I Learned Something…

Awhile back I made three vases, tulipieries. Vases for tulips. (I learned that word from my studio mate, Carolyn Summers, by the way) I threw wide, low cylinders without bottoms then altered the form to be a very narrow oval at the top (one inch wide and about an eight inch long opening) and two or three inches wide at the bottom. I attached the form, sometimes called an envelope vase, to a thin slab bottom. I used my narrative technique on the outside, glazed the inside, and fired them. And every one of them, all three…

1
…cracked. Huge cracks, big time. Look…

2
Now do you believe me?

Cracks happen in ceramics for lots of reasons. One of the first things to think about is how the kiln cooled. In this case the kiln cooled slowly so I knew that was not a problem. Another thing to think about is how the form was altered. I did move the walls of the cylinder in a lot to achieve the very narrow form. But I did it in a way that I felt comfortable with…it was done when the clay was still very soft, right off the wheel. I didn’t think there had been enough stressing to cause that major of cracking. But an obvious solution would be to not alter the form so much. I didn’t want to make that concession so I kept thinking. Wall thickness is another consideration for cracks. There is a lot of stress placed on pottery when it is subjected to such high temperatures, about 2300 degrees. Yes, the walls and bottoms of these forms were very thin. They were also quite even and I was not willing to give up that quality. I could have stopped here and made the walls a little thicker and altered the form a little less and been able to produce a good looking vase. But that was not what I wanted. “I want what I want, not what I need” I once heard a seven year old say when he got pajamas for Christmas. So… I kept thinking and then I did a really smart thing…I ran all this by Bowie. He suggested that it could have happened because the inside was glazed and the outside was not. That made sense. Glaze on just one side of a very thin wall would create extra stress. So I should just glaze the outside of the vase, too, right? Nooooo! I want what I want, remember. It seemed to me that I was using what potters call a tight glaze. Very strong, dense…hard. So I decided to try a different glaze for the liner. I chose a shino and look what I got…a nice vase and a perk!

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The perk is that with a shino glaze, the soda ash migrates through the wall of the clay and shows up on the other side. In this case, it became visible in my carved lines. Cool!

4

So was the cracking caused by the use of a too tight glaze? Or did it happen because a very thin walled form was highly altered and glazed with a very tight glaze and fired on a Tuesday? The Combo Plate!
hmmmmmmmm…

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