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This melt fluidity tester demonstrates that the cone 6 G1214Z recipe fires highly melt fluid, much more than one might expect from a matte glaze. When applied as a thin layer on pottery it does behave as a matte glaze. A quick look at its chemistry reveals the reason: High CaO and a low Si:Al ratio. Unfortunately, many matte glaze are such simply because they are not melted enough.
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The clay is a buff stoneware. The upper two samples are G2934, an MgO matte. The one on the right has 10% zircon added to opacify.
The bottom two are G1214Z1 (transparent version and opacified-with-zircon version).
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The body is Plainsman M390. The firing schedule is Plainsman PLC6DS. The tin is on the right.
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Double-layered on top half. Fired with the PLC6DS schedule.
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G1214Z1 is a popular cone 6 calcium matte glaze recipe. It has very high melt fluidity, enabling a fine grained crystalline matte surface to develop during cooling. A potter was steered to this recipe after finding that G2934 magnesia matte fired too variegated when stained blue. However, her first effort with this failed a leaching test. She had a secret weapon: An account at Insight-live.com, where recipes and their calculated oxide formulas can be compared side-by-side. Leaching glazes are most often runny because they contain excessive fluxing oxides. She simply increased the SiO2, it is the glass that makes up the lion's share of all glazes (higher amounts of it characterize glossy glazes). Al2O3 couples with it to improve durability (and the Si:Al ratio is a factor in the degree of matteness). With an accompanying small increase in the B2O3, the magic glass:flux that makes most cone 6 glazes possible, the got the result on the right. The good news: It passed the GLLE test for leaching. There is a lesson here: She had to compromise the degree of matteness a little to get the food safe product. A benefit is that it is also less prone to cutlery marking. Happily, it turned out that much less blue stain was needed.
Recipes |
G1214Z1 - Cone 6 Silky CaO matte base glaze
This glaze was born as a demonstration of how to use chemistry to convert a glossy cone 6 glaze into a matte. |
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