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bubble in my slab pottery ... feeling so frustrated

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Hello! i must have had beginners luck for my first few firings but lately everything I take out of the kiln has bubbles bettwen the size of 3/4 cm to 1.5 inches wide. I'm getting explosions during the bisque firing and then during the glaze fire all these bubbles show up :(

I am using the clay straight out of the bag. I cut a slab about an inch thick and slide it through my slab roller.  No sign of bubbles and yet after the glaze fire..... bubbles!

I am also taking scraps and wedding and then slab rolling and bubbles show up. So i cut and press and cut and press over and over til no sign of bubbles, then in the kiln, explosions and bubbles. 

I am self taught and feeling discouraged. Any tips?

Many thanks! 


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Air bubbles do not blow up. Moisture turning to steam is what blows up pots. So if you pot isn't totally dry before going into the kiln or if you're firing too quickly and the last bit of remaining moisture is turning to steam before it evaporates, then you get explosions. So either longer drying times, a preheat in the kiln, or a firing schedule that goes slower in the first few hundred degrees will solve that problem.

If you're getting bubbles in the clay, not in the glaze, after the glaze firing, we call that bloating, and it's usually from firing the clay too hot. What cone are you firing to, and what type of clay are you using?

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Slab work is usually thicker than thrown pieces,  when ever you have explosions it comes from moisture.   If you live in a humid area your slabs may not be totally dry,  sometimes Kansas can be humid for days.  Often  setting a piece in the sun would finish drying it,   if my kitchen oven was big enough I would heat it at the lowest temperature of the oven.  If it is large work you can candle it over night in your kiln.   Candling on a manual kiln is putting one control on low overnight and then continue with the firing in the morning.   Denice

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