Pinholing and blisters in a loaded electric kiln
An electric kiln of 60 litre capacity is used for cone 6 stoneware mugs. The kiln is loaded with three shelves; mugs are placed rim-to-rim and within 20 mm of the side elements. The firing schedule is 150°C/h to 600°C, then 250°C/h to 1200°C with a 10-minute soak. Glaze is applied by dipping to about 1 mm thickness; the glaze slurry is not sieved between batches. After firing, about 30% of mugs show pinholing and small blisters, mostly on the side facing the kiln door and on the top shelf. The same glaze on test tiles fired in a small test kiln is smooth. The workshop has a repeat order and no time for a full re-design. The kiln's thermocouple was replaced last year and the controller is new; the owner blames the glaze. No cones are used.
1. Is the glaze or the kiln the more likely main cause? Justify your answer using the distribution of the defects and the test-tile evidence.
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The kiln and loading are the more likely main cause, although the glaze may contribute. The evidence is the distribution. If the glaze batch were at fault, defects would be random across the load or concentrated on particular mugs, not consistently on the top shelf and the door side. Top-shelf ware is close to the lid and elements and can be overfired; door-side ware is often cooler and may be underfired. Both conditions interfere with glaze maturation and bubble release: overfiring can boil the glaze, while underfiring leaves pinholes that never heal.
The 10-minute soak at 1200°C is short for a loaded kiln, and the final ramp of 250°C/h is fast for stoneware. The kiln is packed with mugs rim-to-rim and within 20 mm of the elements, which restricts circulation and creates hot and cool zones. The smooth test tiles in a small test kiln prove only that the glaze can behave; they do not reproduce the big kiln's loading and atmosphere.
The unsieved glaze is a secondary risk: lumps and dust can cause pinholes. But the location pattern and the small-kiln result make firing the first variable to control. The controller being new does not prove the kiln is even; thermocouples and elements can drift. Without cones, the workshop has no independent check of heatwork.
2. Propose a one-load diagnostic plan and the schedule and loading changes needed for the repeat order. Keep the plan practical for a workshop with limited time.
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Diagnostic load:
- Sieve the glaze through an 80-mesh screen and check dipping thickness; keep the glaze variable constant so it does not confuse the trial.
- Load one shelf at the bottom, one in the middle and one at the top, with test mugs and pyrometric cones at each level and near the door. Leave 50 mm clearance from elements and do not place mugs rim-to-rim.
- Fire a slower schedule: 150°C/h to 600°C, 150°C/h to 1100°C, then 80–100°C/h to 1200°C, with a 30-minute soak. Record controller readings and cone results.
- Compare defects by position. If the top shelf still overfires relative to the middle, reduce top loading or adjust elements/controller offset. If the door side underfires, improve circulation with a shelf gap or a small soak.
Changes for the order:
- Do not pack the kiln; leave gaps between mugs and between shelves and elements.
- Use cones every firing until the kiln is mapped; do not rely only on the controller.
- Keep the final soak long enough to heal pinholes, and cool naturally; do not open the kiln hot.
- Sieve glaze between batches and control thickness; dust and thick glaze are common pinhole causes.
- Fire a small confirmation batch before committing the whole order.
The judgement is to fix firing and loading first, then check glaze thickness. Replacing the glaze immediately would waste time and may not change the result.