If your latest polymer clay creation is cracking, there could be a few different reasons why. You want your projects to look smooth and last a long time without wasting materials on broken pieces — but rest assured that cracks happen to the best of us! Whether you're new to clay crafting or an experienced creator, this guide will discuss how and why polymer clay cracks, how to fix it and tips for preventing cracking in the future.
The Main Reasons Your Polymer Clay Is Cracking
Polymer clay consists of polyvinyl chloride (PVC) combined with plasticizers and pigments, making it an ideal modeling material. It remains flexible and easy to shape until it's baked — also known as cured or hardened — in an oven. The heat causes the PVC particles to merge and results in a robust, durable creation. However, certain conditions before and after the baking process can leave clay vulnerable to breakage.
Why Your Polymer Clay Cracks Before Baking
If your Sculpey clay is cracking before baking, it might be due to improper preparation and handling. Here's a breakdown of the common reasons why your polymer clay may be cracking:
- Insufficient conditioning: Clay conditioning is when you knead or work polymer clay until it becomes soft, pliable and evenly textured before you start to shape it. This process helps to distribute the plasticizers evenly. When you skip this step, the polymer clay is less flexible.
- Clay rolled too thin: Thin layers of clay stress and crack more easily, especially if they're not evenly conditioned.
- Mixed clay types: A common reason why polymer clay may crack before baking is mixing different brands or types of clay with varying formulations. Mixing polymer clay with air-dry clay, Play-Doh or plasticine can also cause problems, as each material has a different curing process.
Why Your Polymer Clay Cracks After Baking
Polymer clay can also crack due to several factors related to the baking process. Here are some explanations why this happens:
- Incorrect baking temperature or time: Baking at the wrong temperature or for too short a time can prevent the clay from fully curing. Underbaked clay remains weak and brittle, which means it will crack more easily.
- Air bubbles: Air trapped within the clay expands when heated, which causes bulges that can crack the surface.
- Thick or uneven pieces: Thick or nonuniform pieces do not heat evenly, which can lead to improper curing and breakage.
- Baking without support: Large or heavy items can collapse under their own weight if they aren't constructed or baked with a supporting base.
- Rapid temperature changes: When the temperature changes suddenly — called thermal shock — the clay may contract too quickly and crack. Do not move the clay immediately from the hot oven to the fridge or freezer in an attempt to cool it down faster.
- Porous substrates: If the clay is applied over materials that expand with humidity, such as wood, it can crack over time.
How to Prevent Polymer Clay From Cracking
Preventing cracks involves a combination of proper preparation, baking and handling. Here are some steps to make sure your clay projects remain crack-free:
- Condition your clay properly: Unconditioned clay is more prone to cracking. Knead the clay thoroughly by hand or use a clay conditioning machine.
- Check for air bubbles: When kneading your clay, firmly press and stretch the material to avoid trapping air between layers. If bubbles appear from overworking the clay, pop or cut them immediately.
- Keep the thickness consistent: Variations in thickness can create weak spots prone to cracking. Ensure clay layers are uniform and smooth.
- Use a proper baking surface: The type of surface you bake your project on can affect how it cures. Use a metal sheet, oven-proof glass or ceramic tile for best results. If using a mold, Sculpey Tools™ Oven-Safe Molds are made of silicone and available in a variety of patterns and shapes.
- Maintain consistent baking temperature: Temperature fluctuations can lead to uneven curing and cracks. Bake your clay at the recommended temperature and time specified on the packaging, and use an oven thermometer to get a more accurate reading before and during the baking process.
- Bake a test piece: Shape and bake a small test piece before putting your entire project in the oven to cure. If the test sample has dark spots, your oven is likely too hot. Lower it by 10 degrees Fahrenheit and try again.
How to Fix Polymer Clay That Has Cracked
If cracks still happen, don't worry. With the right tools, materials and techniques, you can repair your polymer clay creations. Do not attempt to fill and re-bake the crack with uncured clay, as this could cause the crack to expand. Instead, create a “sawdust” mixture from your Sculpey clay by filing or grating a piece of your baked project to create dust to combine with epoxy glue. Fill the crack with the mixture and allow to harden without heat.
You can also pick up some Plumber's Putty from your local hardware store to carefully fill cracks — this epoxy-like material hardens in just a few minutes, no oven required. Once it's cured, carefully sand to smoothen before painting to match the rest of your creation.
Since cured clay doesn't fuse with uncured clay, any new pieces you create to replace broken ones will need to be glued on. If your repairs are still visible after painting the filled cracks or attaching the new piece, consider adding decorative elements like small flowers or patterned lines to act as a camouflage.
Get Creative With High-Quality Sculpey Polymer Clay
Now that you have a better idea of how to prevent and fix cracks in polymer clay, it's time to choose the perfect clay for your next project. Sculpey is committed to offering top-quality polymer clay in a wide variety of colors to help crafters of all ages and abilities bring their ideas to life.
Explore our Sculpey range today, or check out our online tutorials to find inspiration for your next creation.
