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An example of how calcium carbonate can cause blistering as it decomposes during a fast firing in our electric kiln. This is a cone 6 borosilicate glaze with 15% calcium carbonate added (there is no blistering without it). Calcium carbonate has a very high loss on ignition (LOI), and for this early-melting glaze, the gases of its decomposition are still escaping after melting begins. Another factor is also involved: Although the glaze has good melt fluidity, bubbles survived till near the end of the firing, resisting rupture (likely because of the high surface tension of the melt). When the bubbles finally did burst, there was inadequate time for healing to occur.

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This cone 04 flow tester compares two commercial low-fire transparent glazes. Their different chemistry strategies are revealed by the shape of these melt flows. While 3825B appears to have the higher melt fluidity, it also has much higher surface tension. This is evident in the narrow, rope-like stream and the way the flow meets the runway at a high angle before pulling into a rounded bead. A, by contrast, spreads and wets the runway, meandering downward in a broad, flat and relatively bubble-free river.
This difference is important in low-fire ware because these glazes must pass far more gases and bubbles than high-temperature glazes. The lower surface tension of A aids bubble release and healing after bubbles break. A is Amaco LG-10. B is Crysanthos SG213 (Spectrum 700 behaves similarly, although flowing less). Both approaches have advantages and disadvantages and are worth testing in your application.
| Glossary |
Decomposition
In ceramic manufacture, knowing about the how and when materials decompose during firing is important in production troubleshooting and optimization |
| Glossary |
Surface Tension
In ceramics, surface tension is discussed in two contexts: The glaze melt and the glaze suspension. In both, the quality of the glaze surface is impacted. |
| Materials |
Calcium Carbonate
In ceramics, calcium carbonate is primarily a source of CaO in raw stoneware and porcelain glazes. |
| Articles |
Firing: What Happens to Ceramic Ware in a Firing Kiln
Understanding more about changes taking place in the ware at each stage of a firing helps tune the curve and atmosphere to produce better ware |
| Troubles |
Glaze Blisters
Questions and suggestions to help you reason out the real cause of ceramic glaze blistering and bubbling problems and work out a solution |
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