Reading a glaze fault: six defects and what actually causes them

A glaze fault is evidence. Pinholes, crawling and crazing all look like surface problems and all have different causes, and treating the wrong one wastes a kiln load. This is the diagnostic table: what each defect looks like under a lens, which of the four variables — application, bisque, firing, or fit — is responsible, and the one change that fixes it without breaking the glaze.

Pinholes and blisters: gas, not holes

Pinholes are small craters with a visible base; blisters are raised bubbles that may or may not have broken. Both are gas escaping through a glaze that has already sealed. The gas comes from three places: air trapped in the clay body, carbon left over from an incomplete bisque, or the glaze itself decomposing.

Diagnose by location. Pinholes scattered evenly across the whole surface mean the glaze is too thick or the bisque was under-fired. Pinholes concentrated around the base and the foot mean the body was not dry when it went into the bisque. Pinholes in one patch mean a lump of unmixed glaze material. Fix by re-bisquing higher, thinning the glaze, or sieving to 80 mesh.

Crawling: the glaze pulled back

Crawling shows as bare patches of clay with the glaze drawn back into beads around the edges. It happens when the glaze cannot adhere to the body: dust or oil on the bisque, glaze applied too thick, or a glaze whose shrinkage in the early part of the firing is much greater than the body's.

The fast tests: wipe the bisque with a damp sponge before glazing, drop the thickness to 0.35 mm, and check whether the glaze has a high clay content. Adding 2 percent bentonite to a crawling glaze usually fixes it by giving the dried layer some flexibility. If the crawl only happens over a waxed area, the wax was not fully burnt out.

Crazing and shivering: a fit problem

Crazing is a network of fine cracks in the glaze; shivering is the opposite, where flakes of glaze come away from the edge. Both are thermal expansion mismatches, and they point in opposite directions: crazing means the glaze contracts more than the body on cooling, shivering means it contracts less.

Fix crazing by increasing silica in the glaze by 3 to 5 percent, or reducing the alkali — sodium and potassium are the biggest contributors to a high-expansion glaze. Fix shivering by adding alkali or reducing silica. Test on tiles before committing a kiln load: a fit test at cone 6 takes one firing, and a crazed glaze on a functional pot is a defect you cannot sell.

Pinholing versus speckling versus underfiring

Speckling — small dark dots — is usually iron or rutile particles that were not sieved out; it is sometimes desirable, and if you want it, sieve to 60 mesh rather than 80. A glaze that looks dry and rough rather than glossy is usually under-fired: check the witness cones before changing the recipe, because a shelf that ran one cone cool will make every glaze on it look wrong.

The diagnostic order that saves the most time: check the cones first, then the thickness, then the bisque, and only then the recipe. In a studio that keeps records, the recipe is almost never the problem.

Steps

1. Examine the fault under a 10x lens and classify it as crater, bubble, bare patch, crack network, flake or speckle.
2. Check the witness cones from the firing before changing anything about the recipe.
3. Measure the glaze thickness on a test tile with a scale test; anything over 0.6 mm explains most pinholes and crawls.
4. Wipe the bisque with a damp sponge and re-glaze one test tile to isolate a dust or oil problem.
5. For crazing, add 3 to 5 percent silica; for shivering, add 2 percent feldspar, and test on tiles.
6. For crawling, add 2 percent bentonite and reduce the thickness to 0.35 mm.
7. Re-bisque at one cone higher if pinholes cluster around the base and foot.
8. Record the change and the result against the firing number so the same fault does not get re-diagnosed.

Recipe and schedule numbers here were recorded on the workshop’s own kiln. A different kiln, a different thermocouple position or a different clay batch will move the result, so treat every firing schedule as a starting point and keep your own log beside it.

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