Bowls Start round, become oval when fired

[Joe Roach]

Hello, I have been making hand built bowls from slabs. When I bisque fire (cone 04) the round bowls become slightly oval shaped, which becomes even more pronounced after the glaze firing (cone 6). It doesn't seem to matter what clay body I've used, although it seems worse with porcelain. I use a slab roller and I'm careful to roll and compress the clay equally in both directions and wait until bone dry before a slow bisque fire. I have even noticed that a bowl I made from coils had become slightly oval shaped. I've only had my own kiln for a short time (a small used Skutt) and I'm wondering if the warping could be caused by a problem with my kiln. The warping doesn't effect the bottom of my pieces, they don't wobble. They are just going from a nice round bowl to an oval shape disappointment!

[liambesaw]

How do you de-mold them? Are they pretty thin?

[liambesaw]

I feel like it's a problem with plastic memory.  It is deforming in bisque which is I think that.  

[Joe Roach]

I have used various bowls as molds, mostly thrift store finds. I use things plastic wrap or paper towels to keep the clay from sticking until it's dry enough to take out. I do tend to roll the slabs pretty thin.

[Joe Roach]

One of the bowls that started out round and became slightly oval shaped was made by pressing leaves from a plant in my yard into the slab. Then I pressed them into a plaster bowl shaped mold and connected them. They would have been from different directions on the same slab, which is another reason I don't think that it's from the way I've handled the slab. Plus the coiled "bowl" was made like a basket, with gaps in between. I hope this makes sense. Could it be that I'm crowding the kiln shelf?

[Min]

14 hours ago, Joe Roach said:

They would have been from different directions on the same slab, which is another reason I don't think that it's from the way I've handled the slab. Plus the coiled "bowl" was made like a basket, with gaps in between. I hope this makes sense. Could it be that I'm crowding the kiln shelf?

I've found that porcelain bowls can warp if the outside of the bowl is close to an element and the other side isn't. If porcelain is taken right up to it's maturing cone (or close) it can get slumpy, the hotter side of the bowl will move more than than the cooler side and distort the bowl. Clay memory will make a difference too, how you handle the slabs or coil built pot. If there was any warpage showing up in the greenware it will be more pronounced by the time the glaze firing is done.

Welcome to the forum. 

[GEP]

When you roll out your slab, make sure you are not stretching the clay in only one direction. If you do, that direction will rebound back more than the other direction. If you are using a slab roller, put the clay through in one direction making the slab twice as thick as you want it. Then rotate the slab 90 degrees and put it through the slab roller again to achieve the finished thickness. I see in your original comment that you say you are “compressing the clay in both directions,” but I’m hoping to spell out technique more specifically. 

I make a lot of square plates. Before I figured this out, many of them would emerge from the kiln as rectangles. 

As for your coiled pot coming out oval, are you working on a banding wheel, which will allow you to address the pot equally from all 360 degrees as you are building it? If you’re not using a banding wheel, this could also be affecting your slab pots.