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Cups cracking in the final glaze firing!


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After several years of successful cup making I have recently had a rash of cups that come out of the glaze firing (electric, cone 6) with invisible cracks in them.  The cracks run generally top to bottom and are hard to see until you know they are there.  But if you thump them the flat sound gives it away.  Some are ok at first and crack only upon being filled with hot coffee for the first time.

I'm looking for an explanation.  My hypothesis is this:

I have recently been trying to get more weight out of them at trimming.  I start with 16 oz of clay and try to trim down to less than 10 oz at leather hard stage.  To do this I have been trimming more aggressively on the top half of the cup and not just at the base.  So, as a result, the wall thickness of the cups issomewhat thinner.

Does anyone have experience with this sort of thing?  Do you think the relatively thin walls could be the reason they are cracking in the kiln?  Comments and speculation welcomed.

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3 hours ago, Rick Wise said:

Do you think the relatively thin walls could be the reason they are cracking in the kiln?

Sharp edge to the cracks, not rounded into the crack? If the glaze is on the borderline of putting too much compression on the glaze with the wall thickness you used to make them and now they are thinner it could be enough to push them into a cooling dunt. Clay can't take the compression the glaze is now putting it under. Break one open and see how thick the glaze is (especially on the bottom) compared to the clay thickness. I'ld take your remaining mugs and freeze them overnight then put them in the sink and pour boiling water into them. Clay:glaze mismatch of COE's can show up days/weeks after unloading, freezer/boiling water test will stress them so (hopefully) if they are going to dunt they will do so with this test.

I'm currently working on porcelain casting slip recipes with clear glaze. Little test pot below came out the kiln in 2 pieces, ill fitting glaze.

IMG_0764.jpeg.c15c30b367f6fe14ecab0d97df8b1089.jpeg

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5 hours ago, Rick Wise said:

Do you think the relatively thin walls could be the reason they are cracking in the kiln?  Comments and speculation welcomed.

It’s certainly possible. Why not make a couple the old way and a couple the new way and see if it’s the clay or the trimming. Personally when I want very light thin wares I try and throw them that way, then no trimming at the top  or excess stress on it trimming inverted. It’s too easy to introduce really minor cracks  or make it go out of round. My thin stuff needS to be mostly free of any major trimming as thrown. Based on the outcome of your test above, you might then try experimenting with  throwing thinner and pre trimming the bottom as much as possible before removing from the wheel.

Edited by Bill Kielb
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@Babs, so for sure the pots are getting dunting with the new clay and this glaze? Easiest way to increase the COE without adding any new materials would be to increase the 3110 frit. It's going to throw the flux ratio off a bit but should be okay still. Glaze some test cylinders (thinly thrown or slabbed) on the inside only, apply the glaze thickly, you want to create the perfect storm for dunting here. Take X amount of the glaze and add 5% more 3110, glaze a test cylinder and repeat. I'ld keep doing this until you get up to an additional 20% of 3110. After firing freeze them then pour boiling water into them and see if they dunt. If you don't want to do such an extreme test then glaze both the inside and outside with the glazes. Adjust the tin once you figure out how much frit you need to add.

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