Week 7 Readings

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Introduction

Do you struggle with cracks in your pottery? Few things are more frustrating than opening a kiln to discover that a large portion of your work has cracked.

In this lesson, we will look at eight of the most common types of cracks in pottery and how to prevent them.

The root cause of all cracks is stress. Stress may come from how the clay is handled, how it dries, how it is fired, or from incompatibility between clay and glaze. When the stress exceeds the strength of the pot, cracks occur.

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Part 1: General Causes of Cracking

Before discussing specific crack types, it is important to understand the general causes of stress in ceramics.

1. Over-firing

Firing clay too high or for too long can weaken the structure. Always follow appropriate firing schedules for your clay body.

2. Repeated firings

Multiple firings increase vitrification and can make clay bodies more brittle over time. Most functional ware requires only two firings (bisque and glaze), unless adding specialty surfaces such as lustres.

3. Drying too fast or unevenly

Drying is one of the most common causes of cracking.

• Drying too fast (e.g., strong sun, excessive heat)

• Uneven drying (thick bases and thin walls drying at different rates)

Important preventative step:
Flip pots over as soon as they are firm enough to support themselves. This allows air to reach the base and helps prevent S-cracks.

If a pot remains upright too long, the base may dry more slowly than the walls, creating stress.

4. Opening the kiln too early

Cooling too quickly causes thermal shock. It is generally recommended not to open a kiln until it has cooled to around 100°C.

5. Uneven wall thickness

Large differences between base thickness and wall thickness create internal stress during drying and cooling.

6. Poor glaze fit

Clay and glaze shrink at different rates. A good glaze fit means both materials shrink at compatible rates. Poor fit causes tension, leading to cracks such as crazing or shivering.

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Part 2: Specific Types of Cracks

Now we will look at eight common crack types and their causes.

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1. Vertical Cracks

Most common in handbuilt work.

Cause:

• Weak points created during forming

• Uneven slab thickness

• Overworking or bending clay excessively

Prevention:

• Use thickness guides

• Avoid over-manipulating clay

• Maintain structural integrity during forming

Cracks may appear late in the process but originate from early handling.

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2. Shattered Bottom

A crack across or through the base.

Causes:

• Base too thick or too thin

• Excess water trapped in the bottom (e.g., leftover throwing slip)

• Uneven drying

• Excess glaze pooling on the interior base

Prevention:

• Remove excess slip after throwing

• Maintain even thickness

• Avoid heavy glaze pooling inside the base

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3. S-Cracks

Common in wheel-thrown work.

Usually caused by uneven drying at the base.

Prevention checklist:

• Remove excess slip from the base immediately after throwing

• Flip pots over once firm enough

• Allow even airflow during drying

There is debate about compressing the base. Some potters advocate compression; others find flipping sufficient. Results may vary by clay body and technique.

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4. Edge Cracks

Cracks near the bottom edge.

Cause:

• Thermal shock during cooling

• Thick bases retaining heat longer than thin rims

As the rim cools and shrinks faster than the base, stress forms at the lower wall.

Prevention:

• Maintain even thickness

• Allow proper kiln cooling

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5. Glaze-Induced Shatter Cracks

Often seen when only the interior is glazed.

Cause:

• Poor glaze fit

• Tension between glaze and clay

• Thin walls under stress from glaze contraction

Prevention:

• Test glaze fit

• Adjust glaze or clay body if necessary

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6. Shivering

Glaze flakes or pops off the surface.

Cause:

• Poor glaze adhesion

• Dust, oil, or contamination on bisque

• Handling bisque with oily or sweaty hands

• Dust settling before glazing

Prevention:

• Keep bisque clean

• Avoid touching surfaces excessively

• Clean before glazing

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7. Crazing (or Crackle)

Fine cracks in the glaze surface.

If intentional, it is called crackle.
If unwanted, it is called crazing.

Cause:

• Poor glaze fit

• Glaze shrinking more than clay

Crazing may appear weeks after firing as internal tension releases.

Prevention:

• Adjust glaze chemistry

• Choose compatible clay-glaze combinations

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8. Springing (Handle Separation)

Handle cracks or detaches from the mug body.

Cause:

• Attaching to clay that is too dry

• Inadequate scoring and slipping

• Uneven drying between handle and body

Prevention:

• Attach handles at similar moisture levels

• Score and slip thoroughly

• Cover with plastic for 24 hours after attachment to allow moisture equalization

Allowing attachments to homogenize under plastic significantly reduces failure.

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Conclusion

Cracks are part of ceramics practice. Every potter experiences them.

Understanding the sources of stress—during forming, drying, firing, and glazing—allows you to diagnose problems and improve success rates.

By controlling drying speed, maintaining even thickness, ensuring good glaze fit, and allowing proper cooling, most cracking issues can be prevented.