Slightly Off Center

Brick by brick

Mon, 07 Mar 2022 16:24:07 GMT

The big gas kiln at Club Mud had a problem. Concentrated heat in the fireboxes had baked the softbrick walls to the point where they were cracked, shrunken, or in worst case, curling like potato chips. Pieces were leaning out, blocking the flame path into the kiln, or threatening to fall out entirely. Obviously, something needed to be done.

I budgeted a week for kiln repairs. It took both more and less time than I expected.

I'd actually planned to tackle this earlier, working with Jon, the other potter who fires the kiln regularly. Then COVID set me back a week, and he volunteered to coach high school track in Portland and things got terribly complicated. Fortunately our treasurer, Don, had done a similar project on our previous small gas kiln, and he volunteered to help me out.

I did an estimate on brick needed--nine dozen--and ran up to Hi-Temp in Portland to collect them, just before my quarantine. Now it was time to take the beast apart. We took apart the bag walls--hard brick walls that take the burner flame and deflect it up into the top of the kiln--placing the two sides separately on the kiln car so we'd be able to reassemble them again later. Then it was time to attack the problem.

The walls of the kiln are two bricks deep. Most of the courses are stretchers--laid flat, running parallel to the wall, joins staggered. Every five or six courses, though we had headers--brick laid the short way, running through the wall, tying the inner and outer layers together. Replacing the stretchers was easy. Replacing the headers was like some reverse Jenga: tap the old brick out with a hammer and piece of two-by-four, slide in the new piece, all the while hoping the whole stack doesn't collapse. Then there were complications: the fully-loaded kiln furniture shelf that blocked the first three headers on one side, bricks too tight to shift on the other. We wound up cutting bricks flush and piecing in bits to fill awkward spots. Fortunately, Don is very good at cutting softbrick. I'm not; the saw tends to wander when I try.

It only took us three hours on Monday to do the entire right wall; another three on Tuesday for the left. (We could probably have continued working Monday--it was only 1 pm--but decided tired and stupid wasn't a good idea. I'm glad--the left side was the one with the too-tight bricks. I'm glad we came at it fresh.

Wednesday was a day off--Don had a standing appointment, and we were waiting the the ITC to arrive.

ITC 100 is a high-temperature kiln coating that reflects heat away from the surface it's on. It's amazing stuff--I saw someone make a firebox door for a wood kiln by slathering the stuff on a sheet of plywood. Coating our brick will help prevent it from overheating, cracking and shrinking again. It took two pints, diluted with 50% water (to make three pints total) to cover the entire new firebox surface, with a quarter cup left over.

I did it on Thursday in about an hour, then spent another hour with the shop vac, cleaning up scraps and brick dust. Friday, after everything had dried, I put the bag walls back together, did a little last clean-up, and put the extra bricks away. I'd overestimated, had three dozen left, but there's always need for repairs.

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Parallel

Mon, 25 Oct 2021 03:45:45 GMT

Wheel-throwing potters get understandably bored with circles after awhile. They're so easy, and everybody's doing them! Like my colleagues, I fiddled around with breaking the tyranny of the wheel at varying points in my career. Some of my experiments stayed, like squared and oval bakers, and my stick butter dishes.

Others, like parallelogram vases, have fallen along the wayside. But I was recently asked to make one again, a companion to a piece I made years ago commemorating a fellow KLCC dj's doggy, so I had to dust off my thrown-and-altered-pottery skills.

First, you throw a cylinder with no bottom, to the desired height and circumference. Clean up the rim and shape it with your chamois (a thin, soft piece of leather that gets very slippery when wet). Squeeze some water onto the bat, inside and outside of the cylinder, then run your cutting wire underneath, cutting it free and pulling some water with it. This allows you to slide the clay freely across the bat as you reshape the pot. Using the bat pins a guides, run your finger from bottom to top along the inside, creating the first two corners.

Measure an equal distance from each and do it again, creating a parallelogram. Roll out a slab of clay for the base and let both firm up overnight. The next day, score and slip the two pieces together, and cut off the excess bottom slab about half an inch from the side with a wood knife, angling it slightly toward the wall underneath.

Flip the pot over and paddle the slab onto the walls, beveling outward on the edges. Follow with a wet chamois, folding the excess clay in on itself and sealing the join. Flip the pot upright and attach coils to pull to make crock handles.

Dry slowly, to prevent the bits from wanting to crack and separate.

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Yellow as the sun

Thu, 13 May 2021 21:22:51 GMT

I've got two Zoom meetings today, afternoon and evening, and a lot of pots to trim in the studio that just aren't ready yet. So I can't really get anything done there; instead, I convince Denise to come outside and play!

It's iris season here, the two-tone purples are blooming like crazy, the black is budding, the peach just opened it's first flower yesterday. And half a dozen yellow iris, bright as the sun, are scattered around the yard.

Denise has made iris-dyed paper before. The black blossoms, in particular, make a lovely sky blue paper, and the various purples dry green. We've never tried yellow, though. So that's what we decide to do.

Process: Set up a sawhorse table in the sun. Fill a plastic sweater box with water and leave it to warm up. Get out screens, couching cloths (pronounced "cooch-"), various plastic containers, a blender, the press. While I do all this, Denise looks for a jar of mordanted (treated with alum to bind colorants to fibers) white paper pulp. Then I go pick off all the wilted yellow blooms, throw them in the blender with some water, and reduce to a slurry.

After that, it's mix some pulp with a little iris slurry, pour into the screen and swish about. Pull straight up, drain, lay a couching cloth (usually a sheet of Pellon interfacing) down on the paper, press out the water, and transfer the wet sheet to the post (stack of other couched sheets.)

Rinse, repeat.

I also harvested some of the purple blooms and blended them up, did some sheets with both colors of iris, some only yellow or purple. When we run out of pulp, irises or energy, I put the stack of paper sheets into a press to squeeze out the extra water, and Denise transfers them to blotting paper.

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Light my fire

Sat, 30 Jan 2021 02:31:33 GMT

In the spirit of "before and after," here's a couple of short video clips. First right after I lit the burners, at the start of the firing; the second, as I wait for the last cones to go down. Cone 8 is completely down, cone 9 nearly. Still need the third cone, 10, to drop. (The last cone is a guard cone, to let me know if I've gone too far. At cone 11, the pictures start sliding off the sides of my pots.)

No song this time, just the music of the fire itself.

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Coneheads

Sat, 15 Aug 2020 19:49:23 GMT

I occurs to me, I talk about what cone I'm firing to regularly on this blog, but have never actually shown you the cones.

Pyrometric cones are devices designed to accurately measure heat work in a kiln. Heat work, the interaction of time and temperature in a firing, is more useful than ambient temperature, which can be measured with a thermocouple and pyrometer. (A thermocouple is usually two different metal wires, twisted together or joined at the tip, that generate an electric current proportional to temperature. A pyrometer takes that current and converts it into a read-out, usually digital these days, calibrated to degrees in your preferred scale.) Because pottery materials respond not just to how hot it is, but how long it's been and how fast the temperature is rising. cones provide much more useful information to the potter than a pyrometer.

The idea goes all the way back to Josiah Wedgwood, back in 1782, who used beads of different compositions to determine how his work was faring in the kiln. Hermann Seger perfected the cone for the Royal Porcelain Factory in Berlin, in 1886. Seger cones are still made today, but most potters us Orton cones, ranged on a standardized scale and manufactured since 1896 by the Edward Orton Company in Columbus, Ohio.

The original Orton cones were numbered from 1 to 14; as low-temperature applications became more common, a new range was added. Rather than using negative numbers, they added a "0" to the beginning of the number. Larger "0" numbers denote lower firings, running from 01 (pron. "Oh-One") down to 022. There is no cone zero.

Lower temperature cones are all brown, from iron oxide in the mix. Because iron oxide is refractory (resists melting) in oxidation, but fluxing (promotes melting) in reduction, they're not super accurate in gas firings, but do well enough for my purposes. For many years, all the other cones were all white, which led to adventures like the time at a summer pottery workshop where the instructor put the cone packs together without his reading glasses. One part of the kiln leapt ahead in temperature, late in the firing, and we fiddled with the damper, air and oil supply until the other part caught up. (We later discovered he'd put both cone 8's in one pack, 9's in the other.) In the last decade or so, Orton has color-coded their cones, though the organic die will burn out in the firing.

I use eight cones in each cone pack, in two rows, pointing opposite directions. The lower temperature cones are closest to the peephole; once they've melted/dropped, I can see the hotter cones behind them. There are two peeps in the door, one about 6 inches from the bottom, the other 10-12 inches from the top.

My cone packs, in order from low to high, are 08 (when I do body reduction), 04 (check the chimney for flame), 1 (progress), 4 (more for reassurance than anything), 8 (is the kiln firing evenly? Fiddle with the damper!), 9 (fiddle some more!), 10 (that's it, we're done), 11 (oh shit, what happened?). They're boat shaped, with sideboards, because by the time we finally reach cone 10, the lower cones are completely liquid, and we do not want them spilling out onto the shelves.

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How the other side lives

Fri, 10 Jul 2020 04:37:53 GMT

It seems like all of the pottery videos out there, especially wheel potters like myself, feature throwing. I get it, it's magic. A wet lump of clay, a little water, hands in ballet and presto! A pot appears!

The thing is, that's only part of the process. Before the throwing, there's wedging and weighing the clay (or perhaps even mixing up the clay, or at least recycling the scrap). Afterwards, there's trimming, handling, drying. Loading and unloading kilns. Bisque fire, glazing, decorating, Glaze fire. Sorting and pricing and packing. That's partly why I find it so hard to answer the question, "How long did this take to make?"

Today I thought I'd focus on trimming. I had a couple of dozen toddler bowls at the leather hard stage. (A weird but traditional description, halfway between wet and dry. Think chocolate bar consistency, where you can shave off neat curls with a sharp tool.) I set up the camera again, two different angles (well, three, but one only shows the back of my knuckles) and shot some video.

Traditionally, a pot is turned or trimmed on the wheel, like it's thrown. I put it upside-down on the wheel head, re-center it, and fix it in place. In olden times, this was done by moistening the wheel head, turning the wheel slowly while tapping the pot gently with your right hand. It will, with practice, settle into place in the center, where it can be anchored with three lumps of wet clay.

I hate tapping pots on center. It takes time and patience, and if you tap too hard, it'll slide right off the other side of the wheel. Back about, oh, 1983, I was exposed to a Giffin Grip at a summer workshop in Tuscarora, Nevada. It's a device that clamps onto your wheel head, with rotating top plate and three hands that slide uniformly toward center as you twist it. Quick, neat, easy, fast. I ordered one as soon as I got home, used it until it wore out, about three years ago. At which point, I unboxed the second one I won at Clay Fest 2008 for Best in Show.

Trimming tools come in a variety of forms: spades, blades, ribbons, loops. My favorite is a Bison trimmer, a wedge-shaped blade made of tungsten carbide I paid $50 for at NCECA. It's super hard and tough, though brittle--I have to be very careful not to drop it on the concrete. But it holds its edge well, I've only had it sharpened twice since I bought it in 2006.

So I set up the Grip, put some cardboard around the wheel table to catch scraps, bring a ware board of leather hard pots down. Center one, lock in place, spin. I start from the outside of the foot ring, trimming away and down toward the rim. Afterward, I cut a circle defining the inner edge of the foot, trim out clay. Burnish the inside of the foot with my trimmer, then scribe a quick little spiral. Tweak the profile of the foot, smooth the outside with a medium-hard silicone rib, then dress up the foot with a softer one. Brush off the crumbs, mark it with my stamp, low on the outside, put it back on the board. Repeat.

Like so.

(Music by Rachel Garlin, Spin, from Wink at July/Tactile Records)

You're a human being, not a human doing.

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Fire, fire again

Mon, 08 Jun 2020 02:04:35 GMT

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.

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Rainbows

Sat, 16 May 2020 18:12:59 GMT

Things are kinda monochrome in the studio at the moment, shades of grey and, well, grey. So I thought I'd go outside for a bit, enjoy the brief patch of sun between showers, and show off one of Denise's obsessions.

Irises.

The first set of irises we had were pale purple, brought from our old duplex on Eighth place and planted around the fence and studio door. They still soldier on valiantly, blooming super early. (They're already mostly gone.) Then we salvaged some two-tone purple rhizomes from a garden clean-up at Club Mud, and planted them along the sidewalk.

Where they took over. We've got about ten yards of them now, flowering vigorously, indifferent to weather, crowding, fertilization or lack thereof. They're really bullet-proof, and, honestly? Kinda invasive. I know now why Linda pulled them out at Club Mud. But they make a nice visual barrier for our otherwise wide open front yard.

After that, I started buy Denise roots at the Farmer's Market. A lovely peach, which did not survive. A yellow, that may have? Or these may be from a gift from another vendor. And a black that changed everything.

This being Oregon, we'll frequently find a bud, just on the verge of opening, neatly clipped off by slugs or snails. We usually bring it in, trim the stem, and put it in a bud vase, of which we have several. This particular vase was porcelain, with a crackle glaze. As the bud went to opening to open to wilting down, the juices seeped into the crackle and stained it a super dark blue. Always on the lookout for natural dyestuffs, Denise popped the liquefying blossom into a jar of mordanted (treated with chemicals to help bind colors) paper pulp, and, sure enough, got some lovely sky blue paper as a result.

She has tried every color of iris we have, since then. Most give some shade of green or goldy-brown, and only one particular strain of black iris gives the sky blue. We went up to Schreiners Iris Gardens last May to see the sights, and brought home four or five new varieties, all blooming right now. We'll see what they do in the vat.

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Captain's log

Sat, 28 Mar 2020 00:38:25 GMT

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.

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Flame on!

Tue, 11 Dec 2018 00:12:16 GMT

More pictures from the storage unit: A raku class I taught at the EMU Craft Center sometime in the early 90s. I'm actually still in touch with three of the folks shown here; two more, sadly, are no longer among the living. And at least two I have no recollection of whatever. Hey, I've inhaled a lot of (sawdust) smoke over the years. The brain cells get dusty.

Raku is a ceramic process originating in Japan, where low-fire pots were rapidly brought to temperature in very small, wood-fired kilns, then pulled out red-hot and plunged in water to cool. Raku tea bowls were highly prized for the tea ceremony, and the firings eventually became a social event, where pots were provided for guests to show off their calligraphy on. They were then fired on the spot, and much admired before being taken home as party favors.

American raku usually uses a gas kiln, and adds an extra step: After removing from the kiln, and before cooling, the pots are sealed in a metal barrel with sawdust, leaves, newspaper or other combustible material. (A steel trash can with tight-fitting lid works well.) This post-firing reduction emphasizes crackle in the glaze surface, as carbon is absorbed there. Carbon absorption makes unglazed clay surfaces black as well. And it's also possible to get metallic lusters, mostly from copper, and rainbow-hued matte glazes (also copper) as well.

Credit goes to Denise for taking the pictures. I think I just gave her my camera and told her to be careful not to set it on fire.

On the left, Penny McAvoy bravely pulls a glowing pot from the kiln; Kathy Lee scrubs a layer of ash off a plate to reveal the black figure, wax-resist-on-crackle-glaze pattern beneath.

Kathleen Fitzgerald and I pull still-not-very-cool pots from the reduction chambers, while TK McDonald prepares for the next batch of hot pottery.

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Definitive

Sun, 11 Mar 2018 05:07:30 GMT

I'm currently reading Word by Word: The Secret Life of Dictionaries, by Kory Stamper, and was inspired to go digging through my files for a photocopy of a dictionary page my friend Lyda sent me many years ago. Here it is:

Let's take a closer look, shall we?

You see the resemblance, of course. It was even more pronounced 20 years ago, when I still had all my hair (and a nifty potter's apron). The clincher? He's throwing left-handed.

I am the very definition of potter...

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Terminololololology

Tue, 29 Aug 2017 16:04:58 GMT

It's been a while since I've added any new glossary terms, so here's one:

Pin-holing. A glaze defect leaving small pits, sometimes sharp-edged, in the glaze surface. In stiffer glazes, like my white base, this can be caused by small irregularities in the clay underneath, if the glaze doesn't flow to fill them. In other glazes, particularly those with a high titanium content (Woo's Blue, for instance), pinholes form when gas bubbles burst from the molten glaze but don't smooth over again. These guys can be very sharp-edged.

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