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Help: stress fractures shattering obsidian clay in glaze fire


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4 hours ago, kristinanoel said:

so I'm thinking it's that irregular cooling problem. If my mitigation tactics don't  solve the issue, I'll likely finish out this order and then abandon this glaze on this form! 

 Im not entirely convinced about the severity of uneven cooling because of the mass of shelves and air gaps under the wares etc… it’s just not something personally I have been able to measure using an infrared device periodically. There generally is just not enough air movement in a sealed kiln. Anything  is possible though ……. but removing the site ports during cooling can definitely provide a pathway for enough air flow to cool things unevenly. I would definitely leave them in and let the mass of the kiln brick, shelves and wares give up their heat naturally by radiation and conduction. If you have a downdraft vent, turning it off at the top of the cycle before cooling is probably a solid idea as well and saves energy.

Just a little data to add, we measured the cool down rate of several kilns and firings. The interesting takeaway, the end of firing is probably on the order of 400 degrees per hour so most vulnerable to uneven cooling and by the time of quartz inversion ((prox 1000 deg) say 50 - 70 degrees per hour.

Most wares do just fine firing 200 - 500 degrees per hour on the way up so no surprise quartz inversion issues on cooldown are probably rare and turning off you fan before the end of firing could prove most helpful.

Data for a sealed kiln

45539EDD-ACB7-45C8-9569-8C7DF831A8F0.jpeg

Edited by Bill Kielb
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4 hours ago, kristinanoel said:

how does the new talc behave

It depends on which one. There’s a few possible replacements being tested, and they have different COE’s. Some manufacturers are also opting to eliminate talc from their recipes because it’s become harder to obtain a version that doesn’t occur naturally with asbestos and/or doesn’t fire some shade of amber from iron contamination.

Talc is a source of magnesium and silica, and has been most often added to glazes to create mattes in larger amounts, or control crazing in smaller ones. if you add too much, you might get dunting under the right conditions. If the right structural stresses are in play, a change in talc source that your supplier might not have mentioned could  have tipped the balance from a combo that was working into one that wasn’t. 

Talc has also been added to light coloured low and mid fire clay bodies to adjust thermal expansion rates, and some other things.  It’s not usually found in darker clays like your stated one, so that wasn’t a likely source in this case.

That’s the Cole’s notes version, and if you’d like an expanded rabbit hole with all the delightfully nerdy extra links, Digitalfire does provide! The notes on Tony’s efforts to reformulate a low fire casting body and associated glazes might also be interesting, even if you’re at cone 6. The principles are the same.

 

 

 

Another firing mitigation tactic you can try is to place test tiles, other small pieces or even lay down extra kiln posts around the rim of the piece. They’ll create a heat sink so the rim/lip cools at a rate closer to the bottom of the plates. If you’ve been making this design in smaller versions, sometimes the larger one will require extra babying. I can vouch for it working on my pieces, and I notice kiln pack can affect things you wouldn’t think are a big deal. In any case, it’s easy to try.

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1 hour ago, Callie Beller Diesel said:

Another firing mitigation tactic you can try is to place test tiles, other small pieces or even lay down extra kiln posts around the rim of the piece. They’ll create a heat sink so the rim/lip cools at a rate closer to the bottom of the plates. If you’ve been making this design in smaller versions, sometimes the larger one will require extra babying. I can vouch for it working on my pieces, and I notice kiln pack can affect things you wouldn’t think are a big deal. In any case, it’s easy to try.

+1

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Ah, well I hope you don’t have to abandon the glaze! As for the sand wads, simply place the piece where you want it loaded in the kiln and trace a light pencil mark to show the footprint. I’ve been taught to use a very small scoop to place quarter to half dollar sized piles of sand that can be up to about 1/2” tall as needed. Place as many piles as needed to prevent slumping, and you can sweep it off the shelves and reuse it forever. You may need to sieve out small kiln wash chunks over time. Good luck!

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Ah! The flaking kiln wash reminds me. There are non-flaking kiln wash recipes if you’re frustrated with the one you’re using. I use the no-crack kiln wash from this article.

If anyone has paywall issues or has used up their 3 articles/month, Sue McLeod has a blog post on kiln wash that mentions the same recipe. She gives some handy application tips (I like the paint roller)  and the reasons behind the ingredients. I just use the version with no silica, even in my electric. 

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