Crazing, Crawling or Pinholes? A Glaze Defect Guide
You open the kiln and something is wrong with the glaze. Before you panic, work out which defect you are looking at. Different faults have different causes, and the fix for one can make another worse.
Use this guide to name what you see, then follow the most likely cause.
Quick identification
| What you see | Name | Most common cause | First thing to try |
|---|---|---|---|
| Fine web of cracks across the glaze | Crazing | Glaze shrinks more than the clay as it cools | Thinner coat, or a glaze with a better fit to your clay |
| Glaze flaking off rims or edges | Shivering | Glaze shrinks less than the clay | A better-fitting glaze, or rounder rims |
| Glaze pulled into bare patches or beads | Crawling | Dust or oil on the pot, or glaze too thick or too dry | Clean the bisque, apply thinner |
| Tiny craters or holes in the surface | Pinholes | Gas escaping from clay or glaze | Slower bisque, a soak at the peak, thinner glaze |
| Larger bubbles that burst and leave rough rims | Blisters | Overfiring or glaze applied too thick | Check cone, lower the temperature, apply thinner |
| Glaze dripping or pooling at the foot | Running | Glaze too thick or fired too hot | Thinner coat, wipe the foot, use a kiln wash shelf |
| Dull, dry or sandy surface | Under-fired glaze | Not enough heat for the glaze | Check the cone and the firing schedule |
Crazing and shivering: a fit problem
Glaze and clay shrink as they cool. If the glaze shrinks more, it is stretched and cracks into a web. If it shrinks less, it is squeezed until it flakes away. Both are about how well the glaze fits the clay body.
Try a thinner coat first, make sure the firing reached the right cone, and test the glaze on the same clay body you plan to use. If crazing keeps returning, the glaze recipe may need changing. A crazed pot is not suitable for food, because liquid and bacteria can get into the cracks. See is my pottery food safe.
Crawling: a surface problem
Crawling happens when the glaze cannot grip the pot and pulls into beads during firing. Common causes:
- Dust or oil on the bisque from handling or dusty shelves
- Glaze applied too thick, so it cracks when it dries and lifts off
- Glaze applied to damp bisque that then dries unevenly
- Gaps in dried glaze that melt into islands
Wipe the bisque with a barely damp sponge, apply an even coat, and let it dry fully. The full explanation is in why is my glaze crawling.
Pinholes and blisters: a gas problem
Gas from the clay or glaze rises through the melted glaze. If the glaze heals over, you get a smooth pot. If it freezes before it heals, you get pinholes. Blisters are the same idea at a bigger scale.
- Bisque a little hotter or slower so more gas burns off before glazing.
- Fire slower near the peak or hold for ten to thirty minutes so the glaze can smooth out.
- Glaze thinner so there is less melt to heal.
- Skip dusty or sandy surfaces. Rougher clay holds more gas.
Running: a heat and thickness problem
Running usually means too much glaze or too much heat. It is also the defect that ruins kiln shelves. Wipe glaze off the foot, keep glaze about 3 to 5 mm up from the bottom, and put a cookie or small tile under pots that tend to run. If one does stick, grind it free carefully, as in grinding drips off a kiln shelf.
Change one thing at a time
When a glaze fails, resist changing the recipe, the clay, the thickness and the firing all at once. Change one variable and fire a test tile. That is how you learn what actually fixed it. Our post on glaze test tiles shows a labeling system that makes this easy.
Keep a record of the glaze, layers, cone and clay for every pot, good or bad. Patterns appear quickly once you write them down, and a pottery log in Claybook helps you find them.