Why does my air-dry clay crack?

You've been working with great enthusiasm on a new piece of air-dry clay – perhaps a small bowl or a figurine – and are eagerly waiting for it to harden. But then, the shock: fine cracks appear in the dried piece! 😟

Don't worry, you're not alone with this problem. Beginners especially often wonder why their air-drying clay cracks. In this blog post, we at pottery supply store Formwerk Berlin get to the bottom of this question and give you tips on how to avoid cracks and save your clay projects.

Why do cracks form in air-drying clay?

Air-drying clay cracks primarily due to drying that is too fast or uneven. If water evaporates too quickly from the clay or if your object has a very uneven thickness, tensions build up in the material – and these lead to cracks.

The reason: Clay contains a lot of water and shrinks by part of its volume as it hardens. If some areas dry faster than others, they contract sooner. This creates tension, and eventually the weakest point gives way – a crack forms. Very thin or unevenly thick areas on your piece are particularly susceptible. For example, if you roll the clay out extremely thin, cracks can easily form as it dries. Furthermore, air-drying modeling clay shrinks by about a quarter as it hardens – so very delicate pieces become even more fragile.

External factors also play a role. The room climate and drying time are crucial: If your clay dries in direct sunlight, on a radiator, or in a draft, the surface hardens too quickly while the interior remains wet. This almost inevitably leads to cracking. A consistent, cool room temperature without strong air currents is best. Patience is your secret weapon – slow, even drying produces the smoothest results.

Water and handling also play a role. If you work the clay with too much water, the mass becomes very soft, and even more liquid evaporates at once during drying – the clay shrinks more and is more prone to cracking. Too little moisture is just as problematic: if the clay dries out during modeling, cracks can appear immediately when shaping (you might be familiar with the unsightly cracks that appear when clay crumbles while kneading). It's all about finding the right balance. Our tip: Keep the clay pliable while working, for example, by moistening your hands or using a fine mist of water. But don't soak the surface – too much water makes the clay mushy and unstable, which in turn promotes cracking later on.

Ultimately, some cracking is almost unavoidable. Larger, thin clay surfaces, in particular, almost always crack a little. Even when air-drying clay is applied to a solid framework or around an object, cracking is nearly inevitable – because the clay shrinks, but the supporting structure doesn't. Don't let this discourage you! Minor surface cracks are common and nothing to panic about. In the next section, we'll demonstrate how to reduce and repair them.

Tips: How to avoid and repair cracks

Now that we know why those unwanted cracks appear, here are the best practical tips from Formwerk Berlin – so your project will be smooth and crack-free:

• Dry slowly and evenly: The most important tip first: Allow enough time for your piece to dry slowly. Never dry it in the sun or directly on a radiator. Ideally, dry it in a well-ventilated, cool room without significant temperature fluctuations. You can, for example, place the piece in a shady cupboard or corner. Cover it lightly with plastic wrap or a damp cloth, especially for the first 1-2 days. This will help the clay dry more evenly and prevent the outer parts from hardening too quickly. Turn your piece occasionally so that all sides can air out and nothing remains too damp on the underside. Patience pays off: The slower the clay dries, the less tension it will have and the lower the risk of cracking.
  
  
• Pay attention to consistent thickness: Keep the wall thickness as constant as possible. Parts of your object that are significantly thinner than others will dry faster and are more prone to cracking. Generally, very thin elements (under ~5 mm) are tricky – it's better to form slightly thicker pieces and sand or file them after drying, if necessary. On the other hand, very thick lumps can also be problematic, as the outside dries much faster than the core. It's better to make thick pieces hollow (e.g., hollow out a sphere) to ensure a more even drying process for the clay. This rule of thumb will keep you on the safe side: Keep everything between about 0.5 and 1 cm thick. The most common causes of cracking during drying are uneven or excessively thin areas of clay.
  
  
• How to handle water correctly: Keep your clay pliable while modeling, but don't over-wet it. A common beginner mistake is either working with bone-dry clay (which causes it to crack when shaping) or constantly dousing it with water (which makes it floppy and loses its stability). Find a middle ground: Keep a small bowl of water handy and moisten your fingertips, or use a spray bottle to lightly dampen the clay as needed. The clay should always remain soft, but not soaking wet. If you notice you've added too much water and the clay is becoming slippery, let it set (dry slightly) before continuing to shape it. Important: Smooth cracks or transitions without using large amounts of water; instead, use gentle pressure or a little clay slip (a clay-water mixture). Too much water on the surface can cause it to dry unevenly later and then crack.
  
  
• Making joints correctly: When assembling several clay pieces (e.g., attaching a handle to a cup or joining figures), treat the joints carefully. Roughen both surfaces (this is called slip or score and slip : using a toothpick or modeling tool to create a rough, smeared surface) and use a little soft clay as "glue." This will ensure the pieces bond firmly. Make sure the pieces being joined have a similar moisture content—putting a soaking wet piece on top of one that's already dried is a recipe for cracking, because the dry piece will draw moisture from the wet one and shrink before the joint holds. So: make them at the same time or let the attached piece dry slightly beforehand so that both pieces are leather-hard before joining. Well-prepared joints are much less likely to crack later.
  
  
• Keep an eye on it while it's drying: During the first few days of drying, observe your piece from time to time. If you discover small cracks, you can intervene immediately: As long as the clay isn't completely hard, lightly moisten the cracked area and press in some fresh clay or slip to close the crack. This will allow everything to dry together, and the crack will usually disappear. If the clay is already completely dry, don't panic – cracks can still be repaired. Mix a little clay and water to create a creamy clay slurry (slip) and fill the cracks with it. Let everything dry thoroughly again. Then you can carefully sand the surface. This way, a cracked piece can still become a beautiful finished piece.

• Use the right materials: Last but not least – pay attention to the quality of your clay. Inexpensive craft modeling clays sometimes tear more easily. High-quality air-dry clays (e.g., certain Japanese air-dry clays) are finer and less prone to cracking. We at Formwerk Berlin are happy to advise you on suitable modeling clays for your project. And if you find that you've caught the ceramics bug, you can always switch to real pottery clay and a kiln in the future. 😉 But for beginners and decorative objects, air-dry clay is a great, uncomplicated choice.

Difference: Air-drying clay vs. "fireable" pottery clay

Many beginners at Formwerk Berlin also ask us about the difference between air-drying clay and regular pottery clay, which is fired . The two materials do indeed behave differently:

• Hardening: Air-drying modeling clay contains binders (e.g., cellulose or similar) that harden in the air. This means your piece hardens simply by drying. True pottery clay (also called firing clay ), on the other hand, consists of clay without such additives. While it does harden when drying, it remains brittle and water-soluble as long as it is unfired. Without firing, you will never achieve the permanent strength of ceramics. Only in a kiln, at temperatures around 900–1300°C, does the clay sinter into a robust, dense body. Unfired clay, however, remains porous – if it comes into contact with water, it softens again or crumbles.
  
  
• Uses: Air-dry clay is perfect for decorative objects – for example, figurines, pendants, small bowls for jewelry, or vases for dried flowers. It hardens completely at room temperature within a few days. You don't need a kiln and can get started right away. But be careful: These pieces are not waterproof and not food-safe! For example, you shouldn't fill a vase made of air-dry clay with water, and you shouldn't drink from such a cup. They are more decorative pieces. If you want to make something functional (such as dishes that hold liquids or flower pots for real plants), there's no way around fired clay. For everyday items, you need a kiln and glazes to make them dense and durable. While you can seal air-dry pieces with varnish to make them somewhat more water-resistant, they will never be truly waterproof or dishwasher-safe.
  
  
• Examples and variations: Traditionally, pottery is made with types of clay such as earthenware, stoneware, or porcelain, all of which require firing. For hobby potters without a kiln, there are alternatives: In addition to air-drying clay, there are also oven-hardening modeling clays (e.g., Fimo or Cernit, a type of polymer clay). These are hardened at low temperatures in a regular oven. You can also find "oven-hardening clay" in craft stores; this can be dried at around 110°C in a household oven, which makes it harder. Important: Please do not attempt to fire regular pottery clay in your kitchen oven – the necessary temperature will never be reached, and your oven (and food) would be ruined by the clay's fumes. If you want to experiment, it's best to use the aforementioned specialized clays.

In short: Air-drying clay is perfect for getting creative and gaining initial experience without a lot of equipment. Fireable clay only reaches its full potential in a kiln and is then essential for durable ceramics. Both have their place – many of our customers at Formwerk Berlin start with air-drying modeling clay to try out the hobby and later switch to proper pottery clay when they want to make dishes, for example. Either way, we're happy to help and advise you!

Conclusion: Practice makes perfect 😉

Finally, we want to encourage you. 💚 Cracks in air-drying clay are completely normal – almost every hobby potter encounters them at the beginning. The important thing is to learn from them and not give up. With the tips mentioned here, you'll be much better at managing your next projects. And if a crack does appear: now you know how to repair it. Every experience makes you wiser and helps you progress in your new favorite hobby.

We at Formwerk Berlin are proud of our growing community of ceramics enthusiasts. Feel free to connect with others, share your progress, and learn from the experiences of seasoned ceramicists. If you have any questions or need support, we're here to help with personal advice – either in our Berlin shop or online. Together, we'll make your clay dreams a reality!

Have fun with your pottery and don't let a few cracks discourage you. With heart, patience, and the support of the community, you'll soon be holding wonderful results in your hands. Happy pottery! 🥰

Your Formwerk Berlin Team