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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). The difference is surface character is visible.

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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 oxide chemistry reveals the reason: High CaO and a low Si:Al ratio. Unfortunately, many matte glazes are such simply because they are not melted enough. Others have to much SiO2 and require very slow cooling in the kiln to achieve results.

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Watch the G1214Z video to see me convert the G1214M cone 6 clear base into G1214Z cone 6 calcium matte using simple glaze chemistry and recipe logic. This first appeared in the Digitalfire desktop Insight instruction manual 30 years ago. It is an understatement to say that this process is interesting if you want to know more about glazes, their chemistry and recipe logic. Watch this video and see me adjust the recipe of my high-calcium transparent cone 6 glaze to convert it into a calcium matte. In an Insight-live.com account, the process is easy enough for anyone. We'll cut the Si:Al ratio, increase the CaO, maintain the thermal expansion for glaze fit and make the recipe shrinkage-adjustable using a mix of calcined kaolin and raw kaolin. We will even compare it with the High Calcium Semimatte from Mastering Glazes.
| Glossary |
Opacifier
Glaze opacity refers to the degree to which it is opaque. Opacifiers are powders added to transparent ceramic glazes to make them opaque. |
| Glossary |
Medium Temperature
These are stoneware glazes that fire in the range of 1200C (2200F). They often contain boron to assist with melting. |
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