Wire, glass, metal foil, thoroughly cured wood, metal screening and Sculpey® UltraLight™ can be used. Do not use armature materials that are painted or glazed. An armature is always necessary for any sculpture of a larger size (over 2″ high or wide), or with unsupported parts or those parts that will carry weight – e.g., legs or arms. Do not attempt to use Styrofoam as an armature material. Heat causes it to expand and emit toxic fumes.
If an object has a variance in thicknesses, an armature should be shaped in order to “bulk out” the thick parts – tightly crushed heavyweight aluminum foil is the best material to use. Cover form with a 1/4″ thick layer of clay. Bake for 15 minutes at 275°F (135°C). Add a second 1/4″ thick complete layer (like a skin), which can be built up slightly to show contours. Re-bake for 15 minutes at 275°F (135°C). Then, apply a third complete layer, adding finishing details, and re-bake for a final 15 minutes. Try to keep all parts of the sculpture of an even thickness of clay. Layering the clay is the best method to avoid cracking and maintain even thickness in a medium or large sculpture. Over baking will cause hardening of the clay, and burning can cause bubbling and cracking.
Do not fill the crack with uncured clay and put back into the oven to re-bake. This generally causes the crack to enlarge. The uncured clay does not fuse with the already cured clay. You can use a product called Plumber’s Putty–a two part epoxy clay-like substance used to repair pipes. It is dark gray, but can be sanded and under painted to match the clay. It hardens in a few minutes without heating. You can also make “sawdust powder” from Sculpey clays and mix it with epoxy glue. Create the “sawdust powder” by taking a piece of baked clay (matching the color of your project), use a very coarse nail file, rasp or grater against the clay to create the “dust” that will be mixed with the epoxy. This makes a paste of the same color as the clay, which hardens without heat. It can also be sanded and painted. It is difficult to add on to any cured clay and re-bake. The cured clay does not re-melt and mix with the uncured clay, so separation will occur. Pieces that break off or were not attached properly prior to baking will need to be glued on.
Cracking is almost always caused by insufficient curing or by baking a sculpture with uneven thicknesses throughout. To ensure you have a consistent thickness in your piece, we recommend “bulking out” with foil or creating an armature from Sculpey® UltraLight™. It is also very important to adhere to proper baking times and temperatures. Using an oven thermometer is the most accurate way to make sure your oven is heated to the exact temperature.
Always use a type of support under the clay if there are raised or extended portions, as the clay softens slightly in the oven before it hardens in the baking process. Without this support, the areas can sag or crack. Fiberfill, crumpled aluminum foil, crumpled up paper towels or tissue paper, glass bowl (to keep a curve in place), wooden dowels or wire supports could all be used as supports under the clay to keep it in place during baking.
If the clay is slightly dry, this can also cause cracking. Add some Sculpey® Clay Softener, work in, and the clay will be reconstituted. Because the clays shrink less than 1 percent when baking, this does not lead to cracking except in cases where the layer is extremely thin, such as over a glass vessel.
To prevent cracking and breaking, we also recommend that you do not expose the baked pieces to large temperature and humidity changes.