There are several easy-to-correct factors that cause polymer clay or sculpy clay projects to burn. All of these factors come down to heat. Heat, is what causes things to burn. Controlling heat and how and where that heat hits your model is the key to consistent clay bakes.
You Don’t Know the Real Temperature
The temperature gauges on cheaper ovens, like toaster ovens, should be read as “average temperature” not “actual temperature”. It’s not uncommon to set a toaster over for 275f and have it get as high as 325f or hotter.
Solution: Get a cheap in-oven thermometer and place it in the middle of your oven, monitor your first few bakes to see what temperature setting on your oven gives you a temperature between 250f and 275f.
Your Sculpt is to Close to a Heating Element
If you are using a smaller toaster oven, it’s not uncommon to have your clay on a sheet pan or wire rack an inch from the heating element. This introduces two problems that cause burning. First, the temperature being given off by the heating element is the hottest spot in the oven. Second, the wire rack or sheet pan are often hotter and placing direct heat on your model.
Solutions: Try one or more of the following things: Cover your heating element with a tile corner piece to prevent direct heat on your clay. Get a bigger toaster oven. Hang your model inside the over versus resting them on the rack.
You’ve Got Shiny Metal or Foil Inside your Oven
Some people swear that putting aluminum foil in your toaster oven helps maintain cooking temperature. I have tested this some and witnessed inconsistent results. Shiny metal or foil, especially if its crumpled or curved in away that can reflect heat may actually intensify your temperatures. I had one case where a 250f setting with foil inside caused the oven to hit 350f.
Solution: Do not put curved, crumpled or shiny metal inside your oven. Use matte finished tile or small bricks to control heat instead.
Your Polymer Clay Model does not Have Consistent Thickness
Thinner bits of polymer clay and sculpy do not need as much heat or as much time in the oven. When you back a model that has thick and thin parts it is not uncommon to burn the edges of the thin parts.
Solution: Bake the thickest parts first, then do a second bake with the smaller pieces attached. You could also try baking at lower temp 230f for longer if you cannot multi-bake your model.