Layered Glazes, a Found Metal, and a Single Kiln
Your high school runs Ceramics I and II in one room with a single 18 × 18 × 27 in. cone-6 electric kiln fired once weekly. Thirty-one students are enrolled; nine want to enter a juried teen exhibition in five weeks. Maya proposes a mug with a commercial zinc-crystalline glaze outside, a commercial copper-red inside, and a thick 3 mm application; she also wants an underglaze band at the lip. Another student wants to embed a found steel washer in a slab box. You have one test tile for each glaze but no layered test. Last firing produced pinholing on several pieces and a greenish ring on one shelf; kiln wash is flaking. Shelf replacement is unfunded until July. The show requires digital images and a 50-word statement by the deadline, and you have one bisque and three glaze firings before it.
1. What technical risks do these two proposals create, and what will you actually load into the next firing? Explain your sequencing and testing decisions.
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The two proposals fail for different reasons. Maya's mug combines a zinc-crystalline glaze, which depends on a specific cooling schedule to grow crystals, with a copper-red glaze that in most cone-6 electric kilns will not reduce properly; in oxidation it can fire green or muddy, and the two glazes may have mismatched coefficients of expansion. A 3 mm application is also thick enough to run, pinhole, or shiver, and an underglaze at the lip may not be food-safe. The washer is an unknown metal: it will expand more than the clay during firing and can crack the box or contaminate the shelf. For the next firing I would load only test tiles on the same clay body: each glaze alone, the layered combination, and the underglaze under the copper-red, all on cookies. I would not fire the washer or Maya's mug. I would fire a slow-cooling segment for the crystalline test and record the schedule, because the copper-red may need a different cycle. The pinholing and green ring suggest thick glaze, dusty bisque, or copper migration; I would clean shelves and bisque, and separate known copper tests.
2. Draft the feedback you would give Maya about her design and the exhibition deadline, including a decision about whether she can still submit.
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Maya needs a clear, respectful decision with a path forward, not a maybe. I would show her the test tiles and explain that the exhibition deadline does not justify firing an untested combination; the show is a real audience, but the kiln is a shared resource and a failed load can damage shelves and other students' work. I would offer three options: (1) simplify the mug to one tested glaze and a dipped lip, which can be fired in the next glaze load if her test tile passes; (2) keep the crystalline exterior but make a second, smaller test cup now and submit digital images only if the test succeeds; or (3) enter a previously completed, tested piece and use the mug as a semester-end exploration. I would set a decision date before the last glaze firing, give her a 50-word statement draft to revise, and document the conversation. This preserves her ambition while teaching that ceramic surfaces are empirical, not guaranteed.
3. What kiln-loading, shelf-preparation, and documentation decisions would you make before the next firing, and why?
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Before loading, I would scrape the flaking kiln wash down to bare shelf, re-coat with fresh wash if any is on hand, and let it dry; flaking wash is what causes shelf sticking and the green residue. Because shelf replacement is unfunded, I would put every questionable glaze on a sacrificial cookie and keep a 1/2-inch clearance from other pieces. I would not fire the washer under any circumstances; if the student wants metal in clay, we can discuss a cold-assembled or epoxy-set element after firing. I would assign shelves by glaze family: known copper tests on a dedicated shelf or catch basin, crystalline tests on another, and no student show pieces in the same load as an untested combination. I would photograph the shelf map, label test tiles, and log results in a shared firing notebook. Then I would submit a maintenance request for shelves and ventilation, because the current condition is a safety and quality issue, not just an inconvenience.