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!
offcntr: (spacebear)
The last time Shelly and I tried saggar-firing, I used stoneware and she porcelain. I got some very nice, rusty red-brown colors; she got amber. So I decided to try again, with some G-mix (a porcelain-like stoneware manufactured by Georgies, up in Portland).

Shelly didn't have anything for this firing, so I used a couple of seconds to form my saggar: a soup bowl that managed to get into the bisque without trimming, and a dessert plate that had a chunk blow out in the bisque. Used a little ceramic fiber to make an air-tight gasket.

Since my test pieces were little birds, I decided to fire them head down, to see if I could get them to look like juncos. (Black head, brown body.) I used three charcoal briquets, and about a teaspoon of soda ash.

I got brown and black, gorgeous warm amber brown. The black, sadly, was bumpy and blistered. I'm going to re-fire one in Jon's kiln, see if I can get the surface to settle. Next time, less charcoal, and maybe put the pieces up on shelf chips to just fume.
offcntr: (spacebear)
Club Mud has seven electric potter's wheels (plus two kick wheels), there for the members' use, but also used by students taking classes sponsored by Maude Kerns Art Center. Recently, two of them stopped working, within days of each other.

Tea tackled the first one, followed by Don and Karen, then Tea again, who finally determined the problem was with a loose connection in the wire bringing power to the motor. Replacing it (and fixing a broken-off ground wire) brought it back to life. They couldn't find anything obvious the matter with the other one, so they dragged it off out of the way with an "Out of Service" sign, and I promised to take a look at it later.

So I came in yesterday with screwdrivers, wrenches, and a multi-tester, determined to, as Matt Damon said in The Martian, "Science the s#*t out of it."

Here's what I did.

First, I diagrammed the wiring in the control box, just so I could see where the power went in what order. (And not mess things up when I started moving wires around.) In the box were three cords: power in from the outlet, power to and from the foot pedal, and power out to the motor. There's also a power switch, circuit board, fuse, reversing switch (this wheel turns both ways), a connector block between the circuit board and reversing switch, and a separate connector block for ground wires. Don't know for sure how everything works, though I suspect the circuit board is a rectifier, converting AC to DC. Then I:

Checked for power. The motor is a variable speed DC motor, so I used the multi-tester to determine that no DC current was actually reaching the motor. So the motor is probably not the issue. More on that later.

Checked the fuse. Swapped the fuse out with one in another wheel, which still ran. Swapped fuses back.

Checked continuity. I used the continuity tester to make sure cords and switches were actually conducting electricity. Power cord was okay. Reversing switch was okay. Power cable to motor was okay. Took apart the foot pedal (speed control) and checked its cable. Was okay. Then I checked the rheostat, a sliding switch that changes the amount of current to the motor. I got no current through at the top position, nor midway down. I finally got a reading way down at full extension. Not sure whether this is a problem or not.

So it's not a broke wire or switch that I can see. Probably not the motor, but I can determine that for certain with a little bodging.

Our seven wheels are all the same manufacturer, Brent, but wildly different ages and models. Some things they have in common--the same fuse, for instance. Others, like the control box and foot pedal, I'm not so sure. Best I can do is pick one with the same motor and hope the rest is compatible.

I find exactly one wheel with matching motor, and get Jon's help lifting it up onto the table. I disconnect the power leads from its motor, and attach them to the other wheel. Yeah! It spins; theory confirmed, the motor is fine. To double check, I hook up nonworking controls to working wheel, which doesn't then work.

Now I need to narrow it down to either the circuit board or the pedal. The pedal connects to the circuit board with two spade connectors; a third goes to the ground block. I mark the connectors for white and black wires on each board (as it happens, they're in different order), then disconnect both pedals. Fortunately, the wires are long enough to reach into the other box without pulling the whole cable free.

Plug in, switch on, press pedal. Hooray! Wheel spins! It's definitely a problem in the original pedal.

At this point, I take a break and call Georgies to see if they have a replacement pedal rheostat in stock, and how much it'd cost. They'd have to order one in, and it looks like we'd have to pay for pedal and all, for around $200.

So I stop and think a bit. There's actually three control switches in the pedal: two rotary switches, blue and red, that control low and high speed limits of the wheel, and the rheostat, which as I mentioned, is a sliding switch, controlled by a forked lever anchored to the axle of the pedal with a set screw. I wonder whether one of them is the problem.

I reconnect the original pedal, turn the rotary switches back and forth to determine their limits--about 210°--then set them right in the middle of their range. Power up and test. No change. I notice that the rheostat controller is set to go from top to middle of its range. You may recall that the continuity tester didn't show any current across it at that range. I loosen the set screw--which already seems slightly loose--and rotate the lever down so it goes from middle (at off) to bottom (at full). Plug in, switch on, press the pedal and it spins! Flip the reversing switch and it spins the other way.

I fixed the wheel! I replace the bottom of the pedal. Reassemble the control box and attached it to the frame. Put the bottom plate back on the motor and wrestle the wheel back off the table. Last of all, I take off the "Broken" sign and replace it with "Fixed."

What happened to begin with? I have two theories. Either someone tried to adjust the wheel speed range from the rheostat rather than the rotary dials, or a loose set screw allowed the axle to rotate within the the end of the rheostat lever, moving it out of its effective range.
offcntr: (radiobear)
Look how cool the inside of the saggar looks after we emptied it out.
offcntr: (spacebear)
Mad science report:

Hard as it was to wait, I didn't open up the saggar firing until the rest of the kiln was completely unloaded, Denise and I'd had a lunch break… and Shelly arrived to see how our experiment had come out.



Results were great fun--some lovely amber-colored flashing on the porcelain slug, with speckles where we'd dusted the copper. Also a brown, ashy glaze on the snail, bunny, and whistles that seems heaviest near the bottom, tapering to almost black on the top. We're speculating on the presence of soda and silica in budget charcoal briquettes, since we certainly didn't sprinkle in that much soda ash.

In any case, Shelly's excited to start making more things out of cone 10 porcelain, and I'm kinda tempted, myself. I've actually got a couple of bags, rock-hard, but if I start slaking them now, maybe by my January firing I'll have some more things to put in the box.
offcntr: (spacebear)
One of my colleagues at Club Mud, Shelly Fredenberg, does lovely, minimalist animal sculptures out of porcelain. The sort of pure, white, simple surfaces that just beg for elaboration by exotic firing processes. At least, to all the rest of us surface treatment fanatics.

Jon and Karen talked her into soda-firing. Tea wants her to try saggar firing. I promised I'd throw a couple of saggars, and figure it out with her.

Saggar firing is basically firing inside a closed pot, as a way to create a special firing environment in a controlled space. It's usually used at low temperatures in electric kilns to get salt glaze or reduction effects. Less often, it's done in high-fire kilns, which is where we're gonna try.

The plan was to nest the work in combustible material, probably sawdust, top-dressed with a little copper carbonate, and see whether we got black or red or white or some wild combination of all of them. Of course I discovered I was out of sawdust, though I did have a small bag of wood chips I'd bought for my sausage smoker. Fortunately, the research I did online--which all seemed to reference the same Byron Temple workshop--suggested that crushed charcoal worked even better, and I have plenty of that.


So yesterday we loaded up the saggar. Crushed charcoal first, about six briquettes, mooshed inside a doubled clay bag with the hammer and two-by-four I used to fix the kiln chimney. (It's a multi-tasker!) Then we nestled our treasures in place, sprinkled about a teaspoon of copper carbonate over the top, and then, because we were feeling a little mad-sciencey, a little bit of soda ash. The last ingredient is used in soda firing, where it vaporizes and makes a glaze with silica in the pot. We didn't use enough to make a glaze, but hope it might help the copper fume up and move around a little bit.

Lastly, we lined the top of the saggar with scraps of ceramic fiber, for a tighter seal, and pressed the top in place. Today, I'll put it in the glaze kiln, and we'll see what happens.

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