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An example of how calcium carbonate can cause blistering as it decomposes during firing. This is a cone 6 Ferro Frit 3249 based transparent (G2867) with 15% calcium carbonate added (there is no blistering without it). Calcium carbonate has a very high loss on ignition (LOI) and for this glaze, the gases of its decomposition are coming out at the wrong time. While there likely exists a firing schedule that takes this into account and could mature it to a perfect surface, the glaze is high in MgO, it has a high surface tension. That is likely enabling bubbles to form and hold better.
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This cone 04 flow tester compares two commercial low-fire transparent glazes. Their different approaches to the chemistry are revealed by these melt flows. While 3825B appears to have a higher melt fluidity, its higher surface tension is the real story. This is demonstrated by how the flow meets the runway at a perpendicular angle. Notice that A, by contrast, meanders down the runway in a broad, flat and relatively bubble-free river. Low-fire glazes must pass many more bubbles than their high-temperature counterparts, the low surface tension of A aids in that. A is Amaco LG-10. B is Crysanthos SG213 (Spectrum 700 behaves similarly, although flowing less). Both 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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