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Skimming my Dreamwidth Reading list tonight, and conuly had a link to an interesting Atlas Obscura article on ceramic repairs through history. I've done a bunch of them over the years, most recently a decorative earthenware tile that I reassembled with the help of white glue, spackle, acrylic paint and gel medium.

My most cherished fix-it piece is this one, knocked off a table by cats, reassembled with epoxy and copper epoxy putty, with an appearance not unlike the kintsugi (gold powder and lacquer) repairs described in the Atlas article.

I've always loved this piece, which I bought on an undergraduate pottery class trip to Warren Mackenzie's studio in Stillwater. It's an unusual piece--I don't know that he did that much with porcelain--with a really nice celadon glaze splashed with a blue ash glaze and Mashiko stone. It's particularly precious to me now, as Warren passed away just before New Years.
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For years now, Club Mud has been firing by ear. Literally; we determined the flame adjustment on our kiln burners partially by sight, but mostly by listening. The sound of the flame at different stages of firing was variously described as a "whoosh", a whistle, a flutter. Experienced firers could tell just by the sound whether the kiln was adjusted properly.
 
Beginners? Not so much.

Add in the fact that, as us veteran kiln-meisters get older, our hearing gets a little less reliable, and you end up with a strong lobby for more objective measurements. So at our November meeting, we voted to install pressure gauges on all the burners of our two gas kilns. Somehow I wound up being the one to figure out how to make this work.

I went online, of course. Quickly found pressure gauges for under ten bucks, if you wanted to measure in pounds per square inch (PSI). Unfortunately, PSI is a huge measurement for natural gas. Normal pressure is a fraction of 1 PSI. What we really need are water-column inch gauges. (You know how atmospheric pressure is measured in inches of mercury? Like that, only with water, which is much less dense, hence measures lower pressures.) I found a supplier priced around $25 per, then hit a Black Friday sale, so we got two sets of four for well under $200.

Then of course, I had to figure out how to install them. They needed to go in the--fairly short--space between the on/off valve and the burner itself. We had to go from this--

--to this. (More or less. All the burners were plumbed slightly differently.) I was able to reuse a couple of 4 and 5-inch pipe nipples, but for the rest, I had to get:

4 1-inch Tee joints
4 1x1/2-inch bushings and 4 1/2x1/4-inch bushings (to narrow down the 1-inch pipe to accept the 1/4-inch gauge stem)
4 2-inch pipe nipples
3 3-inch pipe nipples
And a roll (or two) of teflon tape, the yellow stuff specially for gas fitting.

Also, a small box-end wrench, three pipe wrenches, and a big length of pipe to slip over the end of a wrench when I needed extra torque. Plus our bench vise.

It took me three visits to Jerry's (our local home-improvement chain) to get the parts I needed. Wrong measurements (A 1-inch pipe is actually 1-1/4" across, as they go by internal diameter. Go figure.). Wrong parts. Right parts, wrong size. Last minute fourth run for three slightly longer pieces. And I still have to return three 2-1/2-inch nipples that I got because I wasn't sure 3-inch wasn't too big. That's okay, though. Now that I've finished the big kiln, I need to get the pieces I need to set up the small gas kiln with its own set of gauges.

And it is finished; everything came apart and went back together again! All I need to do is pressure-test all the joints, seal any that leak, and it'll be ready for my end-of-January firing.

Objective documentation! I can hardly wait.

ETA: Pressure test is good! No hisses, no bubbles (you check for leaks by flooding all the joins with bubble soap. No bubbles means no gas leaks). And all the gauges read! Ran them up to warm-up levels, and they were between 2-1/2 and 3 W.C. inches. Things are looking good for my next firing.

Requests

Jun. 16th, 2018 06:55 am
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Had a customer in the booth yesterday afternoon who really liked my work. Was going to send her husband over to buy a particular bowl for her, as it was her birthday. And she asked if I ever did commissions.

Yes, I said, I did the all the time.

Well, I'd like a bowl like that one, with no rim, just a straight side, she said, pointing to the batter bowls. That's easy enough, I replied. And I'd like it in a matte black glaze.

Aaaand that's where I had to say no.

I do take special orders all the time. Every firing has at least half a dozen of them, I've already got three or four in queue for my July kiln load. But they're generally a form I usually do, but with a particular pattern, or a new form but glazed and painted like my standard ware. I do not match colors, I do not test glazes.

Glaze is not like paint. You do not just buy a can at Home Depot and slap it on the pot. There are chemical interactions, with the clay body, the firing, the atmosphere in the kiln. Even the most reliable glaze in your firing situation will need extensive testing to adapt to mine. Best case, it will take at least two or three firings to get a reliable result, and since I fire the famous 50 cubic-foot car kiln only once every six weeks, we're talking a minimum three months, more likely six, before we get a finished pot. (A potter I work with at Club Mud has taken over a year on projects like this.)

Add in the fact that matte black glazes are notoriously tricky--they're just on the edge of devitrified (no longer glassy), and loaded with metal oxides--usually iron, cobalt and manganese--so I can't be sure the resulting glaze, no matter how lovely, is actually food safe.

So I explain the complications to her, and suggest that she find a potter with a black glaze she really likes, then ask them to make the bowl for her. It's far easier to throw a new form than it is to create a new glaze.

I didn't realize it was so complicated
, she said. (Few people do.)

And her husband never came by to get her birthday present.

Mr. Fix-it

Jun. 4th, 2018 02:51 pm
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Firing a gas kiln is an experience of prolonged, but intermittent attention. You need to be there at certain critical moments: starting it, of course, throwing it into body reduction and out again, spot checks of the cones, the pyrometer, the atmosphere every hour or so. There's a long gap in mid-afternoon between cone 4 and 8, after which it's time to pay attention again, particularly if the top and bottom temperatures are uneven. And then the (subjective) eternity between cone 9 and 10, when it's time to shut down the burners, close the damper and ports, and go home for a much needed rest.

Different potters deal with the intermittent part differently. Jon throws dozens of pots, moving them in and out of the kiln room to take advantage of the heat to speed drying. Sookjae and Michiyo share their firing, so they get to take turns leaving or staying, mostly reading while here. Tea is notorious for going to the movies during the long afternoon stretch.

Me, I fix things.

I fidget too much to throw, or even sculpt down here, I'm always getting up to check the cones. I also don't like to actually leave the studio for more than an hour or so at a time, so a double-feature is out. I can only read for so long, do so many sudoku or cryptoquotes, or blog posts before I get bored.

So I've rewired potter's wheels. Installed new shop lights. Fiddled with the pyrometer and thermocouple, got them working properly again.

Today, I fixed the ware cart.

It's a very basic design, probably goes back to the 60's, two 2-by-4 uprights supporting 2-by-2 shelf brackets. Easy to put work on and take it off by the shelf-load. Only problem?

The brackets wiggle. They move like a teeter-totter. Put a shelf of work down on one side and the shelf on the other side goes up. You needed to be very conscious of the relative weights when loading it up. So today, I brought in my drill and driver bits, a bunch of self-tap screws, some pre-cut 2-by-2 support blocks. A small level, a bar clamp.

In a little over half and hour, the job was done. Shelves are now all firm and level, no give, no bounce. A community of potters is gonna be so grateful.

Now what do I do with the rest of my afternoon?

Bleary

May. 25th, 2018 10:06 pm
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Been sick and miserable all week, as what initially seemed to be a very bad allergy attack morphed into a persistent head/chest cold. No fever, oddly enough, but buckets of snot, irritated throat, cough, the works. Fortunately a little ahead of schedule making pots for my next firing, so only had a little studio time, throwing one last batch of pie plates (we're down to three in inventory) and tall mugs. Mostly, though, I blew my nose. Constantly.

Finally started feeling humanish Thursday, so went down to the studio to get organized for glazing and see if I could figure out what was wrong with the left electric kiln. Apparently, it'd been blowing a circuit breaker before reaching cone 6, making it useless as a glaze kiln. This is the one I found, nearly pristine, at an estate sale a few years back, so I didn't think there was anything wrong with it, but you never know what a year in a busy shop will do.

Karen said they'd installed a new breaker, and wondered whether we should swap the right and left kilns to see if that might narrow the problem down a little. Thought I'd do some lower-impact detective work first.

First thing I did was open the kiln control box. (Well, the first first thing I did was make sure the circuit breaker was off.) The new computer-driven Skutt kilns don't actually show you much--well-labeled leads plug into a well-designed switch block, but all the actual circuit boards are hidden behind a sheet-metal and insulation barrier. Still, everything was plugged into its proper place, no signs of heat or wear.

So I pulled out the plug; here I saw a clue. The nylon faceplate showed a little toasty color around one of the three prongs. I took the faceplate off, but didn't see any further sign of damage in the wires or connectors inside the plug, so set that aside. 

Took a flashlight to the outlet itself next. The contacts in the slot corresponding to the over heated prong were black, corroded with copper oxide, so I opened up the box, at which point the problem was obvious. One of the electrical wires had overheated, melting insulation back a good inch from the outlet and frying the bakelite itself. The copper wires were variously pink from the surface melting and resolidifying, and black from corrosion. Messy.

So I disconnected the plug, cutting the wire on that side, as the setscrew was welded solid, and took it to the nearest hardware store.

Where they didn't recognize it, didn't have it, didn't think they were even manufactured anymore. Tried to sell me a different 50A/240V outlet, and when I said the plug wouldn't fit, brought out a clothes dryer power cord. No, guys; just no.

The nearest electrical supply store was in Whitaker, about a half mile further, but they had no trouble rustling up a replacement receptacle. Which didn't fit the circuit box, when I got it back to the Club. Thought I was gonna have to make another round of errands when Jon reminded me that we had a stash of used electrical bits on a shelf in the kiln room, and one of them turned out to be an outlet box in the correct size, complete with a matching faceplate. Success! I pulled out some slack in the cable, cut a couple of inches off the three leads and re-stripped the insulation. Took three hours, all told, and I had to disconnect and reconnect that outlet three times before it all fit together, but in the end, we had both kilns running again. Total cost? $15.26.


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Once upon a time, I was a math major. Well, art and math double major, I took the math classes for fun. And one of the things I remember from math is this: conical forms are self-similar.

What this means is, if you take a regular cone that's, say, six inches high, and slice off the base an inch from and parallel to the bottom, the second, truncated cone is proportional to the original.

Or, to reverse the process, if you continue a cone several inches, following the same angle, the new, bigger cone is also proportional.
should i be reminded of ford prefect's ebony bath?
This is the surviving, base section of my quail sculpture. You'll note the top of the piece. It's conical.

Well, conoidal, actually, a teardrop in cross-section. but the angle is consistent, so if I could extend it by the correct amount--about a quarter inch, by my calculations--I should have a base proportional to the original, wet-clay version of this one. That I can build a complete replacement quail body on. I'm thinking thin, flexible cardboard; Denise always has a stash with her bookbinding supplies.

Watch this space...
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Had a couple of less-than-optimal firings just before Christmas, so I decided to see whether a little kiln maintenance might help. I already tightened up the chimney back in December (the bricks expand and push apart over many firings. Gaps allow in cold air that reduces the draft).

Today's repairs:

1. Take a vacuum cleaner to the burners, clearing out scale and detritus from the venturis.

2. Tear down and rebuild the bag walls. These are hard-brick dividers between the flame trough (where the burners shoot in burning gas) and the ware chamber itself. They're about four bricks high, and divert the flame up to the top of the kiln, where it will be drawn back down through the pots and shelves before exiting through a flue in the floor (and isn't that a tongue-twister). Extreme heat on the outer face has caused them to lean toward the outer wall, constricting the flame as it enters the kiln chamber. I tear them down to the bottom layer, reversing most of the bricks--they've actually started to warp, a little--and shimming them up with ceramic fiber to true them up vertical again.

3. Tighten up the bricks of the door, mostly by banging them with a hammer and two-by-four, to eliminate gaps and get the door face mostly flat again. I also brought in wrenches to tighten down the tie rods that hold the bricks in place in the steel work. We really need to get some new valve springs--the pair we have now are almost crimped flat. Automotive valve springs are often used in kiln frames to allow for expansion and contraction with heat. This set seems to have given as much as they have.

I'm hoping this will make for a better--at least more normal--firing Monday.

ETA: Aaand, it seems to have worked. Cones dropped together, same temperature top and bottom from about cone 1 all the way to the end. Used 64 units of gas, substantially better than the last two outings. Could even trim that down a little next time, as the reduction was pretty heavy throughout, could be a bit less.
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Bat pins!

Was getting ready to (finally) make mugs this morning when I took a close look at the head of my wheel and saw something surprising: the bat pins were nearly worn away.

I didn't even know this was possible. Bat pins are supposed to be forever, sturdy little bolt heads that hold your throwing bats in place. If anything is supposed to wear out, it's the holes in the bats themselves, particularly the masonite ones. They're always getting loose and wobbly; I've started sticking a sheet of old t-shirt fabric on the wheel head to keep the bats from sliding back and forth.

Well, it turns out the bats weren't entirely at fault. Or maybe they were--the back-and-forth friction, along with the abrasive qualities of the clay, seem to have worn a good sixteenth-inch or more off the pins all the way around, on one pin, all the way to the hollow center of the head. I had to take off the wing nuts, lever out the pins with a vise-grips, and take a set, along with a representative throwing bat, down to the nearest hardware store. (I could have gone to the ceramics supply instead, but only if I wanted to pay four times the price.)

I should have taken a flashlight. Socket-headed machine screws were in the back-most layer of a three-deep sliding hardware thingy. (You know the kind, like sliding closet doors covered with tiny, badly labeled drawers full of every screw and fastener except the one you need right now.)

I finally found what I needed, 3/4 inch pins with a 1/4 inch head. Cost me 86¢ for two. And the best part--almost making up for a lost morning's throwing--is that my bats don't wobble anymore.


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Dismantling my old kiln is a project in and of itself.

These old Olympics were an early experiment in modular design. Each ring had its own control box, which plugged into the box below and above it using standard extension cord plugs. This was always a weak point in the design. Expansion would push the plug and socket apart, little by little, until the contacts were far enough apart that they'd arc, and presto, dead plugs.

I'd replaced about three of them, oddly enough always in the top box, before I gave up and just hard-wired them together. It took another ten years before the bottom set blew, at which point they were treated likewise.

So to get the kiln out of the studio, I first had to reverse this process. As there's a chance someone else will be putting this back together again, I carefully labeled all the wires and connections.

After that, it's just a matter of destructive distillation. Disconnect the power cord from the wall box. (Of course the circuit breaker is off. Whaddya think, I'm an idiot?)(Oh, right, I'm a potter. Never mind.) Slide the hinge pin out so I can take out the lid. Carry out the rings, one at a time. Throw away the broken bottom slab (fortunately, I have an extra, so the kiln is still useable). Knock all the rust off the stand and take it outside.

Cough and choke in the dust. Remember to put my dusk mask back on. Vacuum up the rust, dust and cobwebs.

Before I can install the new kiln, I need to commit carpentry. Specifically, to cut a hole in the wall for the EnviroVent.

I kill a 2-inch hole saw and a jigsaw blade making the hole. Back when I installed my first kiln, I thought it would be a good idea to fire-proof the wall, so I covered it with HardiBacker, cement board used in tile-laying. Turns out that stuff is death on saw blades. My neighbor Bob takes pity on me and loans me his SawzAll so I can finish the job. (Now I have tool envy.) A standard dryer vent slides in the hole, and a few nails hold it in place.

Installing the new kiln is comparatively easy, stepwise.

1. Connect the controller to the power supply. Set aside.

2. Bring in the base, position carefully. Set in the EnviroVent, connect it to the wall vent with aluminum dryer ducting. Plug in and test. (Works. Yeah!)

3. Bring in bottom slab, carefully position on the metal base. The points where the shelves post need to be supported by the legs of the base.

4. Put down some 1" stilts, then the bottom-most kiln shelf. (Needs to be raised up so the vent can draw air out the bottom.

5. Bring in the rings, one at a time in the proper order. Line 'em up and latch 'em together.

6. Bring in the lid; slide the hinge pin back in place and secure it. Position the prop and discover the screw that holds it in place is missing. Go looking for another screw to replace it. Find out the screw isn't missing, it's broken off. Fortunately, I have a screw puller, so make short work of this problem.

7. Slip controller box into its hinges. (This is, bar none, the coolest part. Having struggled for years with control boxes that need to be supported with one hand, while two others hold a wire and screwdriver, respectively, the idea that the control box hinges open for access? Genius.)

8. Reconnect the power leads to the bus block. Plug in the thermocouple. Close the box and screw otto the frame.

9. Say a quick prayer.

10. Flip the circuit breaker. Display reads PF; this is normal when current has been interrupted. It interprets it as a power failure. Press Clear and the display reads IDLE and 67 degrees.

Oh my God, guys, I have a new kiln.

Fixin's

Sep. 7th, 2017 06:09 pm
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So, strange things from the very start. The switch on the vent cord is rated at 15 amps. Bi-Mart only has that model in a 3-amp rating; Jerry's has a 6-amp version. The fan motor itself seems to only draw 1.1 amps, so I figured I'd be safe with the switch from Jerry's.

The duct was another thing entirely: it's stainless steel, held on with set-screws and silicone sealant, and didn't want to go back to its proper shape for any amount of squeezing, tapping or vise-gripping. Finally, I stripped off the sealant, unscrewed the screws and pulled the whole tube out. I fortunately had a piece of pipe about four inches longer, left over from a studio project, that I could use as an anvil. I slid the duct over it, suspended both ends from sawhorses, and hammered on it until it approximated round again. Rescrew, reseal, and the vent is ready to go.

For some reason, the previous owner had the kiln up on wheels. Possibly easier to position; definitely harder to load, as the casters raise up the kiln a good four inches. Fortunately, they're only bolted on, so come off easily enough.

I decided to decommission the KilnSitter entirely, which necessitates several calls to Skutt. It looks like I can just join the red, black and white wires together and cap them off, then clip the ground close to the box, but I want to be sure, so I call customer support.

They haven't got a clue either. White wire? What white wire? Also, the red and black wires don't seem to connect where the diagram says they should. I shoot them a couple of photos of the inside of the sitter and computer box, then give up for the weekend.

On Monday, we hear back with good news: He's found the relevant circuit diagram deep in their archives. This is when I find out how old this kiln really is.

It turns out I was mostly right: the red and black leads do connect. The white wire only provides power to the limit timer, so it can get cut out entirely, as can the ground. After some scary discussion about whether I should run the red wire directly to the mercury relay (mercury relay?) instead, I choose the better part of valor (i.e. cowardice) and just cut both wires, twist them together, and screw on cap.

It occurs to me as we're talking that it sounds like the explosives expert coaching the terrified civilian into defusing a bomb via long-distance. First you cut the red wire. Then disconnect the black wire from the relay and attach the red one there. Clip and cap the white…

After finishing the duct work and electrical, it's time to be a brick mason…

Well, not quite. I have most of the bricks I need to replace, neatly bundled with the kiln, although I still need to order one more down from Portland, so there's another week gone.

Replacing the brick is not that complicated. First, I have to tease out the elements from the grooves in the bricks, remembering to first pull out the element pins holding them in place. This needs to be done right-side-up, as the pins are nigh onto invisible otherwise.



Afterwards, I get Denise's help to flip the ring over, as all the broken bricks are at the bottom. I loosen the stainless steel jacket by turning the screws on a set of hose-clamps that are split in half and spot-welded to the frame. Once that's done, it's not too difficult to slide out the broken brick and slide in the replacements. Two of them aren't too badly busted, and all the pieces are here, so I stick them back together with Sairset, a high temperature kiln cement. Tighten down the jacket, thread the elements back in, and press them into their grooves with a plastic putty knife while replacing the pins. And throwing in a few extra, for good luck.
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Testing clay bodies again.

The last time I did this, I was in crisis mode. The clay body I'd relied on for years--Pine Lake White, from Seattle Pottery--had started throwing out chips. This sometimes happens when the fireclay component has insufficiently-screened bits of limestone in it. They calcine down to quicklime in the bisque firing, then rehydrate to lime from atmospheric moisture, expanding just enough to pop a little chip out of the surface. You can tell that's you problem because there'll be a little white bit at the bottom of the hole, lime-pops they're called.

This time, there were all the symptoms of lime-pops but the lime. Mystery pits were showing up after bisque, sometimes even during the glaze firing, which is worse. (You can sand/grind down the pits to minimize them in your bisque. If a chip pops out in the glaze firing, it leaves a bare hole and, often as not, a flake of clay stuck to the glaze.) My potter friend Pete Meyer suggested the problem might be bits of lignite (soft coal), which would burn out entirely. Whatever it was, it was driving me nuts.

Worse was the supplier's reaction. They claimed no knowledge of the problem; apparently, I was the only potter in the Northwest who had this happening. I decided to take my business elsewhere. After testing a bunch of clay bodies that looked similar (based on sample chips on their website), I settled on Clay Art Center's DWS.

I've used DWS for a couple of years now. It throws and hand builds well, iron spots nicely, but it's difficult to trim. This summer, in particular, I've noticed that it's very hard to smooth out the pits left behind by the coarser particles, and that can sometimes lead to pin-holing in the glaze. I don't know if the particles are coarser, or I'm trimming differently--dryer, perhaps, since it's summer, or my recently re-sharpened tungsten carbide trim tool has different cutting characteristics. In any case, it's wasting my time and occasionally, my pots.

So I'm testing again. Two candidates this time: Dakota White, which is basically the same clay without the coarse bits (DWS is apparently short for Dakota With Sand), and CAC White, recommended by the salesman in Tacoma, which I suspect will be too white for my glaze palette. I want a warm tone with iron speckles in my glaze, and I don't think I'll get them.

So far, I've done shrinkage tests, flat tiles scribed with a 10 cm line that I'll then measure as they dry, are bisque fired, and glaze fired. I've also thrown and trimmed some bowls--both trim nicely smooth. I'll also need to do some tests throwing larger forms, to see whether I really need the sand or not. I could theoretically buy both Dakota and DWS, but I hate to complicate my life like that. One clay body, please.

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I've been poking around the archives, trying to find the posts on glaze and overglaze recipes, because every now and then someone asks for them. I've finally found both, and have retagged them glaze recipes, so they'll be easier to find.

But recipes aren't the only important information. There are details of mixing and application that need to be considered.
 
Consistency. Glaze recipes tend toward dairy metaphors. A glaze should be the consistency of milk, cream, half-and-half. I found this maddeningly imprecise, and a real problem when painting on vertical surfaces. Too thin a glaze coat, and the base is brown and unattractive from the iron in the clay body, but too thick, and the picture runs off the pot. After far more years than I'd like to admit, I finally bought a hydrometer. It's a glass tube, like an oversized thermometer, with a lead weight in the bottom and a paper scale, graduated to measure specific gravity. After mixing the glaze thoroughly and waiting for it to stop spinning around in the bucket, I lower the hydrometer into the glaze and let it float, then read off the scale where it's at the surface level. The specific gravity of water is 1.000; I've found 1.500 works perfectly for me. (As an aside, this means there's twice as much water as dry materials in my glaze, so for my standard batch of 14,000 grams, I'm actually mixing up 42,000 grams of glaze--a bit over 90 lbs.)

 I also find that the glaze gets thicker with use, whether from evaporation or absorption of water into the bisque, so I check a couple of times a day, and add water as necessary.

Thickness. The thickness of application of a glaze depends on several factors: how you apply it, for how long, and how much water the pot can absorb. I dip or pour the glaze--spraying leaves too fluffy a surface for my overglaze decoration--for a count of four (one, two, three, four--out again), which I long ago determined is actually about three seconds. Apparently, I count in waltz time, 3:4. Smaller pots, especially thinner pots like soup or toddler bowls, may need a little more time in the glaze to get a proper coating. I usually dip lids for casseroles, cookie jars, honey pots twice to get a proper coating.

Settling. If the glaze isn't fully suspended, you're not going to get an even coat. If your glaze is high in slightly soluble minerals, like nepheline syenite, it'll want to settle to the bottom of the bucket and form a rock there, while leaving just enough in suspension to fool you into thinking you're still applying glaze. Mix early, mix often. And a teaspoon of epsom salts, and a bit of aging, doesn't hurt either. I now mix my glaze for each firing during the previous firing--four to six weeks before I actually start glazing again. The extra conditioning time does wonders for glaze suspension.

Overglaze application. If you're used to underglazes, you're gonna put the stains on too thickly. You want it the consistency of india ink, maybe a little thinner. And that's for the initial line drawing, at full strength. Dilute with water to create washes, shadows, lighter tones. There's almost homeopathically small quantities of red stain in the water when I glaze bunny ears. I want only the palest of pink. Some colors have more tinting strength than other. Cobalt carbonate can be diluted many times and still give you blue.

If your stains are blistering, or, worse, burning out to a scabby grey scale, you'll really want to dilute them down some.

Firing temperature. There's a lot of variation in the way potters fire to cone 10. I know a potter who fires cone 10 flat on the pad, 11 nearly touching down to get the glaze effects he wants. If I did that, every picture on every vertical surface would be a blurred smear. I want cone 10 just touching at its tip. If the kiln is firing unevenly, I may fiddle with the damper to try and catch up the cold end, but not past that point. I can usually count on carry-over to drop that last half-cone, particularly if the top of the kiln is cooler than the bottom at shut-down.

Other firing ranges. I'm firing cone 10 in reduction; I've also used cobalt, iron and rutile at cone 6, reduction. Not sure how they'd work in oxidation; some metals, iron in particular, act as a flux in reduced form, but a refractory in oxidation. Experiment with a reliable white/whitish base glaze, moderately glossy, and be prepared to add Gerstley Borate or Ferro Frit 3134 to your oxides and stains. I wouldn't overglaze on a matte glaze if I intended to use it for food; I'd worry that the oxides wouldn't be sufficiently absorbed into the glassy matrix.
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Most potters in the world (Japan excepted. Don't ask me why.) throw with the potter's wheel turning clockwise.

But I'm left-handed. I grew up in a world of Of course,  that's if you're right-handed. Left-handers reverse these directions. So when I first started to throw, I kicked the wheel clockwise.

If the regular pottery professor had been teaching, or if Viterbo had had electric wheels, it might have been otherwise. Throwing is very ambidextrous,  equally easy (or difficult. For me, more the latter) to learn turning either direction, and back then, few electric wheels had a reversing switch. But Tim was on sabbatical, Jan was occupied with teaching an unfamiliar class, and the studio was committed to Leach-style treadle wheels, so by the time anyone noticed, I'd gotten used to the direction I was throwing.

I continued throwing on kick-wheels, left-handed, after college, all through graduate school, and into my days at the Craft Center. I did teach myself to throw counter-clockwise as a teaching assistant in grad school, the better to do demos, but it never came as easily as clockwise.

Fast forward to my days with Slippery Bank Pottery. I was committed to making nine dozen hummingbird feeders as week, while continuing my teaching load. Kick-wheels weren't gonna make it.

I mentioned Japan earlier? For some cultural reason, right-handed potters throw clockwise there. Fortunately for me, the Craft Center had a Japanese-made electric wheel, an old Shimpo that nobody used much because the speed control was either a very stiff, inconveniently placed pedal, or the attached gearshift-style lever. It quickly became my wheel.

Wheels with reversing switches are more-or-less standard these days, so I guess I'm not the only lefty out there. I have two electric wheels now, a Pacifica with factory reverse and an old Soldner with a switch bodged in by the previous owner (who I once made hummingbird feeders for). But I still get the occasional confused look as I throw, from spectators who aren't quite sure what's different, but know something ain't right.



Bang!

May. 29th, 2017 07:56 am
offcntr: (Default)
Reluctantly started glazing for my next firing yesterday. I've gotten spoiled by my unplanned week off, want to play in the studio or lie around reading. Unfortunately, deadlines wait for no one. The back room is still full of Sookjae's pots until this morning, so I do the easy stuff--elephant banks, one dip in clear glaze--and fussy stuff--dragons, multiple layers, lots of wax resist. Afterwards, I wax a bunch of teapots and casseroles for the morning, and look for other work to do.

The chimney looks a little loose, gaps between the bricks. During a firing, the bricks get hot enough to actually expand a few millimeters, push each other apart. As they cool, they don't move back, so each firing finds them a little further apart. Gaps between the bricks allow air to be pulled into the chimney, diluting the pressure drawing gasses from the kiln itself. Less draft means a less efficient, less controllable firing. More reduction when I don't want it, more fuel burned, more time wasted.

The solution? As the Hitchhiker's Guide once said, the secret is to bang the rocks together. Or in this case, bricks. All you need is a hammer and a scrap of 2x4--and a dust mask--to bang in from the corners. Back to back, side to side. Can't push back from the front, sadly, chimney's too close to the kiln wall, but I can still make the kiln visibly tighter by the time I'm through.

Before and after.

Timing

Mar. 25th, 2017 06:38 am
offcntr: (spacebear)
In my normal firing schedule, we load the kiln during the day, go home and collapse, then I rouse myself to come back and light the kiln at 8:30 pm. With the right adjustment of gas, it'll fire on its own until I return around six the next morning. With luck, I'll find the kiln orange hot and nearly ready for body reduction. This firing was different.

We had tickets to see Alton Brown at the Hult Center last night (if you were there, I was the one who shouted Temperature control! during the mixed drink bit), so had the choice of starting the kiln early, before the the show, or waiting until after. I didn't want to fire super slow and waste fuel, so opted for the latter.

Finally got things started at 11 pm, did my best to gauge flame speed to give me enough heat, but not too much. And of course slept terribly all night, worrying that I'd got it wrong.

Arrived at 5:54 am to find orange heat, cone 08 down on top but not bottom, so went straight into body reduction. Thirty minutes later, 08 has dropped on the bottom, 04 top, and I'm well into the firing, still dark outside.

Yay, timing!

Evolving

Feb. 23rd, 2017 06:56 pm
offcntr: (rocket)
Round about sixteen years ago, when I started making the family room of our new-mortgaged house into a pottery studio, I knew I needed a big shelving unit, something I could slide ware boards of freshly-thrown or trimmed or drying pots on and off of easily. My studio down at Club Mud had a system made of two-by-four uprights and pipe cross-members, simple, flexible and easy to duplicate. Or so I thought.

Almost immediately, I got it wrong.

I put the back set of uprights right against the wall, for stability. Front set about forty-four inches out, nearly to the other end of the four-foot ware boards. Used scaffolding pipe I bought on the cheap from BRING Recycling. It worked. Sort of.

The problem is balance. When you slide a ware board onto the rack, it goes in easily until just past half-way, then the front end starts to droop, more and more the further in it gets. Since the back pipe support was on the back wall, it took superhuman strength to keep the shelf from crashing down on the pots below it before it reached the next support. Clearly, a solution was needed.

At the time, I had a surplus of ware boards. I'd bought a bunch from a potter who was closing shop, so I could just leave a set permanently mounted on the rack, slide the full boards in on top. Sure I lost 3/4" of height on each level, but I could live with that. And it's not like I'd ever run out of boards, right?

Fast forward a few years. Suddenly, I've got three galleries, eight road shows, and Saturday Market is doing unexpectedly well. I'm running out of ware space, I'm running out of ware boards, I have to do something. I briefly consider replacing the base set of boards with something thinner, like 1/4" paneling, but that just seems like a Panda's Thumb solution.

(For those of you who aren't natural history geeks like me, The Panda's Thumb is an essay and book by Stephen Jay Gould that tells how evolution jury-rigs solutions to problems. The panda was a bear that had become strictly herbivorous, and needed the dexterity to strip bamboo shoots from stems so it could concentrate on the high-nutrition bits and toss the chaff. A thumb would be a wonderful help, but unfortunately, the toe that developed into a thumb in primates (and us) was already in use as a toe in the panda's bear feet. Since evolution doesn't go backwards, they evolved a thumb-like appendage from an oversize wrist bone. It works, but it's limited. Kinda like my shelving system.)

Fortunately, I can reverse course. My initial plan was to leave the back uprights in place, for stability, install a new set ten inches back from the front ones, then move them to the back, ten inches from the wall. This would mean no ware board would need support more than 28 inches before the next cross-pipe caught it, and I'd only have to buy hardware and two-by-fours for one more set of uprights.

The more I looked at the existing ones, though, the less I liked the spacing. I'd originally placed holes 5" apart, on center, but never actually used every set, because inch-and-a-half of shelf. With pipes in every other set of holes, my basic space was 10" high. If I only needed one shelf, though, I could use the closer spacing and get twice as many dinner plates, desserts, or pie dishes. With an extra inch, though, I could fit soup bowls. And I throw a lot of soup bowls.

So I went back to Jerry's for two more uprights. Drilled 32 inch-and-a-quarter holes, inhaling copious amounts of sawdust. Installed new uprights, dis-installed old ones, reusing the lag screws. (The photo shows the halfway point. Lighter colored 2x4's are the new set, darker colored ones the old.) Moved pipes into their new places, shifted shelves around, dusted, cleared fifteen years of detritus, and guess what? I have a storage system that works.

Also? Sixteen extra ware boards.

If only I'd done this sixteen years ago.
offcntr: (maggie)
The nice thing about procrastinating is that it gives me time to think about things.

Every time I thought about this sculpture project, tentatively titled Baba Yaga Takes a Lodger, I thought of another problem, found another solution. Case in point: Chicken legs.

I'd gotten a scaly texture on clay before with lace, rolling it into the surface. Unfortunately, that was in my Craft Center days, and I foolishly left all those bits and scraps behind when I left the job in 1998. Nothing at the fabric store looked convincing. One of the potters at Club Mud had a strip of lace that might have worked, but wouldn't let me borrow it, and I wasn't ready to start the project then and there during my last firing.

The roulette was a total accident. I made a completely different one to make roof tiles. It was scalloped with popsicle-stick depressions, and kinda worked, but not really. Too cute, too Beatrix Potter. Also, the impressions were ragged-edged, a little erratic. I decided I either needed thatch roof, or wooden shakes. The latter are easy enough to model with a piece of end-grain wood, so that's what I decided to do. I consigned my roofing roulette to the recycle bucket where it dissolved to slip.

But the idea stayed. I pulled out all of my modeling tools. Made a set of long scale impressions on one side of a roll, smaller, deeper ones on the other. Filled in the space with dimples of a couple of different sizes in a random pattern. Dried it and tried it and it worked. Far better than I had any expectation of.

That wasn't the only custom tool I made for this job. I usually use tools sanded from bamboo chopsticks to model facial features. These figures were so tiny that I ended up making a new set of miniature tools from bamboo skewers.

Other problems were easier. How do make a brick chimney? Press the edge of a ruler into the slabs to make horizontal courses. Make seams between the individual bricks with a flat-cut popsicle stick.


How do I make clapboard siding? Press the edge of a board into the slab, many, many times. How do I get the siding to line up at corners? Don't bother. This is a witch's house. Decor is not a priority. I almost made it a good deal more rickety, but decided if the house was alive (which I assumed. Chicken legs, remember?), it would probably heal from minor wear and tear.

Figuring out how to safely support the house on the legs took most of last year. It finally occurred to me to make the house and legs separately, fire them separately, assemble them later.


Some questions didn't get answered until I actually cut into the clay. I had three different ideas for the front steps, actually cut out two different templates, and then didn't use any of them. I realized the steps as visualized were just too big. If I made the steps shorter than the bottom of the door, a) they'd be free-standing and easier to fire without breaking, and b) it'd give the impression that the house was in the act of squatting down to meet its front stoop.

So here's Baba Yaga Takes a Lodger. Time to recycle the scrap, start the long, slow drying process, think about glazing.

Oh, and probably? Start making pots again.
offcntr: (spacebear)
I've had the weirdest problems with my glaze over the years.

Don't get me wrong, I love the glaze. When it's right, it's a beautiful cream color with iron speckles that reacts wonderfully with the overglaze colors. It's tough and durable on functional pottery, cleans up easily with just a little soak.

In the application… well, there's been problems.

It was originally designed for single firing, going on bone dry un-bisqued pots, so it had a lot of clay in it, nearly 20%. This allowed it to expand slightly with the raw clay under it, shrink again as it dried. When I started bisque firing, it'd crack and peel off with too thick an application, drying up like a mud puddle in the sun.

Calcining (pre-firing) half of the clay solved that problem. I now load a big bisque bowl of powdered ball clay into the kiln every few months, then use it when I mix my glaze.

The next problem wasn't with the glaze itself, but with the glaze suppliers. The original recipe was about 20% Gerstley Borate, but around 1998, Hammill & Gillespie, the suppliers, announced that they'd mined out the source and it would no longer be available. Every potter in the world promptly stocked up on it (myself included), after which they say Oh, my bad, here's another warehouse full, and apparently, they're still finding warehouses, because it's still on the market.

I didn't know this at the time, so I fired up my glaze-calculating software and re-jiggered the glaze to use Ferro frit 3195, which is a more reliable composition, and also isn't hygroscopic (water-retaining), which meant that I no longer had to worry about stray patches of not-quite-dry glaze peeling off on my fingertips when I picked a pot up. The surface was a little powdery, harder to paint on, but adding another percent of bentonite fixed that.

I still had a settling problem, though. Nearly half of the glaze is a sodium-bearing mineral called Nepheline Syenite, and it likes to settle like a rock on the bottom of the bucket. Digging it out and crumbling it back into the liquid part of the mix didn't help. Adding about a teaspoon of epsom salts to every bucket did.


But not completely. It tended to settle, though not as fast nor as hard. But I could still wind up with brown pots, thinly glazed because the glaze minerals were in a mud on the bottom of the basin and not in suspension where they belonged. And mixing before each pot gets dipped gets tedious and time-consuming.

I found the solutions more-or-less by accident. I normally mix a new batch of glaze at the beginning of the glazing/firing cycle, two 7000 gram (dry weight) buckets. This time, though, I knew I wouldn't have time; too many shows in too short a period. Since I'm always looking for something to do while firing the glaze kiln, I mixed up glaze for my next cycle, five or six weeks out.

Starting the next glazing was lovely: walk in, stir the bucket up and start dipping. After a while, I realized something was different. The glaze wasn't settling! I'd still give it a stir every once and a while to keep from developing a sheen of water at the surface, but the bottom of the bucket stayed clear, all the glaze materials suspended.

I'm not sure of the chemistry or physics involved, which component needed the extra time for hydration, but I'm not gonna question it. The kiln is firing, I've mopped the floors and put away my stains and brushes. Time to mix more glaze.
offcntr: (spacebear)
Had a fellow come into the booth yesterday with an... interesting request. He was in possession of his buddy's ashes. Cremation remains. Now if he'd wanted me to make him a container for the cremains, I'd have been totally ready. Said buddy was a hunter and fisherman, and the patterns I paint would've been perfect for the project. But he didn't want me to make an urn for the ashes.

He wanted me to make an urn with the ashes.  Mix them into the clay, or the glaze, or both.

Uhm, no.

Quite apart from the ewww! factor, there are technical issues involved. Cremains are not ceramically neutral. They're mostly bone ash, with a bit of iron oxide and other trace minerals. Bone ash is a flux, a chemical that lowers the melting point of ceramic blends. Mixed into the clay body, it might lower the firing range by a couple of cones. It might change the thermal expansion of the fired clay, causing the glaze to craze, or worse, shiver. Unless it's milled very fine and sieved, it might cause lime pop-outs, little chips popped out of the clay when calcium carbonate is calcined to calcium oxide, then rehydrated to calcium hydroxide, expanding and blowing bits of clay out of the piece.

A better use might be in a glaze. Bone ash is a component of a lot of microcrystalline iron glazes: Kaki, Shaner red. With some testing, I could probably even coax a workable version of my beloved Best Possible White out of it, subbing ash for the talc and whiting.

But there's the rub. Testing. I'd need to mix up tests, fire them, adjust the recipes, fire again. With my fifty cubic-foot kiln and once-every-six-weeks firing schedule, this project could take all year.

More importantly, not all tests turn out. The failure rate is high, unsuccessful tries are thrown out. And I just don't feel comfortable treating the remains of a person like that. Like raw material.
 
offcntr: (spacebear)
I've been struggling for a while with the firing atmosphere in my glaze kiln. My glazes need to fire in a reducing atmosphere for best results: creamy color, iron speckles, best oxide and stain color reaction.

The problem was getting the atmosphere consistent. There was always an oxidizing spot, usually low and toward the front left, near the door jamb. I assumed an air leak through the door, around the frame, or even through gaps in the floor, but no amount of tightening brick or packing/wadding with ceramic fiber made any difference.

Then one day a couple of months back, while unblocking the burner ports prior to unloading a particularly unsuccessful firing, it occurred to me that the gas pipe along the wall supporting the burners was not actually parallel to the wall of the kiln. The tip of the front burner was actually a good two inches further from the burner port than the back one. Light bulb time!

As I've explained before, you get a reducing atmosphere in the kiln by controlling the ratio of air to fuel in the kiln. Too much air, oxidizing; too little air, reducing. But there's two different sources of air in the kiln, and two ways to control it.

Primary air is pulled through the burners themselves, by the low pressure caused by fast-moving gas coming out of the aperture into the mixer. (This is the venturi effect, so we call them venturi burners.) A disk threaded onto the back of the burner limits the amount of air drawn into the kiln. Twisting the shutter open allows more air, closed, less.

Secondary air comes in through the burner ports, mostly, but also through the peep holes, cracks in the door, gaps in the floor, what have you. The draft from hot gases exiting up the chimney pulls secondary air into the kiln, where it combines with unburned fuel from the burners. The amount of secondary air is controlled by the damper, a kiln shelf that slides in and out of a slot in the chimney. Close the damper, the kiln reduces. Open it up and the atmosphere becomes neutral or even oxidizing, as the excess fuel is burned more thoroughly.

The problem is, while it's possible to control primary air at the individual burners, secondary air has only one control for the whole kiln. Normally, this isn't a problem, as the gaps and openings are evenly distributed around the kiln, but here I had a burner with a lot more space around it for secondary air. And it happened to be in the corner of the kiln where oxidation was a persistent problem.

But how to fix it? The gas pipe is old, joints solid with rust at this point, so moving one end closer to the kiln would be a major project. I tried just wedging a piece of 2-by-4 between the burner and wall to push it closer, but it didn't move appreciably. I could try to compensate by cutting back the primary air, or even by running extra gas through that burner, but how much is difficult to estimate.

So the only thing left is to move the kiln wall closer.

No, not really. What I did is take some thin sections of soft brick, each about an inch thick, and build a collar around the burner port, closing the gap between burner tip and brick.

My first firing after the changes was astonishing. The lower corner was reduced perfectly. There were a few random oxidation spots, both about chest high. One was in the middle of the back wall, the other at the left door jamb, but they were very small, affecting about half a pot each.

The second firing, which I unloaded last night, was perfect. No oxidation anywhere in the kiln, and no over-reduction (which causes excessively brown pots, and sometimes bubbling in the black and green stains). And the cones? Even, top and bottom, to within millimeters.

I've shown the fix to Jon and Beth, two of the other potters who fire this kiln, and explained my repair. I'm not sure how long it will be before I load high-value items (teapots, special orders) into what used to be the death zone. Maybe one more firing? I need to convince myself that this thing is in fact repeatable.

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