Glaze Results That Do Not Match the Test Tile
A ceramics student applies a commercial brush-on glaze to six identical stoneware test tiles and fires them to cone 6 in the school kiln. The tiles come out as expected: a satin teal with fine speckling. The student then applies the same glaze, three coats, to a thrown bowl with a thick, rounded rim and a carved foot ring, and fires it in the same kiln load. The bowl emerges with the teal colour but a dry, rough, pitted surface on the rim and foot, while the belly looks correct. The student concludes the glaze is faulty and wants to switch brands.
1. Identify the most likely causes of the dry, pitted rim and foot, and explain why the flat test tiles did not reveal the problem.
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The most likely cause is glaze thickness variation interacting with form. On a rounded, convex rim, glaze flows away during application and during firing, thinning on the high point, which produces a dry, rough surface (a 'starved' glaze). On a carved foot ring, glaze pools in recesses and can crawl or pit where it is too thick or where dust/kiln wash contaminates the surface. Pinholes and pitting also come from trapped air or organic material under a glaze that is applied too thickly or over a dusty bisque.
Flat tiles hide all of this: they have no high points, no recesses, and receive an even coat. A tile tests colour and maturity, not the behaviour of the glaze on three-dimensional form. The glaze is probably not faulty; the application and form are the variables. The student should also consider whether the bowl was bisqued to the same temperature and whether the rim was wiped clean.
2. Recommend a revised process for the next bowl and state what evidence would confirm or refute the diagnosis.
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Revised process: (1) wipe the bisque with a damp sponge and let it dry to remove dust; (2) apply thinner coats, three light passes rather than three heavy ones, drying between coats; (3) on the rim, apply slightly more glaze and smooth it with a finger to build thickness at the high point; (4) on the foot ring, avoid pooling, wipe excess from carved recesses, and wax-resist the very base; (5) fire a small vertical test piece (a folded or curved slab) alongside the bowl rather than relying only on flat tiles.
Evidence that confirms the diagnosis: the revised bowl fires with a smooth, fully matured rim and no pitting while the original glaze batch is unchanged. Evidence that refutes it: the revised bowl still pits on the rim despite correct thickness, which would point to kiln atmosphere, contamination, or an incompatible clay body (e.g., high iron or sulphur in the stoneware causing blistering). Comparing a curved test piece against a flat one isolates form from glaze chemistry.