixed, the background glaze got browner, and on vertical surfaces, mugs, for instance, the picture would start to slide. What I really needed, I decided, was the ability to fire cooler, in oxidation, maybe cone 9?

Then I discovered (by blowing circuit breakers) that my new kiln was in fact, a professional model, rated for up to cone 10. So I tried refiring some blistery pots, and not only did they smooth out nicely, several of them brightened up considerably. It's still not perfect--the background glaze looks a little dirty, to me--but it's a huge improvement. And it only costs around five or six bucks per firing. So far, I've done two loads of brown pots, and I think I'll do at least one more. Here's a before and after comparison.

I wish I had a solution for the opposite direction.

I've been keeping firing logs forever; if I dig far enough into my filing cabinet, I can probably find logs from my Craft Center days, possibly even graduate school. They're a good way to keep track of how often I fire, how much gas I use, what firing strategies. Sometimes they're even useful in predicting what I should do next time.

Sometimes.

Our big gas kiln is weirdly unpredictable. I don't just mean the oxidation spot, which seems to move around from firing to firing. Firing strategies that work for months, even years, will suddenly crash, for no perceptible reason. For the longest time, I had a schedule that fired reliably at eight hours overnight pre-heating, twelve to thirteen hours more the next day to reach cone 10, with the kiln about half a cone hotter on the bottom. I could shut down at cone 10/9.5 and rely on heat rising to drop that last half cone on top on carry-over.

Jon couldn't match my experience. No matter what he tried, even if he tried the exact same settings (possible after I installed gas gauges on all the burners), he'd always fire hot on top, cooler bottom. I privately felt a bit smug, thinking I had a better handle on firing this kiln than he did.

And then, about six months ago, it abruptly switched on me as well. The bottom was staying as much as two cones cooler than the top, necessitating a lot of fussing with the damper, trying to slow down the top long enough for the bottom to catch up, wasting time, wasting gas.

This is where the logbook comes in handy: trouble-shooting. I have a record of a series of attempts, what helped, what didn't. What to try next time. I record the times of adjustments, gas, primary air, damper. Pyrometer reading, and notes on what I'm trying to accomplish at any given point.

Gas pressure is read on gauges on the individual burners, as mentioned. There are valves on each burner, plus a big emergency shut-off valve where the pipe comes in from the meter, in case we have to shut off everything at once. Pressure is measured in water-column inches; I generally pre-heat overnight at 1.6", then turn up to 2" (or more recently, 1.8") for the day's firing. For now, I keep pressure the same at all burners, though I suppose I wouldn't have to.

Primary air is the air going directly into the burners. Speeding gas pulls in air via the venturi effect. The amount of air is effected by the constricted shape of the burner, the speed/pressure of the gas, and the size of the opening permitting in the air. We can't control the first--burners are cast iron--and already talked about the second, above. The third is controlled by a disk-shaped cover, threaded over the gas pipe at the back of the venturi. By spinning the disk, you can move it closer or further from the opening, changing the amount of primary air. I generally fire with the air open 2 turns overnight, move it to 2.5 in the morning when I turn on the gas.

Secondary air is what comes in around the burners, through the burner ports, peepholes, gaps in the brick. Hot gases in rising in the chimney create draft, which pulls in this secondary air. By opening and closing the damper, a piece of kiln shelf that slides in and out of a slot in the chimney, we control the cross-section of flue, and hence the secondary air. Body reduction and the neutral-to-reducing atmosphere later in the firing are generally controlled with the damper.

Londonderry Air is something else entirely.

The pyrometer is a digital read-out attached to a bi-metallic thermocouple that's slid into a ceramic sleeve, protruding into the kiln. Heat from the kiln causes a current in the metals that's then converted into temperature in degrees Fahrenheit or Celsius. It's not actually that accurate, which is why we use pyrometric cones we can watch through the peepholes to determine when we've hit temperature. However, the pyrometer, like an instant-read thermometer in cooking, can give us a snapshot of the firing, tell us whether we're still gaining heat or have stalled. Like the time, it can also be a handy reference when you're comparing different firings.

My notes are usually things like "cone 08 is down, start body reduction" or "tweak the damper for flames in the chimney" (a way to gauge kiln atmosphere), and occasionally "why won't the bottom heat up, dammit!" Descriptions of what's happening, as the other columns give the when and why.

I've been struggling a little with my black stain, the last two firings. For years, I've been using a chrome-free black stain, Mason 6616, mixed with Gerstley Borate, 50:50 by volume. It was a little fluid on vertical surfaces, but overall reliable. Which is why I was so unhappy when I learned last spring that Mason had stopped making the stuff.

The thing is, chrome is a refractory--it resists melting. Because I want the stains to melt into the surface of my glaze, using as little chrome as possible is a good idea. I had to use it for green, but at only a quarter of the volume, it was unlikely to be a problem. Problem being defined as bubbling, blistering, refusing to melt smoothly.

Which is a problem I have with my new black stain. I ordered the one with the least chrome content, but it's still around 25-30% chromium oxide.

So I changed the proportions. I no longer mix with Gerstely Borate half-and-half. Now I use two parts Gerstley to one part stain. Even so, I have to be careful not to apply too thickly, else blisters appear. I can grind them down, but can't really sell the resulting pot as a first.

I considered refiring them to smooth them out, but I've had problems with that in the past. Glazes get more fluid on refire, more likely to run. I also tend to see them get darker brown, as reduction brings more iron out of the clay. If only I had some way to fire them cooler, say, cone 9, in oxidation.

Then last week I had an inspiration. As I mentioned, after having to rewire my studio, the new electric kiln is a production model, a Skutt 1227 PK. What this means, besides it requiring a bigger circuit breaker and heavier wiring, is that it can fire to cone 10. In oxidation, because an electric kiln doesn't burn fuel.

So I boxed up a dozen unsuccessful pots, brought them home, ground down the blisters. Dabbed on a little straight Gerstley, to make sure they smoothed out. I also had a couple of pots that had gotten scratched in handling, so I put them in as well. Set the kiln Fast Cone Fire mode, going to cone 9, turned on the vent and pressed the Big Red Button.

The next morning, the kiln flashed Complete. It'd taken a bit over 5 and a half hours to reach temperature, was already back down to 665° (from a high of 2300°). It took the rest of the day to cool it safely down to open, but the results are gorgeous.

Blisters and bubbles gone, glaze still creamy, with iron speckles and warmish tone. And one scratched casserole, which had been a little over-brown, brightened up nicely. I'd rehabilitated over $350 worth of pottery, for about 80 kilowatt-hours of electricity.

Wow. Just wow.

I'm gonna have to reign in the tendency to refire everything that comes out of the big kiln with problems, but for a specific problem set, I've got a new solution.

Well, it's been three days since I unloaded the test firing. I can finally talk about it without flinching. It was a qualified success.

We reached temperature, top and bottom largely even. We didn't take much more time than usual, and the gas usage was right on average, 34 units.

The reduction was awful.

I've talked it over with Linda, who fires next, suggested some ways to make it better. Reduce the primary air (air through the burners, controlled by opening and closing a threaded shutter), maybe use the peephole in the chimney as a passive damper. Don says you really need to be able to smell the unburned gas, which is way more reduction than I'm used to in the inside kiln. Whatever happens, it's out of my hands now.

The firing wasn't a total disaster. I had lots of white pots in my stuff, little or no copper red in anyone else's. But Nicole's cobalt crawling glaze mugs looked good, Jon's servers and some of his mugs were fine, and Linda said her two mugs and fox vase, while not ideal, were certainly sellable. Don's two test bowls looked normal to me, but Brian got mostly cobalt in his bowls, though there was a little copper red inside two. A couple of my pots, near the top back, showed the opposite effect, over-reduction giving a peachy bloom to my white base glaze. Go figure.

There was one spot of perfect reduction in the entire kiln, on the back left side, just above the bag wall. Just big enough for one pot; guess which one?

Squirrel!