Nepheline Syenite is a common flux chemically similar to a soda feldspar. However, nepheline syenite contains a varying blend of nepheline, iron minerals, soda feldspar and potassium feldspar, as well as silica, alumina, and calcium. Nepheline syenite is a low silica, high soda and high alumina mineral. Its high alumina content strengthens glass, and the high soda makes it a great flux.
Added to glazes, nepheline syenite increases the firing range of low-fire and mid-range glazes. and may increase crazing in glazes due to its high thermal expansion.