Pinholing and Cracking in a Cone 6 Glaze Load
An art teacher fires 18 student mugs made from buff stoneware. The mugs were bisque fired to cone 04 with a 10-minute hold, then glazed to roughly 1.5 mm thickness with a commercial cone 6 oxidation glaze. The glaze firing went to cone 6 at medium speed with no hold; the top peephole stayed open until 900 °C. Results: 11 mugs have pinholes, 3 show glaze crawling near the foot, and 1 mug has a crack running from the base upward (dunting). The kiln is a manual electric Skutt with a kiln sitter and no digital controller. The teacher has one week before the school art show and 20 more greenware mugs to fire.
1. Distinguish the likely causes of pinholes, crawling, and dunting, using the firing and application details.
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Pinholes are likely from too-thick glaze (1.5 mm is heavy), insufficient bisque maturity or hold (cone 04 with 10 minutes may leave organics or carbonates), and no hold at cone 6 so bubbles do not heal. Crawling on 3 mugs suggests glaze too thick, dusty bisque, or oily contamination near the foot. Dunting-a crack from the base upward-usually comes from uneven heating or cooling, especially a fast ramp or cooling through quartz inversion around 573 °C, or from a thick base. The open peephole until 900 °C helps with early carbon burn-out but does not solve a late soak. The manual kiln with kiln sitter means the teacher must control ramps by witness cones and time, not a controller.
2. Propose a revised firing schedule and application protocol for the remaining 20 mugs; include specific cones, ramps, holds, and thickness targets.
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Bisque to cone 05 with a 20-minute hold, or at least cone 04 with a 30-minute hold, to burn out carbon. Sieve glaze, apply 0.75–1.0 mm (about the thickness of a nickel), and wipe the foot clean. For the glaze firing: slow ramp 100–150 °C/h to 600 °C, 150 °C/h to 1000 °C, then 80–100 °C/h to cone 6; hold 10–15 minutes at cone 6; cool no faster than 100 °C/h to 700 °C. Use witness cones on each shelf. Leave the peephole open only until 600–700 °C, then close. For the remaining 20 mugs, fire a test tile with the same glaze thickness before committing the load.
3. What should the teacher do with the cracked mug, and why is re-firing it risky?
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Do not re-fire the cracked mug for food use. A dunt crack compromises structural integrity; reheating can propagate it or cause it to break in the kiln. Use it as a glaze test or a pencil cup with a saucer, clearly labeled 'not food-safe.' If the crack is only in the glaze, it might be a crazing line, but a base-up crack is a body crack. Document it as a loss and use it to teach thermal shock. The risk is not just losing one mug; a piece that fails in the kiln can damage adjacent work and the kiln shelf.