The 8 most common types of cracks and how to avoid them.
1. What Causes Cracks in Pottery?
Pottery cracks because clay is under stress. Clay always shrinks as it dries, shrinks again during firing, and then expands and contracts during cooling. If one part of your pot moves differently from another, stress builds up, and if the clay can’t handle that stress, it cracks.
Stress can come from:
-Firing too hot, too long, or too many times
-Drying too quickly or unevenly
-Cooling too quickly in the kiln
-Variations in wall or base thickness
-Poor clay/glaze fit (shrinking or expanding at different rates)
-Weak joins between attachments
In other words: cracks are rarely “random.” They’re nearly always the result of uneven stresses in the clay body. The key is recognising which type of crack you have.
2. Which Type of Crack Do You Have?
S-Cracks
Introduction:
S-cracks are one of the most common problems potters run into, and they almost always appear at the bottom of wheel-thrown pots. They’re called “S-cracks” because of the typical S-shaped fracture line that forms. This happens because the bottom of the pot dries unevenly compared to the walls. Clay shrinks as it dries, and if the base is drying or shrinking at a different rate, the stress radiates outward in that distinctive S pattern.
Root cause: Uneven drying and insufficient compression of the clay base.
Bad practices that lead to it:
-Leaving slip or water pooled in the bottom while throwing
-Not compressing the base during throwing or trimming
-Letting the pot dry upright without flipping it
Best way to avoid:
-Wipe out slip while the wheel is spinning
-Compress the base carefully during throwing and trimming
-Flip the pot over as soon as it’s firm enough to handle so the base can dry evenly
Edge Cracks
Introduction:
Edge cracks appear where the base of the pot meets the kiln shelf, often looking like small splits or fissures around the edge. These are usually not caused during throwing but during firing and cooling. The reason is thermal shock: the kiln shelf retains heat longer than the walls of the pot, so while the walls may start cooling and shrinking, the thicker base and shelf are still hot. This difference in temperature means the base and walls shrink at different rates, and the stress shows up as cracks at the edges.
Root cause: Thermal stress during cooling, often worsened by thick bases.
Bad practices that lead to it:
-Making bases thicker than walls
-Cooling the kiln too quickly
Best way to avoid:
-Keep base and walls similar thickness
-Allow the kiln to cool gradually, especially through critical silica inversion temperatures
Vertical Cracks
Introduction:
Vertical cracks are cracks that run straight up the walls of your piece. Unlike S-cracks, which start at the base, vertical cracks reveal weak spots in the walls themselves. These often happen when walls are pulled too thin in places, bent excessively, or have uneven thickness that leads to concentrated stress. Sometimes the clay is simply overstretched and loses its structural integrity. These cracks may appear during drying, firing, or even when handling the pot before it’s fully dry.
Root cause: Weak points or uneven thickness in the walls.
Bad practices that lead to it:
-Pulling walls too thin or inconsistently
-Over-bending or distorting clay walls
Best way to avoid:
-Keep wall thickness even (use gauges if needed)
-Avoid overstretching clay when throwing
Shattered Bottom
Introduction:
Sometimes, instead of a simple crack, the entire base seems to break apart. This is what’s often called a “shattered bottom.” This usually points to serious unevenness between the base and walls, or to trapped water at the bottom of the pot. Because the base is thicker and holds water longer, it shrinks differently than the walls as it dries and fires. Glaze pooling at the bottom can also contribute, creating extra stress as the glaze pulls the clay with it during firing.
Root cause: Inconsistent thickness or water/glaze trapped in the base.
Bad practices that lead to it:
-Base thicker than walls
-Slip or water soaking into the bottom during throwing
-Glaze pooling at the bottom and drying into the clay
Best way to avoid:
-Keep thickness of base and walls consistent
-Always wipe out slip from the bottom of thrown pots
-Avoid glaze pooling at the bottom
Shatter Cracks (Dunting)
Introduction:
Dunting describes long, sharp cracks that appear either during cooling or even months after a piece has been fired. These cracks often cut cleanly through the pot, with sharp edges. They usually happen because of thermal shock during cooling, especially when passing through silica inversion points (~573°C and ~226°C). Sometimes, dunting is caused by a poor fit between clay and glaze: the clay and glaze expand and contract differently during temperature changes, and eventually the stress splits the pot.
Root cause: Thermal stress during cooling or poor clay/glaze fit.
Bad practices that lead to it:
-Cooling the kiln too quickly
-Using clay and glaze with incompatible expansion rates
Best way to avoid:
-Fire and cool slowly through silica inversion ranges
-Test clay and glaze combinations for compatibility
Shivering
Introduction:
Shivering is when small flakes of glaze pop off, usually from rims or sharp edges. Sometimes flakes even take bits of clay with them. This isn’t really a drying problem. It’s usually a surface preparation or glaze fit issue. If the bisque surface is dusty, over-sanded, or contaminated, glaze adhesion is compromised. Alternatively, the glaze itself may not fit the clay body properly, meaning it can’t shrink and expand together with the clay.
Root cause: Glaze failing to adhere to the bisque properly.
Bad practices that lead to it:
-Leaving dust on bisque ware (from sanding or storage)
-Poor underglaze application
-Using incompatible glaze/clay combinations
Best way to avoid:
-Thoroughly clean bisque before glazing
-Avoid over-sanding without cleaning
-Test glaze/clay fit
Crazing (Crackle)
Introduction:
Crazing, or crackle, appears as a network of fine lines across the glaze surface. Unlike other cracks, crazing doesn’t actually weaken the pot structurally, though it may make it less food-safe. Some potters even aim for crazing as a decorative effect. It happens when the glaze shrinks more than the clay body during cooling, causing it to split into fine patterns.
Root cause: Poor clay/glaze fit, with glaze shrinking more than the body.
Bad practices that lead to it:
-Using glaze/clay combinations with mismatched expansion rates
-Applying glaze too thickly
Best way to avoid:
-Use clay and glaze designed to fit together
-Apply glaze more evenly and in thinner coats
Springing (Handle Cracks)
Introduction:
Springing refers to cracks at the points where handles or attachments (like spouts) join the main body. This usually happens because the handle and the pot were at different moisture levels when joined, so they shrink at different rates as they dry. Sometimes it’s simply poor attachment; not enough scoring or slipping. These cracks tend to show up during drying, not firing.
Root cause: Weak join between handle/attachment and pot body.
Bad practices that lead to it:
-Attaching handles when one part is drier than the other
-Not scoring and slipping properly
-Drying the attachment faster than the pot
Best way to avoid:
-Join when both parts are the same moisture level
-Score and slip thoroughly
-Cover the piece in plastic for 24 hrs so moisture levels even out
3. Summary of Best Practices
Cracks happen to everyone - they’re part of pottery. But there are patterns you can learn to recognise and prevent:
-Dry slowly and evenly: use damp boxes, cover with plastic, and avoid heat or direct sun.
-Keep thickness consistent: walls, bases, and attachments should all be similar.
-Compress bases and clean up slip: avoid pooled water at the bottom.
-Support attachments properly: score and slip, match moisture levels.
-Control firing and cooling: fire steadily and cool slowly, especially through inversion points.
-Match clay and glaze: use compatible combinations to avoid stress.
The more you make, the more you’ll notice the patterns: certain cracks almost always come from certain mistakes. With practice, you’ll get better at preventing them before they happen - and when they do, you’ll have an idea of what caused it.