When the Glaze Test Lies
A student is making a series of five ceramic vessels for assessment. They tested a new commercial glaze on three small tiles fired to cone 6 and recorded a glossy teal result with no defects. Confident, they glazed all five vessels and fired them together in the same kiln load. The kiln was packed more densely than during the tile test because a second class also loaded work. Results: two vessels show crawling (the glaze pulled back from the clay, leaving bare patches), one shows pinholing across one side, and two are acceptable. The student's process folio contains the three tile results and a single line reading 'glaze works, fired cone 6'. The assessment criteria reward documented testing, analysis of variables, and evidence of iterative decision-making.
1. Diagnose the most likely causes of the crawling and pinholing, and explain why the tile test did not predict them.
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Crawling usually arises from poor glaze-to-body adhesion, surface contamination (dust or oil on the clay), an overly thick application, or excessive shrinkage of the glaze layer during drying and firing. Pinholing typically comes from gases escaping through a glaze that has sealed too early, often linked to firing too fast, thick application, or organic material in the clay. The tile test did not predict these because the test changed at least two variables simultaneously: the substrate (flat tiles versus three-dimensional vessels, where glaze thickness varies on curves and rims) and the kiln environment (a sparsely packed test load versus a densely packed shared load, which changes heat distribution, soak time, and cooling rate). A single successful tile at cone 6 is not evidence that the glaze is robust across forms and firing conditions. The folio line 'glaze works' overstates the evidence and would be marked down for failing to record application thickness, clay body, kiln packing, and firing schedule.
2. Recommend how the student should revise both the making process and the folio to meet the assessment criteria, given only one kiln firing remains before submission.
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With one firing left, the student should not re-glaze all five blindly. Prioritise: re-fire only the two crawling vessels after grinding back the bare patches, cleaning the surface, and reapplying a thinner, more even coat; leave the two acceptable vessels untouched. For the pinholed vessel, a thin re-glaze and a slower firing with a longer soak may help, but accept the risk that it may not resolve. Crucially, load the kiln with deliberate spacing and record the packing. In the folio, replace the single line with a structured record: the original tile conditions, the changed variables (substrate, thickness, packing, schedule), photographic evidence of the defects, a labelled diagram of the kiln load, and a written analysis linking each defect to a probable cause. This converts a failure into documented iterative decision-making, which is exactly what the criteria reward. The student should also note what they would test next time, such as a thickness gradient tile and a controlled packing comparison.