How to Harden Modeling Clay

Modeling clays don’t harden. Made out of wax or proprietary plastic derivatives, clay animators use modeling clay because it stays soft. The clay remains flexible, so the artist can reform the clay figure as needed when filming action sequences. Hence the name: modeling clay. Potters and clay artists use natural earthen clays cured in a kiln at high temperatures. The only type of clays you can harden at home are polymer clays -- designed to cure in a home’s oven -- homemade or cold porcelain clays, and self- or air-hardening clays. Warning: If you try to heat wax-based or plastic lays, they will melt in the oven.

Even among the various types of clays you can harden at home, each brand has its own requirements and handles differently. Because there are so many brands from which to choose, you must select the product that meets your craft project’s requirements by carefully reading the package instructions. Polymer clays bake in an oven in temperatures that range from roughly 200 degrees Fahrenheit to over 300 degrees, depending on the brand. Brands of self- or air-hardening clays all have specific handling instructions. Take the time to find the clay that works best for your craft projects.

Polymer clays harden in the oven at different temperatures set by the clay manufacturer. While you can cure polymer clays with different heat sources, the results can be devastating if you make a mistake. Stick with what works: the oven. Preheat the oven to 20 degrees below the brand’s recommended temperature. Once it reaches the set temperature, place the piece in the oven and bake it for 10 minutes, recommends award-winning polymer clay artist Nancy Ulrich before increasing the temperature to its recommended setting.

Before you bake the item, complete a test run with a sample piece of clay sized to the same thickness as your project. Baking times also vary across polymer clay brands. The problem arises when your clay project is thicker. The basic rule of thumb is to add 10 to 30 minutes to the baking time for every 1/4 inch of thickness, based on the brand. Stay in the kitchen as your item bakes so you don’t scorch it. When it comes out of the oven, it may still feel soft; it harden as it cools.

Ulrich also recommends baking your clay polymer craft item on a glazed porcelain tile. Since the tile has a glossy surface already cured in a kiln at a much higher temperature, it offers a nice flat surface on which to bake. Heat the porcelain tile as you preheat the oven because the tile retains the heat and diffuses it evenly to your craft piece. When you open the oven door to check on your project, it won’t lose heat because of the heated tile. Transfer your item to the heated tile by setting it onto a card stock -- like an index card -- or a strong white paper. Wax paper is not the recommended surface, as the paper can burn or curl onto your item.

If you're using self-hardening or air-hardened clays, set them on wax paper atop a flat surface, such as a glazed ceramic tile. Place them in a location where they have good circulation and even temperatures. Let the item harden over the course of several days. Air-dried or self-hardening clays may shrink a bit after drying; this is normal. Remove from the wax paper and paint with acrylic paints, if desired.