You lift the lid on a cooled kiln and there it is: a patch of bare clay where glaze should be, or a rim pitted like a sieve that looked perfect on the test tile. Glaze faults are rarely mysterious once you know what you are looking at. Nearly every defect comes down to one of three things—glaze fit against the clay, application thickness, or the firing schedule—and the trick is narrowing down which one. Here are seven of the most common faults, what usually causes each, and the adjustments that fix them.

Crawling: Glaze That Pulls Into Islands

Crawling looks like the glaze retreated, because it did. Surface tension pulls the molten layer into beads and patches, leaving bare clay between them, often with a rolled, thickened edge. It almost always starts where the glaze could not grip the pot.

• Dust, oil, or fingerprints on the bisque. Glaze cannot wet what it cannot touch.
• Glaze applied too thickly, especially in one heavy coat.
• A very smooth or burnished surface—rims are frequent offenders.
• Too much raw clay or gum in the recipe, which cracks the dried layer.
• Firing too fast early on, before the layer has bonded.

Wipe bisque with a damp sponge and handle pots with clean hands or gloves. Two or three thin coats beat one fat one. Sieve your glaze before use, add a little CMC gum to help the raw layer stay put, and slow the first half of the firing. If crawling appears on one shape only, look at the form: tight inside corners and sharp rims are where it begins.

Pinholes: When Gas Escapes Through the Glaze

Pinholes are small craters, sometimes with a dark speck at the bottom. They form when gas pushes through the glaze while it is still fluid enough to move but too stiff to heal over.

Why gas gets trapped

• Carbon and carbonates burning out of the clay or glaze late in the firing.
• Bisque fired too low or too fast, leaving carbon in the body.
• Thick glaze, which gives gas a longer path to the surface.
• Over-firing, which thins the glaze, or under-firing, which leaves it viscous.

Bisque slowly to a proper temperature and soak at the end so carbon has time to leave. Sieve the glaze, apply it thinner, and slow the glaze firing through the range where burnout happens—roughly 800°C to 1050°C for many recipes. A ten- to twenty-minute hold at top temperature helps, and a drop-and-hold (cool 30–50°C, then hold) lets bubbles rise out and the surface level.

Shivering: Glaze That Flakes Off the Edges

Shivering is crazing's opposite. The glaze sits in compression—it wants less space than the clay gives it—so it lifts and flakes away, usually at rims, sharp corners, and foot rings. You will find small crescent chips with glaze still attached, and clean bare clay underneath.

This is a glaze-fit problem, not an application problem, though thickness makes it worse. The glaze's thermal expansion is too low for the body. Practical responses: apply thinner, ease sharp rims with a damp sponge before bisquing, and fire a little lower or shorten the top hold. If you mix your own, raise the glaze's expansion with more alkali flux—soda or potash feldspar—and less silica, in small steps. Or simply pair a commercial glaze with a clay body rated for it. Always test on an upright rim, because shivering often shows up only on edges.

Crazing: Fine Cracks in the Surface

Crazing is that web of hairline cracks, and it can appear months after a pot leaves the kiln. Here the glaze is in tension—it contracts more than the clay on cooling. Lower the glaze's expansion or raise the body's:

• Add silica to the glaze, or cut back on sodium and potassium fluxes.
• Fire higher, within the glaze's range, to develop a better fit.
• Reconsider the clay body; some have unusually high expansion.
• Resist the urge to apply thicker, which worsens the mismatch.

For a quick check, run a test tile through a few dishwasher cycles or a boil-and-quench. If the piece is decorative, crazing can be a deliberate look. If it is a mug, fix the fit or refire with a glaze known to work on that body.

Blistering and Bubbling

Blistering produces raised bubbles rather than craters. The usual suspects are over-firing, a fast climb through the middle of the firing, thick glaze, and soluble ingredients such as borax or soda ash that migrate to the surface as the pot dries. Carbon left in the bisque contributes too. Slow the firing, thin the application, and check whether your glaze contains a lot of soluble material—switching to a fritted source often settles it.

Running and Dripping

A glaze that runs off the pot is simply too fluid for the clay and the temperature it reached. Thick application, over-firing, and a high flux content all make it worse. Reduce the number of coats, fire lower, and stiffen the recipe with a little alumina hydrate or kaolin. Wax-resist the foot ring so the glaze breaks cleanly above the shelf, and always use kiln wash or a catcher tile under suspect pots.

Orange Peel and Stippling

A pebbled, matte-looking surface on a glaze that should be smooth is almost always an application fault. Spraying too dry, holding the gun too far from the pot, or applying over dusty bisque leaves a granular layer that never melts flat. Air whipped into the bucket does the same. Stir gently, spray closer with a wetter coat, dust the bisque first, and try a dip or brush application to compare.

Work the Variables One at a Time

Faults often share a cause, so resist changing three things at once. Keep a notebook next to the kiln and record the glaze, thickness, bisque temperature, firing schedule, and results for every load. Glaze a few test tiles alongside each batch and keep them with your notes; a tile from two years ago is worth more than a memory. When something fails, change one variable, refire, and compare.

And a practical word on the studio side: raw glaze materials are fine dust, so wear a fitted respirator when mixing and sieving, wet-mop rather than sweep, and keep the kiln area ventilated. Good habits there make troubleshooting a lot more pleasant.

Photo: MAG Photography / Pexels