Nepheline Syenite is an anhydrous sodium potassium alumino silicate. Although feldspar-like in its chemistry, mineralogically it is an igneous rock combination of nepheline, microcline, albite and minor minerals like mica, hornblende and magnetite. It is found in Canada, India, Norway and USSR. Thus it does not have a simple theoretical formula like soda feldspar (we have provided representative chemistry of a Canadian nepheline syenite).
Nepheline Syenite has been a standard in the ceramic industry for many years, and is very popular for its whiteness. Nepheline syenite melts lower than feldspars. For example, it is possible to make a very white vitreous medium temperature porcelain (firing as low as cone 4, but more practically at cone 6). Up to 50% nepheline syenite will be needed at cone 4, 35-40% at cone 6. 20% silica is needed or glazes will craze. The rest is clay, preferably kaolin. The whiter the kaolin, the less plastic it will be, and the more there will be a need to add a plasticizer. White plasticizers are expensive (VeeGum T at 3-5% will be more expensive than all the other ingredients combined). Bentonite plasticizers are cheaper, but will darken the color. You might consider using a ball clay instead of a kaolin, it will reduce the need for plasticizer additions, but the body will not fire as white as with kaolin.
Like feldspar, nepheline syenite is used as a flux in tile, sanitary ware, porcelain, vitreous and semi-vitreous bodies. It contributes high alumina without associated free silica in its raw form and fluxes to form silicates with free silica in bodies without contributing free silica itself. This stabilizes the expansion curve of the fired body. It is an excellent tile filler and melter, especially for fast firing. Nepheline syenite is valuable in glass batches to achieve the lowest melting temperature while acting as a source of alumina.
Like talc, this material can also be used at low temperatures to increase the thermal expansion of bodies to make glazes fit better (up to 50% talc is used for this purpose, but it cuts plasticity drastically unless vacuum pugged). Nepheline Syenite does not do this, and potentially less is needed. And it fires whiter.
Since nepheline syenite can be slightly soluble, in pugged bodies it can be responsible for stiffness changes during aging (although admittedly many other factors can also contribute to this). It can more challenging to maintain stable deflocculated slurry bodies using nepheline syenite than with feldspars. However, the place where you may note the solubility of nepheline the most is in glaze slurries containing significant percentages, they can gel over time and the addition of more water to thin the slurry can wreak havoc with application performance (try adding a few drops of deflocculant instead).
Because of its sodium content, high nepheline syenite glazes tend to craze (because of the high thermal expansion of Na2O). Also, since nepheline syenite has more alumina than most feldspars, substituting it into recipes means that on one hand a lower melting temperature is achieved while on the other a more viscous melt results because of the extra alumina.
The picture of the flow test here shows that nepheline syenite by itself is barely beginning to flow and melt at cone 9. However when combined with other materials it will promote melting to a much greater degree than is suggested by its performance alone. Notice that the 400 and 270 mesh particle size versions do not melt differently at this temperature.
Source; Digital Fire