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Jul 13

More crazing

in Watertown Discussion

I wonder if there is a larger issue? Two+ of my pieces also had crawling.

6 comments

6 Comments

Ned Levering

Jul 13

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Hi Laura, Renee, and Carolyn. I'm happy to address all of this :)

I just responded to two posts regarding flakiness/crawling which you can check out and should help you navigate that issue.

We had a couple of kiln misfires in June that were frustrating. We fire our four kilns non stop and so maintenance issues are expected. With that said, there are definitely some design quirks to our kilns that we feel are challenging. Regardless, when a kiln fails to reach temperature at a particular range, it leads to extra crawling. It's the 1,800-2000 degree range. Whenever we have a misfire that results in this, we always group the work together and leave a note.

@Laura Mintz, crazing is a chemistry issue. Glazes that have high proportions of alkaline metals (sodium and potassium primarily) have high shrinkage rates and result in crazing. In early IF days, I steered away from crazing prone glazes because they can potentially affect how food safe a glaze is. We've decided that there are too many awesome glazes/colors to ignore though. As a reminder, all of our glazes are considered food safe if the glaze is well melted and free of major defects. The glazes that are chemically prone to crazing are:

Amber Crackle :)
Archon Blue
OFF Pink
Assad Black

It's also worth noting that crazing occurs more dramatically on white clay bodies.

@renee zale, unless your pot was in a section of the glaze pickup shelf that was marked as a misfire, the kiln got up to the right temperature. There are always very minor temperature differences in particular spots of the kiln (mostly the top and bottom shelves) that we monitor.

@Carolyn Emberley, in an attempt to alleviate the infrequent, but consistent bloating we were seeing when we first introduced the charcoal brown clay, we modified are firing cycles slightly. We fire very slightly cooler, with a longer hold at the top of the firing. We did many tests early on to determine that the glazes were all melting properly with this cycle.

Crawling, running and pitting are all individual glaze issues, probably with unique causes. Crawling and running often go hand in hand of course because the glaze is falling off the pot.

Glaze running is basically related to two variables(assuming the kilns aren't over firing which they haven't been): thickness and glaze selection. We have a lot of runny glazes and glaze combinations available. With the right glaze combination, sometimes the glaze is destined to run no matter how quick the dip. Remember that larger pots are much more likely to run because they soak up more glaze. And beware of the new OFF Pink! It's a runner.

Glaze pitting and pinholing(or blistering) are challenging and slightly different issues. Pitting is usually the result of a rough surface texture that creates tiny air pockets when the pot is dipped. If the glaze isn't melty enough, that spot remains bare. Smoothing these areas can help, but often there's already a layer of glaze over the tiny bubbles.

Pinholing is when air bubbles, either trapped or from off-gasing (it's a hot topic in the ceramics world) rise to the glaze surface at the top of the firing, burst, and are then frozen there upon cooling. It's a tough issue without a quick fix. Certain glazes and combinations are more prone than others. Pinholes after the firing don't correlate to the tiny pinholes you see after applying a glaze.

As much as I have to say about glazes and firing, I often get stumped by weird, spooky results. So I'm always learning like you all are.

Thanks for your feedback.

Ned

Carolyn Emberley

Jul 13

Replying to

Thanks for such in depth and quick responses! It's always fascinating to learn more about the physics and chemistry of the glazing process

Laura Mintz

Jul 14

Replying to

Thanks Ned

Is this information posted in the studio? Before I sent the note and you replied I had just used

Archon blue on everything with Assad black on top all on porcelain…I’m glad to know for the future. I hope the kiln gods look favorably on the firing(s).

Ned Levering

Jul 15

•

Replying to

Hi Laura! Yes, there is a glaze/food safety flyer posted on the wall at each studio. There's also a collection of helpful information sheets at the glaze area in each studio which includes a Glaze Defects sheet, and a 'Glaze Card' for each glaze. The glaze cards have a general description of the glaze which will include defect tendencies, a drippiness scale, behavior in the bucket, and application tips.

Really though, the easiest way to assess your a glaze combination is to scrutinize the tile. If the combination tends to craze, it will show up on the test tile.

In regards to your Archon/Assad pots, the primary issue with crazing is that bacteria can live in the cracks. If you put the pots through the dishwasher, that does a good job killing the bacteria.

Have a great week!

Ned

renee zale

Jul 13

I haven’t had crawling but I did have two recent pieces turn out very differently than I expected. I used glazes that I have used consistently but they were almost unrecognizable. I wonder if there is an issue in getting up to the right temperature?

Carolyn Emberley

Jul 13

Yeah, I was just going through some more of my stuff from the past handful of weeks, and I have a lot of crawling, a lot of running, a lot of pitting (and I'm really diligent about smoothing out pinholes in the dry glaze), and a lot of colors not being the result I've gotten from them in the past. And I pay close attention to how thick the glaze is in the bucket, and always do incredibly quick dips. I too wonder if there's something going on. I heard we changed what cone we fire to when we got Bmix and/or charcoal, I wonder if that's related? But I think we dropped the cone, and I'd think more crawling would be more likely at higher temps, not lower. I also wonder if we ever got the broken kilns fixed, since turnaround time never went back up and feels at times even slower.