1220°C — The point where clay loses its identity.
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I took a common clay, formulated for low-temperature firing, and pushed it beyond the limit it was designed for. Not to obtain an object, but to register the exact moment when matter stops being what we assume it is.
At 1220°C, the clay no longer belongs to ceramics. It doesn’t vitrify, it doesn’t stabilise — it turns into slag, a glassy, brittle mass with gas pockets trapped inside, like a frozen industrial reaction.
In the Thomsa archive, this result is not classified as a failure, but as material evidence. This is the point where clay breaks away from its form — and beyond that break, something else begins. A new matter, no longer ceramic in language, but rich in technical meaning.
ATELIER NOTE — 1220°C / THERMAL COLLAPSE POINT
Material tested: Red earthenware clay — official specification: 960–1150°C
Glazes used: Jupiter, Rustica Gold, Albastru de Noapte, K21 Matte, Taiga, KGS 92 Fossil — glazes formulated for 1200–1260°C (stoneware/porcelain range).
Experiment objective: To determine the behaviour of this low-fire clay when pushed beyond its declared maximum temperature, in the presence of high-flux glazes.
TESTED VARIANTS
| Variant | Clay condition before firing | Procedure |
|---|---|---|
| V1 | Clay bisque-fired at 1000°C | Glazing → final firing at 1220°C |
| V2 | Raw clay, fully dry | Direct glazing → single firing at 1220°C |
FIRING PROGRAM — KITTEC KILN
| Segment | Ramp rate | Target temperature | Hold time | Functional observation |
|---|---|---|---|---|
| S01 | 100°C/h | 600°C | 10 min | Physical water elimination |
| S02 | 80°C/h | 800°C | 10 min | Organic burnout and progressive degassing |
| S03 | 120°C/h | 1000°C | 15 min | Equivalent to in-process bisque stage |
| S04 | 150°C/h | 1220°C | Hold 10 min | Glaze enters full flux |
| Cooling | 100°C/h | down to 200°C | — | Controlled cooling, no thermal shock |
POST-FIRING OBSERVATIONS
Bisque-fired variants (V1):
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The clay did not enter a controlled vitrification phase but began to partially melt.
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The glaze reacted with the iron oxides → resulting in a brittle glassy mass.
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Outcome: fragile structure with no internal cohesion, visually similar to ceramic slag.
Raw clay variants (V2):
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Intense reaction: violent degassing beneath the glaze, leading to complete structural collapse.
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The glaze, once in full flux, accelerated the melting of the clay body.
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The resulting material is a porous ceramic slag, partially fused to the kiln shelf.
TECHNICAL EVALUATION
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This type of clay cannot perform as stoneware at 1220°C.
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At this temperature, it does not vitrify gradually, but transitions directly into an unstable glass phase.
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The ceramic body loses structural identity, turning into a brittle vitrified mass.
KILN CONDITION
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The kaolin protection layer successfully contained the flow.
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No significant infiltration was observed towards the heating elements.
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The kiln shelves can be mechanically cleaned.
ATELIER CONCLUSION (FINAL DECISION)
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Firing this clay to 1220°C will not be repeated in its current form.
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To explore the high-temperature range (>1200°C), two pathways are considered:
• Option 1: Clay + 40–60% refractory grog (0–2 mm)
• Option 2: Transition to a stoneware clay body (red or black), formulated for 1200–1260°C -
The resulting vitrified mass will be archived as an official laboratory sample within the Thomsa Experimental Lab, under internal code:
→ EXP/1220/01 — Thermal Collapse Point / Slag Formation
EXPERIMENTAL VALUE
This test establishes the definitive thermal limit of this low-fire clay body. The melted mass is not considered a ceramic piece, but a material collapse specimen, essential for defining future Thomsa protocols in the high-fire ceramic range.
CONCLUSION:
Firing a low-fire clay body to 1220°C, in the presence of glazes formulated for stoneware, does not produce a stable ceramic structure. Instead, it results in an unstable vitrified slag, brittle and lacking internal cohesion.
From a technological standpoint, this experiment defines the operational thermal limit of this clay: beyond 1140–1150°C, the material loses structural integrity and enters a phase of uncontrolled melting.
The outcome is not suitable for functional use, but it is valuable as a boundary material sample. Within the Thomsa Lab archive, this fragment will be retained as a thermal collapse reference, relevant for comparison with future compositions and for defining firing protocols in the high-temperature range.