The kaolin is implicated for this glaze crawling problem. 30% EPK is a lot, a tipping point to issues with shrinkage and cracking on drying. But although #6 Tile kaolin in its place (panel #2) doesn't disrupt the chemistry of the glaze, it does disrupt the drying shrinkage. It is just too plastic for that high a percentage in a recipe. The fix is to substitute part of it for calcined kaolin.
Calcined kaolin has no LOI (vs 12% for raw kaolins), so we could call it a purer aluminum silicate. Without overthinking it, round this off and just consider that 10% less is needed. So 20% #6 Tile and 9% calcined kaolin sources the chemistry of 30% EPK. The right panel shows the calculated chemistry of the recipe with this change; notice it is very close to the same.
There are other fix options. NZK and Grolleg are better choices than #6 Tile; using either might mean that no calcined material is needed. Also, the failure happened on multi-layering of dipping glazes; crawling is common with that. The base glaze could have been mixed as a base-coat dipping glaze.

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The reason the G1214Z recipe became G1214Z1 was the high clay percentage (which increased drying shrinkage, thus cracking, leading to crawling). The partial healing of the drying cracks during melting produced these unusual wrinkles (rather than the bare patches normally associated with crawling).
The original recipe called for 37 kaolin. That is too high. The fix is to supply kaolin as a mix of raw and calcined (to reduce drying shrinkage). I adjusted to 20 EPK and 15 calcined kaolin (15 being an LOI-adjusted amount).
If you need to prove this, glaze identical test pieces with all-raw G1214Z and raw/calcined G1214Z1, at the same measured thickness and SG. Before firing, inspect them with a magnifier, not just when they first appear dry, but again 5-15 minutes later (high-clay coatings can continue shrinking after they look dry).

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#6 Tile kaolin is a "designer kaolin"; they add bentonite to the material during manufacture to increase the plasticity far beyond what Mother Nature can make. This 10-inch vase is made from the pure kaolin, it is very thin-walled and light. This would be impossible with almost any other kaolin. The fired colour is darker than some other North American kaolins, but less of this (and less added bentonite) is required in a recipe to get the same plasticity. Thus, in the end, a plastic porcelain body can often fire just as white using this as other whiter-burning, but less plastic kaolins.
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Converting G1214M Cone 6 transparent glaze to G1214Z matte
I converted a glossy transparent pottery glaze into a matte glaze using glaze chemistry and recipe logic. Watch me do it in my account at Insight-live.com. We'll cut the Si:Al ratio, increase the CaO further, maintain the thermal expansion for glaze fit and make the recipe shrinkage adjustable using a mix of calcined and raw kaolin. See the landing page to learn every detail: https://digitalfire.com/picture/3787. |
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Glaze Chemistry
Case studies where glaze chemistry was used to solve a problem. |
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