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Alternate Names: Roasted Alberta Slip
Oxide | Analysis | Formula | |
---|---|---|---|
CaO | 6.40% | 0.38 | |
MgO | 4.20% | 0.35 | |
K2O | 3.80% | 0.14 | |
Na2O | 2.40% | 0.13 | |
TiO2 | 0.30% | 0.01 | |
Al2O3 | 16.50% | 0.54 | |
SiO2 | 58.20% | 3.26 | |
Fe2O3 | 4.90% | 0.10 | |
LOI | 3.00% | n/a | |
Oxide Weight | 325.21 | ||
Formula Weight | 335.26 |
Fully calcined Alberta Slip (fired at 1900F) has an LOI of 9%. However it is not necessary to fire it that high to remove the plasticity (to reduce shrinkage during drying). The albertaslip.com website outlines this 1000F roasting process. This version looses only 3% of its weight on firing, so the analysis totals 97.
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Roasted Alberta Slip (right) and raw powder (left). These are thin-walled 5 inch cast bowls, each holds about 1 kg. I hold the kiln at 1000F for 30 minutes. Why do this? Because Alberta Slip is a clay, it shrinks on drying (if used raw the GA6-B and similar recipes will crack as they dry and then crawl during firing). Roasting eliminates that. Calcining to 1850F sinters some particles together (creating a gritty material) while roasting to 1000F produces a smooth, fluffy powder. Technically, Alberta Slip losses 3% of its weight on roasting so I should use 3% less than a recipe calls for. But I often just swap them gram-for-gram.
This picture has its own page with more detail, click here to see it.
If I put this on social, I might get 100 comments on the cause. Most would say it's applied too thick. That is not correct. At the top, it is too thick, but not at the bottom (where it is still cracking). Crawling during firing is almost certain.
Before offering any opinion, one needs to sanity check the recipe. This one, GA6-B, contains a lot of clay. Why do dipping glazes contain clay, usually kaolin, ball clay? To suspend the slurry, slow down drying, gel the slurry to make it thixotropic and harden the layer on drying. But their chemistry is also important, clays supply the all-important Al2O3 and SiO2 to the melt (which would otherwise have to come from feldspar). Certain clays excel at suspending. 15-20% EP kaolin, for example, is all that is needed. It’s not that plastic, but sticky and thixotropic (Grolleg and New Zealand kaolins are similar or even better). 10% of a gelling ball clay might be enough. However, 50% of a silty non-plastic clay might be needed. When the clay has too much influence, glazes shrink too much as they dry and then crack like this. Ones lacking clay (or employing one that is not suitable) have poor application properties (for dipping) and are powdery. This glaze has 80% pure raw Alberta Slip. That is a plastic clay, no wonder this is happening! 50% of it needs to be roasted.
Materials |
Alberta Slip
Albany Slip successor - a plastic clay that melts to dark brown glossy at cone 10R, with a frit addition it can also host a wide range of glazes at cone 6. |
Materials |
Alberta Slip 1900F Calcined
|
URLs |
http://albertaslip.com
AlbertaSlip.com |
Troubles |
Crawling
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms. |
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