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Although Nepheline Syenite and Custer Feldspar are used as effective body maturing agents and fluxes in glazes past cone 6, curiously, neither of them melt well by themselves. Thus, both of these come 6 melt fluidity tests add 20% Ferro Frit 3134 to get them flowing. This is a 2021 shipment of the feldspar and a 2022 shipment of the nepheline.

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What would it take to cast pure Nepheline Syenite? A clay addition. This 10:90 bentonite:Nepheline has enough shrinkage and leather-hard strength to enable pulling itself away from the plaster mold. This bowl was just extracted; it cast without cracks to 3mm thickness in about 15 minutes. The defloculated slip required more water than normal but it pours and drains really well. This is a demonstration of how much control technicians can have over casting rate. The amount and type of plastic clay added can take this material from being completely water permeable, zero leather-hard strength and non-castable to water impermeable, very high leather-hardness but very slow casting. Only a testing program is needed to determine the minimum bentonite percentage needed to achieve reliable casting (it is likely less than 10%).
For example, 95%+ nepheline slurries can be made using Veegum. How did this bowl fire? As a vitreous product around cone 1. And, on aging for a month, the rheological properties of the slurry did not change.

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It is near stoneware strength. How was it possible to make this? Actually, it is 90% nepheline syenite and 10% bentonite. The latter imparts enough plasticity that it can be thrown easily on a potter's wheel. By about cone 1 it begins to warp. This is fired to cone 02 with standard Spectrum low fire glazes. No crazing is evident and the coverage is normal. This really demonstrates the amazing ceramic properties of this material. We aged this for several weeks before throwing and it was stable and unchanged in softness or plasticity.
| Materials |
Nepheline Syenite
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| Materials |
Custer Feldspar (pre-2000)
The most common potash feldspar used in ceramics in North America. It was a standard for many decades but is no longer available. |
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