Pottery cracking almost always comes down to one root cause: stress from uneven shrinkage. Clay shrinks as it dries, shrinks again during firing, and then expands and contracts during cooling. When one part of a piece moves differently than another, that stress eventually shows up as a crack. This guide covers the real causes behind that stress, and exactly how to prevent it.
Pottery Cracking From Uneven Drying
Clay contains roughly 20 percent water. That water dries off unevenly across a piece. A thinner wall dries faster than a thick base. The outer surface also dries faster than the layers underneath.
This uneven moisture loss creates real stress throughout the piece. Slow the process down instead. Cover your work loosely with plastic, which lets moisture escape gradually instead of all at once.
Inconsistent Wall Thickness
Uneven walls dry and shrink at different rates. This creates the same kind of internal stress uneven drying causes. A piece thick in one section and thin in another is more likely to crack. Even walls hold up far better.
Keep thickness as even as possible while shaping too. Watch closely where a wall transitions into a base, or where two sections join together. These transition points are where cracks form most often.
Handles and Attachments Are Common Trouble Spots
Handles and other small attachments dry faster than the main body of a piece. They’re thinner and more exposed to air. This mismatch in drying speed puts real stress right at the joint.
Wrap handles in plastic wrap to slow their drying down and match the rest of the piece. Some potters also paint wax resist onto leather-hard handles, which burns off harmlessly during the bisque firing.
The Role of Grog in Preventing Cracks
Grog is clay that’s already been fired once, then ground into small bits and mixed back into fresh clay. It genuinely cuts down cracking. Fired clay holds no water, so it simply shrinks less than regular clay.
Grog also opens up space between clay particles. This lets moisture inside reach the surface and dry off more easily. Less shrinkage plus easier moisture escape makes grogged clay much better at resisting cracks.
Poor Wedging Causes Real Problems Too
Clay that isn’t wedged well ends up with moisture spread unevenly through the block. Some parts end up denser and drier than others. That sets up the exact same uneven-drying stress described earlier.
Wedging also gets rid of trapped air bubbles, another real source of cracks. This matters most during firing, when trapped air can expand fast. Thorough wedging genuinely solves two problems at once.
Cracks That Appear During Firing, Not Drying
Dunting describes long, sharp cracks that appear during the cooling phase of a firing. Sometimes they even show up weeks after the piece comes out of the kiln. This happens from thermal shock, when a piece cools too quickly and different sections contract at different rates.
Firing too fast, cooling too fast, or opening a kiln before it’s fully cooled all raise dunting risk. A proper firing schedule genuinely prevents most of this. Don’t rush either end of the process.
Reliable Firing Starts With a Reliable Kiln: Skutt KM-1027
Consistent, programmable firing schedules genuinely reduce thermal shock too. This directly cuts down on dunting and other firing-related cracks.
Weather and Environment Play a Real Role Too
Humid conditions slow drying down naturally, while a hot, dry room speeds it up unevenly. Neither extreme is ideal, since both make it harder to control moisture loss consistently across a piece.
A fan or air conditioner blowing directly on drying work often creates uneven cracks too. One side simply dries much faster than the rest. Keep drying pieces away from direct airflow whenever possible.
Clay Choice Genuinely Matters
Highly plastic clay bodies tend to crack more during drying than less plastic ones. This doesn’t mean plastic clay is a bad choice, just that it needs more careful, even drying to avoid problems.
Say you’re new to a specific clay body. Ask your supplier about its typical drying behavior before committing to a large or complex piece. Some clays are simply more forgiving than others for a beginner.
The S-Crack: A Real, Specific Pattern Worth Knowing
An S-shaped crack on the underside of a thrown pot usually points to one specific cause. A lack of compression at the bottom during throwing is often the culprit. This pattern is common enough that most experienced potters recognize it immediately.
Compress the base thoroughly while throwing, pressing firmly across the bottom before pulling up the walls. This single step prevents a genuinely common and specific crack pattern from forming later.
What to Do If a Crack Already Appeared
Moisten and smooth over small cracks that appear during drying. Let the piece keep drying slowly after that. This works well for minor surface cracks caught early.
Skutt’s own firing guidelines stress following a proper, steady schedule too. This matters even more once you’re trying to salvage a piece that already shows early signs of stress.
A Simple Checklist Before You Dry or Fire
Wedge your clay thoroughly, keep wall thickness consistent, and compress your base well while throwing. Slow your drying process down with loose plastic covering, and protect handles and attachments separately.
Follow a proper firing schedule, and let the kiln cool fully before opening it. These few habits address nearly every real cause of cracking covered in this guide.
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