[Troubleshooting] 7 Common Pottery Glaze Defects: Causes & Solutions
Opening the kiln is always an exciting moment, but finding cracks or peeling glaze can be disappointing.
In pottery, "failures" are common, but understanding their causes will help you create better pieces next time.
This article explains the causes and specific solutions for 7 common glaze and firing defects.
Table of Contents: Which defect are you facing?
1. Crazing: Fine cracks in the glaze
[Symptom]
A network of fine hairline cracks appearing on the surface of the glaze.
[Cause: Thermal Expansion Mismatch]
This occurs during cooling when the glaze contracts more than the clay body.
The glaze is stretched by the clay until it cracks under tension.
[Solutions]
- Adjust the Glaze: Add ingredients that form the glass structure (like silica or kaolin) to lower the glaze's thermal expansion.
- Adjust the Clay: Use a clay body with lower thermal expansion, or add feldspar to the clay.
- Glaze Thickness: Apply the glaze thinner and more evenly, as thick application exacerbates crazing.
💡 Want Crazing as a Decoration?
While crazing is often considered a defect, beautiful crackle patterns (like in Celadon) can be desired as artistic "Kannyu".
Fukaumi Shouten offers glazes specifically designed to create beautiful crackle effects even on porcelain.
*Note: Search results may include some products other than porcelain crackle glazes (e.g., Irabo glaze). Please check the product name.
2. Crawling: Glaze peeling or beading up
[Symptom]
Bare patches of clay visible where the glaze has pulled back, often forming bead-like lumps.
[Cause: Poor Adhesion & Shrinkage]
- Drying Shrinkage: High clay content in the glaze causes it to shrink too much during drying, detaching from the body.
- Dirty Surface: Dust, grease, or oil on the bisque ware prevents the glaze from adhering.
[Solutions]
- Clean the Bisque: Wipe off dust with a damp sponge and ensure no grease gets on the surface before glazing.
- Adjust Density: Ensure the glaze is not too thick.
3. Pinholes: Tiny holes like needle pricks
[Symptom]
Small holes remaining on the glaze surface, resembling skin pores.
[Cause: Outgassing]
During firing, organic matter or impurities in the clay/glaze burn off, releasing gas. If the gas bubbles burst through the glaze but don't heal over before cooling, pinholes remain.
[Solutions]
- Bisque Fire Properly: Fire bisque slowly and to a slightly higher temperature to burn off all organics.
- Remove Impurities: Sieve the glaze to remove dust. Keep glaze containers covered.
- Firing Cycle: Soak (hold) the kiln at top temperature longer to allow the glaze to smooth over.
4. Blistering: Glaze bubbling
[Symptom]
Large bubbles or craters in the glaze, looking like boiling lava.
[Cause: Over-firing & Gas]
- Over-firing: Firing too high causes the glaze to boil.
- Trapped Gas: Air trapped in the clay body expanding.
- Rapid Cooling: Not allowing time for gases to escape.
[Solutions]
Lower the firing temperature or use a glaze with a higher melting point.
5. Shivering: Glaze flaking off
[Symptom]
Glaze flaking or chipping off, often at the rim or edges, sometimes taking pieces of clay with it.
[Cause: Compression]
The opposite of crazing. The clay shrinks more than the glaze during cooling.
The glaze is under immense compression and pops off to relieve stress.
[Solutions]
The fit between clay and glaze is extremely poor. Change the clay body or adjust the glaze formula.
6. Dull Surface (Devitrification): Lack of gloss
[Symptom]
A glaze that should be glossy appears cloudy, rough, or matte.
[Cause: Under-firing]
The glaze has not fully melted due to low temperature, short firing time, or thin application.
[Solutions]
Increase the firing temperature, add a soak time, or apply the glaze slightly thicker. Reducing glaze density (adding water) may also help if application is too uneven.
7. Metal Marking: Grey streaks from cutlery
[Symptom]
Grey or black pencil-like lines appear where knives or forks scratch the surface.
[Cause: Micro-abrasion]
Matte glazes often have a microscopically rough surface that acts like sandpaper, scraping off metal from cutlery.
[Solutions]
For dinnerware, choose glazes designed to be smooth and resistant to metal marking.
Best Glaze for Dinnerware!
Fukaumi Shouten's "126HMS White Matte Glaze" is specially formulated to resist metal marking while maintaining a beautiful matte texture.
Ideal for cafes and restaurants.
Summary: Failure is the First Step to Adjustment
Pottery defects arise from a complex interplay of clay, glaze, firing, and forming.
Even with professional-quality glazes, defects can happen depending on the clay pairing or kiln conditions.
The key is to ask "Why did this happen?" and test by changing conditions one by one.
Fukaumi Shouten supports your creativity by providing "stable, high-quality materials" that serve as a reliable baseline for your experiments.
