How do you bake clay castings? Always remove the clay from the molds before baking. Never put the molds themselves in the oven. Follow the clay manufacturer's recommendations for baking your clay castings. Don't ever bake polymers at more than 300 degrees F. They will produce toxic fumes. Molds are not to be used in a kiln, but can be removed from the mold prior to curing in the kiln. We offer sample sizes of Sculpey Polymer clay on this page.