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Hello everybody! :)

Today I had a surprise when I opened the kiln and need help to figure out what went wrong.

I made some dinner plates with this clay: Sibelco WMS2502 (stoneware clay). I used a △6 glaze that I found on glazy.  I made some test tiles with the clay & glaze combo in a small oven in my studio and they looked perfect.

When I fired the plates the first time it was a disaster. Basically the whole oven was underfired and the plates came out looking really matte and not finished. Also all other pieces with this particular glaze looked weird, some crawled and some clay completely couldn't handle this glaze.

My studio fires to △6, but it seems like the oven is quite old and because it's so big it fires really unevenly and doesn't reach the right temperature. Some pieces came out okay that were on the top.

So I thought I will fire the plates again to see if its the oven, but in a medium sized one, that seems to fire a little bit hotter . I fired a small underfired piece (little bowl)  a second time and it came out perfectly. When I fired the plates the second time - △7 - they all came out broken, except one!  -.-   (See pictures)

Now my problem is, that I have no idea where I went wrong. Is it the glaze/clay combination? Is it the the second firing? Is it because I only glaze them on one side? Could it be because some of it is recycled clay? Is the application too thick? 

I really appreciate any hints, as I'm  a bit lost...




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That's called dunting. You'll be able to tell if it happened during the heating cycle or the cooling cycle by looking that edges of the cracks. If they are sharp, it happened in the cooling. If they are melted and rounded, it happened during the heating. Either way, the process needs to be slower. 

You kind of have a perfect storm for that to happen: glaze refiring, one side glaze application, maybe a too thick glaze layer. You may not have the best glaze fit for that clay too.

If you're set on using that glaze/clay combination, I'd add a foot ring to your plate to glaze both side and apply a thinner glaze layer. If you know there's a spot in your kiln that reaches the right temperature, make sure it goes there. 

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There’s the factors that Kswan already mentioned, and all of her fixes are what I’d suggest too. Additionally,  when I looked up your clay’s specs, I notice that the manufacturer lists a cone 6-9 firing range. It doesn’t seem like much, but it means that the clay won’t be fully mature at cone 6. It can create glaze fit issues that will be exaggerated by uneven glaze application, and going too fast through the wrong point in the firing/cooling. Some clay bodies also don’t like to be re-fired, with each firing creating more crystobalite in the clay. That makes pieces more susceptible to thermal shock. 

In addition to Kswan’s fixes, I would suggest considering either firing the clay to maturity in one firing and using a cone 9 glaze, or choosing a clay body that has a top end maturation point of 1200C.

I find it odd that the bowl looks so similar to the other pieces in terms of thickness, but seems otherwise fine. I would do a flick test on that bowl, just to make sure there aren’t any hairline cracks that aren’t immediately visible. If the piece is whole, it will ring. If it is cracked, you’ll hear it. It makes almost a clicking sound.



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@Callie Beller Diesel Thanks for your input. I'm sure the refiring didn't help, they probably would have been fine with one firing. Another plate that was half the size also came out fine after the second firing, same glaze & clay. It might be the fact that they are big, flat dinner plates. I will try out different things and keep you posted about the results.

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If it was only the big flat things, then the most likely fix is to add a foot rim.

It’s still a good idea to do some porosity testing on that clay/glaze combo. If the clay isn’t mature, you could have issues if you want to be able to put pieces in the dishwasher or microwave.  Digitalfire has some good tutorials on how to. https://digitalfire.com/test/shab

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