|Monthly Tech-Tip |
Although Nepheline Syenite and Custer Feldspar are used as effective fluxes in glazes at cone 8, curiously, neither of them melt well by themselves. Thus, both of these 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.
Well, actually, it is not completely pure - I had to mix in 10% bentonite to give it enough shrinkage and leather hard strength to be able to pull itself away from the plaster mold (this bowl was just extracted). It cast without cracks to about 3mm thickness in about 15 minutes. The defloculated slip required more water than normal but it pours and drains beautifully. I have made higher-percentage slurries in the past using Veegum but the casting time was too long. On aging, the previous slurry I made did not change its rheological properties after a month of storage. Nepheline syenite is a fantastic material for ceramics, but it is not used pure like this of course (that being said, firing tests have already shown promising results, with a vitreous product around cone 1).
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.