Cups explosion in the kiln during glaze firing

[ellirogi]

Hello, It is my first time here. I have had some issues with stoneware cups exploding during glaze firing, which I do not understand at all. Any advice, help or ideas on what could have happened will be very welcome! 
 

I first noticed a similar issue with a porcelain (firing to vitrification at 1240-1250 according to the seller), with many test cups exploding in the kiln after a second firing (I put it on the fact that this happened because I fired a second time). I also recently poured hot coffee in one of the surviving cups and it split and shattered into several pieces. I am only mentioning this first occurrence because I used the same glaze on this porcelain and the stoneware.

 

However, these were just cups I made for my pleasure. I have had more problematic explosions with a stoneware (from the same seller, and same firing temperature). I followed the firing schedule of the seller. I had made 24 cups, and only managed to get 8 left intact (I haven’t poured hot liquid in them). I am attaching a photo. Nothing stuck together at all. 

I haven’t written to the sellers of the clay or the glaze. The firing ranges are supposed to overlap completely. I fired all these in the same kiln.
 

Is it a problem with both clays? Or with the glaze shrinking at a different rate? Another issue I am not aware of…? I am not disappointed anymore, but cannot understand what happened.

 

[Callie Beller Diesel]

The good news is, I don’t think it’s a mistake you’re making in the forming or anything else. The only reason a pot would “explode” in the glaze fire is if the coefficient of thermal expansion (COE or CTE) are really different. The firing temperatures can overlap nicely, but if the clay and the glaze shrink at different rates as they go through inversion points, one can pull the other apart. This is especially true if you’re using a porcelain and throwing it very thinly, or if your white stoneware has a lot of free silica. Most porcelains have a lot of silica.

Unless you’re crash cooling through quartz inversion (573*C), your problem is likely a result of glaze chemistry, especially since you have this happening across 2 clay bodies. Manufacturers usually aren’t willing to share recipes, but I’m sure if you call your supplier and tell them about the issues you’re having, they can suggest a better alternative.

[Pres]

I also wonder about your glazing/firing steps. Are you firing the glazed pots immediately after glazing and clean up? If so, the absorbed moisture in your glaze ware could cause explosions in your kiln especially if firing fast. Another question might be what your bisque temp is to.

Pictures may help, as would a more in-depth description of your process.

 

best,

Pres

[Min]

Just to add a couple points to what has already been said by Callie about glaze fit. If the pots are only glazed on the inside (as seems to be the trend the past few years) this will exacerbate any glaze fit problem insofar as the pots dunting like you are experiencing. Going forward I would test for dunting by freezing half a dozen or so cups overnight then place them in the sink and immediately pour boiling water into them. If the glaze has too low a coefficient of expansion (COE) you will see dunting. (pots splitting)  A low COE glaze is one where the glaze is larger than the pot it's on. Shivering (sharp slivers of glaze coming off the pots, usually on the rim) and dunting can result if the mismatch of fit is more than the clay can absorb. Crazing is the opposite, the glaze is smaller than the pot and needs to stretch to fit the pot as it cools. 

Welcome to the forum.

[ellirogi]

Dear Callie, Pres and Min, 

Thank you so much for all you help and answers. It is so useful and generous of you…I can finally understand what is happening and how to move forward!

I will definitely test for dinting in the future.

Many thanks again,

e