I was making these round, polymer clay boxes when I learned again how clay can warp.  The bottom of the box was perfectly flat against the paper when I put it in the oven, but it rose up in the middle up when baked.  So I tried to force it flat, this time while warm, and it still cracked. (The box on the top left is the flat look I was going for.)

Closeup of warping and cracking.


These small, round, ceramic pie weights are now in my polymer clay toolbox because they prevent this problem.

I figured they’d make little dents in the clay, so I first put a circle of card stock inside the box to protect the bottom.  If it’s the first baking, the metal form the clay is wrapped around will protect the sides. Then I poured in the pie weights and baked.

Pie weights retain heat longer than polymer clay, so I pour them in a tin can to cool.

After I removed the warm clay box from the metal form, I decided to add a veneer to the inside bottom, so I needed to bake the box again.  This time I put a securely Scotch-taped card stock cylinder around the inside to keep the sides smooth and prevent them from bulging out.  (See photo above.)  The metal form never seems to fit again because the clay shrinks as it cools.

If you can get the bottom of the box to bake flat the first time, it might not warp in subsequent bakings, but no promises.

What I learned from this:

  1. This was Kato clay.  It needs to be baked at 300˚ or it tends to crack. Other brands, like Fimo, are more flexible.
  2. If clay is still quite hot and you’re careful, you can slightly change its shape.
  3. Pie weights will keep the bottom of a box flat when baking.
  4. Polymer clay shrinks when it cools.