Thursday, March 09, 2006

Two Bags Down, No Big Bowl

I tried another 25 pound bowl.

It started cracking on the inside, while I was finishing the rim.

I cut it back to about twelve pounds.

I'll try again this weekend.


Blogger Maureen McHugh said...

Cracking? Okay, what causes that?

3:42 PM  
Blogger SquidgePa said...

Too much stress on the clay.

Looking downward into a pot from the top, the piece is pulling radially outward from the center. Centripedal acceleration from rotating the wheel also is a factor. The thickness of the clay walls, and the tooth of the clay fight this tendancy. Sometimes, just adding a thick rim can fight this, like an iron band on a barrel.

Looking from the side, a piece that flares outward from the base is pulling downward any place that is not butressed by clay underneath. The more butressing clay, the more trimming the piece will need.

Allowing the bowl to dry out a bit on the wheel head for a bit can improve the tooth of the clay, before the bowl is given its final shape.

To throw a large bowl, there needs to be a balance between the thickness of the clay, the tooth of the clay, how much the piece flares out, and how much butressing from below there is.

If a flared bowl is thrown improperly, the piece will sag, and the clay will start to stretch along the innder surface as gravity and centripetal acceleration pull on it. The clay will start to crack.

11:16 AM  
Anonymous Anonymous said...

I've thrown large bowls ... and then end up cutting lots away ... I have been thinking of using a torch to harden the inside as I throw ... I've seen others doing that ... any experience with that? -Kevin

3:23 PM  

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