When it comes to polymer clay, it seems that many people assume that it’s fragile and will easily crack. Nothing can be further from the truth. It’s a durable vinyl plastic when properly cured. It should never break under normal use. (If your finished items break when flexed, you need to address your baking process.) However, cracks and small fissures that happen during or shortly after baking are a whole ‘nother ball game with a very different cause. While sometimes the cause is obvious, other times the cause is more cryptic and can leave the artist shaking their head (and their confidence). Here are some major causes of cracking in polymer clay pieces.
Causes of Cracks in Polymer Clay
Anytime there is pressure on the clay while it is hot, cracks will result. The pressure can result from expanding air, gravity, or even from internal stresses within the clay, itself. Here are some common reasons that you might see cracks in your polymer clay items after baking.
Air Bubbles – Hot air expands, so air trapped within a piece will cause a bulge that often cracks or breaks.
Thick Pieces – If you’re making something that is thicker than ½ inch (13 mm) or so, you’ll likely see cracks on the surface. Instead, bake in layers or with a compressed foil core. This is common with beads or small sculptures.
Hollow Pieces – If you make a hollow item, you MUST leave an air hole. Otherwise the air expands when hot, causing cracks, and contracts when cool, causing a collapse.
Baking Without Support – Hot clay is weak, so large items must be supported so they don’t collapse under their weight and crack apart in the oven.
Stretching – Although polymer clay is a putty, it does have some elastic memory. If you stretched the clay during construction (such as when covering a glass jar), it can sometimes “relax” in the oven and crack as the clay contracts.
Porous Substrate – This is a long-term problem that tends to happen much later, not during the baking process. If the clay is covering something that expands with humidity (such as a wooden picture frame or box), the clay often cracks with time as the seasons fluctuate.
Gremlins – Truth be told, the cause of cracking isn’t always clear. Sometimes it just happens. It could be due to the clay batch, weather conditions, or Mercury being in retrograde. Who knows.
Fixing Cracks in Polymer Clay
Thankfully, cracks in polymer clay are fairly easy to fix. Large bubbles with cracks can usually be carved out and the clay repaired by filling with more of the same color clay. If the cracks are tiny, fill them by smearing some matching clay into the cracks and smoothing. Then bake again. (Yes, you can bake polymer cay multiple times!)
Use translucent clay if the area is multicolored and you can’t match the exact color of clay. You can also use liquid clay if the cracks are tiny. Sadly, if the entire item collapsed or broke apart, the item will usually require “reconstructive surgery”, which can be quite the ordeal!
If you can’t fix the cracks invisibly, another strategy is to cover the offending area with something that complements the design, such as a small sculpted flower or a wiggly line of clay.
Cracks During Second Bakes
Larger items will need support during the baking process if they are baked a second (or third or fourth!) time. Hot polymer clay is fragile, and if gravity is pressing down, cracks can appear as the item collapses under its own weight during the baking cycle. Always support larger pieces with paper towels, cardboard, washcloths, toilet paper, or similar.
Hopefully, this covers most of the main causes. But if you still get cracks, be assured that it happens to everyone and there isn’t always a clear cause. Clay’s like that!
Crackle Is Something Different
Now that you know how to deal with unintended cracks in polymer clay, did you know that you can also create cracks intentionally to make a crackle effect in your polymer clay? This can be a very striking way to make contrasting accents or bands of color. A very subtle crackle can be a great way of adding interest to your jewelry pieces without adding the motifs and style elements that come with using texture stamps and silkscreens. You can go bold and striking or subtle and barely there and crackle can be made in any color. Go grungy and distressed. Go finely textured like china. Such a versatile set of techniques!
You can create many, many types of crackle effects with polymer clay. I’ve introduced many of them in the Crackle Compendium, which is a guide to exploring this intriguing process. It’s a deep dive course that will change the way you see the art materials in your studio. Enjoy!
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