Reading the surface
A glaze defect is a message. The surface tells you what went wrong — whether it was application thickness, firing rate, body preparation or glaze chemistry. Learning to read it turns a ruined pot into information.
Most defects fall into two families: those caused by how the glaze was applied, and those caused by how it was fired. Sorting a defect into the right family is the first diagnostic step.
Pinholes and blisters
Pinholes are small craters in the fired glaze surface. They form when gas escapes from the body or the glaze layer after the surface has already sealed. Blisters are the same phenomenon at a larger scale — a bubble that froze in place.
| Cause | Why it happens | Fix |
|---|---|---|
| Thick glaze application | Trapped gas cannot escape through the melt | Apply thinner — two coats, not four |
| Over-fast bisque | Organic matter not fully burned out | Slow bisque to 600 °C, candle at 100 °C |
| Dusty bisque surface | Dust creates gas pockets | Wipe bisque with a damp sponge before glazing |
| Over-firing | Glaze boils at the peak | Reduce top temperature or shorten the soak |
If a glaze is prone to pinholes, drop the temperature 30–50 °C at the peak and hold for 20 minutes. The glaze becomes fluid enough to heal over the holes.
Crawling
Crawling looks like the glaze has pulled back from the pot, leaving bare patches with raised edges. It is a wetting failure: the molten glaze does not bond to the body, so surface tension pulls it into islands.
- Too-thick application is the most common cause — keep coats thin.
- Dust, grease or wax on the bisque surface stops the glaze adhering. Handle bisque with clean hands and wipe it down.
- Very high clay content in the glaze recipe causes excessive shrinkage on drying and pulls the layer apart.
- Firing too fast can prevent the glaze from wetting out properly.
Before reformulating, check your thickness and your bisque surface. Nine out of ten crawling cases are solved by applying less glaze to a cleaner pot.
Crazing and shivering
These two are opposites and both are about fit. Crazing is a network of fine cracks in the glaze — the glaze is shrinking more than the body, so it cracks to relieve tension. Shivering is the reverse: the glaze is in compression and flakes off at edges.
| Defect | Glaze vs body | Typical fix |
|---|---|---|
| Crazing | Glaze shrinks more (too much compression) | Add silica, reduce alkali flux, or fire hotter |
| Shivering | Body shrinks more (glaze in tension) | Reduce silica, increase flux, or fire cooler |
A crazed glaze on functional ware harbours bacteria and can allow leaching from the body. If a liner glaze crazes, do not sell or use that piece for food.
Runs, drips and thick edges
- A run is a glaze that became too fluid — reduce the flux or fire cooler.
- A drip line on a dipped pot comes from pausing on withdrawal. Keep the motion continuous.
- A heavy bead at the rim comes from dipping past the rim and letting glaze pool there. Dip to just below the rim.
- Glaze that fuses to the kiln shelf is a run that reached the base. Wax the foot and leave a bare margin.
Fire a test tile with every glaze batch, on the same body, at the same schedule. Most glaze defects are caught on a tile that costs nothing instead of a pot that took an hour.
Ceramic work involves respirable crystalline silica, kiln temperatures up to 1300 °C and glaze materials that can be toxic in powder form. This guide is an educational reference — it is not a substitute for the instructions supplied with your equipment. Always ventilate, wear appropriate respiratory protection when handling dry materials, and read our studio safety guide.
