Even experienced potters run into glaze defects. The good news? Most problems have a clear cause and a clear fix. This guide will outline three common glaze defects: crawling, pin holing, and crazing. Understanding why they happen is the first step to preventing them.
1. Crawling
Crawling appears as bare patches on your fired piece where the glaze has pulled away from the clay surface, often leaving a beaded or rolled edge. It can look dramatic and unfortunately it usually mucks up your piece!
Why it happens
- Glaze applied too thickly (this is almost always the cause from work we see in the studio)
- Dusty, oily, or contaminated bisque ware
- High-shrinkage glaze materials (especially raw zinc oxide or high-clay glazes)
- Glaze hasnt dried before the piece is fired
- Under fired bisque ware
How to fix it
- Wipe bisqueware with a clean damp sponge before glazing to remove dust
- Apply glaze in thinner, even coats, two - 3 thin coats beat one gluggy thick one
- Check your specific gravity if dipping (lots of dipping glazes work best between 1.40–1.50)
Crawling is especially common with cone 6 and cone 10 stoneware glazes. If you're troubleshooting a stoneware firing, browse our Stoneware Cone 10 Glazes range for reliable, well-tested commercial options.
2. Pinholing
Pinholing shows up as tiny holes or craters in the glaze surface after firing. Unlike crawling, the glaze is still covering the clay, but the surface texture is pitted and rough. Damn.
Why it happens
- Gases escaping from the clay body or glaze during firing that don't have time to heal over
- Firing too fast through the 573°C quartz inversion or through the carbon burnout stage
- Glaze applied too thinly, leaving insufficient melt to fill the holes
- Organic material in the clay or glaze
How to fix it
- Slow your firing rate between 500–700°C to allow gases to escape before the glaze seals
- Hold (soak) at peak temperature for 10–15 minutes to give the glaze time to flow and self-heal
- Try applying glaze slightly thicker
- Ensure your kiln atmosphere is clean, avoid firing organic materials alongside glazed work
Pinholing is particularly common with midfire cone 6 glazes. Our River Rock Midfire Cone 6 Glazes and Mayco Midfire Cone 6 Glazes are formulated to melt smoothly and minimise pinholing when fired correctly.
3. Crazing
Crazing is the network of fine cracks that appears in the glaze surface sometimes immediately after firing, sometimes days or weeks later. It's caused by a mismatch between how much the glaze and the clay body expand and contract.
Why it happens
- Mismatch between clay body and glaze (shrinkage issue)
- The glaze has a higher coefficient of thermal expansion (COE) than the clay body and it contracts more on cooling and goes into tension, cracking
- Firing too high or too low for the glaze's intended cone range
- Cooling too quickly through the 573°C quartz inversion
How to fix it
- Match your glaze to your clay body, always use glazes rated for the same cone range as your clay
- Slow your cooling rate, especially between 600–500°C
- If reformulating, increase silica or decrease flux to lower the glaze COE
- Try a different glaze from the same cone range that is known to fit your clay body
Crazing is a sign of a glaze-clay fit issue. If you're firing earthenware, our Earthenware Glazes are matched to earthenware clay bodies for a reliable fit.
Quick Reference: Glaze Defect Troubleshooting
| Defect | Main Cause | First Fix to Try |
|---|---|---|
| Crawling | Dusty bisque or thick application | Wipe bisque, apply thinner coats |
| Pinholing | Fast firing, gases not escaping | Slow firing 500–700°C, soak at peak |
| Crazing | Glaze-clay COE mismatch | Match glaze cone to clay body |
Still Struggling?
Come in for a chat about your work and get free advice about how to trouble shoot repeating glaze woes.
OR sometimes the easiest fix is starting with a glaze that's already well-tested for your clay and cone. Browse our full glaze range including River Rock Cone 6, Mayco Cone 6, Stoneware Cone 10, and take the guesswork out of glaze fit.