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I was using a red earthenware clay body covering it with white slip and decorating over that with a variety of underglazes.   I was using Deb's clear as my glaze. I fired to cone 04. Then the glaze began to shiver off. So I switched to a white clay body eliminated the slip fired a bit lower soaked the kiln for 30 minutes and you guessed it the problem persists. My preference is to use a red clay body with slip and the underglazes. I used Pete Pinnell's slip recipe. I know its a glaze body  not fitting but wonder how to fix the problem. Plus not all the pots all the time are affected but I need a reliable glaze.  Am at at a loss.

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Hi Karen and welcome,

 

Sounds like you need to raise the COE (coefficient of expansion) of the glaze. I would either try adding the high coe frit 3110 or run some tests with the G1916Q recipe from here. For stress checking for shivering I make cylinders as thin as possible, bisque them then glaze thickly on the inside only, right up to and including the rim but not the outside of the pot. Fire then put in the coldest part of your freezer overnight. Then take the still frozen cylinders, place them in the sink and pour boiling water into them. I would try over all the combinations of your clays and slips. I would also run different glazes through that test to find what coe figure fits your clay. Whichever ones pass the shivering test I would then go on to craze test to make sure you haven't gone too much in the higher coe direction.

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For a first test I would just try adding increments of 3110 to your existing recipe. I would start with adding 3 grams to your existing 100 gram base, then add another 3 grams then a third addition of another 3 grams.  Scale up the amounts to make enough to do some test cylinders. (you are using the original Deb’s clear, same as the recipe on that link I posted right?) Try a shivering test and see how it fits. It’s likely to be a bit runnier but should be okay. If none of those tests work then I would try some alterations of the 1916 recipe from that link, as he suggests, decrease the 3195 by the same amount you increase the 3110. I find it’s hard to know how much info to put into a post without having an idea of how much glaze chem someone knows but hopefully this makes sense. I would also suggest asking your supplier for a clear glaze recipe, they should be able to help you out or if not contact the manufacturer.

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Guest JBaymore

Presently I bisque at 08 and fire at 04. I think according to the article I should bisque higher can I bisque and fire at the same cone?

 

Yes.  As long as the body is still absorbent enough to get the glaze coating you desire in a reasonable fashion.

 

best,

 

............john

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