Alternate Names: Neph Sy
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|MLRG - Frit Melting Range (C)||1100C|
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.
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.
Nepheline syenite is not available in many parts of the world and the INSIGHT ceramic calculation software instruction manual contains a lesson on how to calculate a substitution using a soda feldspar. The chemistry of nepheline is quite different from other feldspars and this is thus well worth while.
Comparison between Canada, Norway and theoretical materials:
SiO2 60.0 56.0 41.1
Al2O3 23.2 24.2 34.9
Fe2O3 0.10 0.11
CaO 0.25 1.2
Na2O 10.8 7.8 15.9
K2O 5.1 9.1 8.1
LOI 0.5 1.5
This Nepheline Syenite flow test did not demonstrate much of a difference in melting at cone 9 between 270 and 400 mesh materials.
Left: Cornwall plus 10% Ferro Frit 3134. Right: Nepheline Syenite plus 10% of the same frit. These are fired at cone 6.
Left: Cone 10R (reduction) Plainsman P700 porcelain (made using Grolleg and G200 Feldspar). Right: Plainsman Cone 6 Plainsman Polar Ice porcelain (made using New Zealand kaolin and Nepheline Syenite). Both are zero porosity. The Polar Ice is very translucent, the P700 much less. The blue coloration of the P700 is mostly a product of the suspended micro-bubbles in the feldspar clear glaze (G1947U). The cone 6 glaze is fritted and much more transparent, but it could be stained to match the blue. These are high quality combinations of glaze and body.
These are porosity and fired shrinlage test bars, code numbered to have their data recorded in our group account at Insight-live.com. Plainsman P580 (top) has 35% ball clay and 17% American kaolin. H570 (below it) has 10% ball clay and 45% kaolin, so it burns whiter (but has a higher fired shrinkage). P700 (third down) has 50% Grolleg kaolin and no ball clay, it is the whitest and has even more fired shrinkage. Crysanthos porcelain (bottom, from China) also only employs kaolin, but at a much lower percentage, thus is has almost no plasticity (suitable for machine forming only). Do H570 and P700 sacrifice plasticity to be whiter? No, with added bentonite they have better plasticity than P580. Could that bottom one be super-charged? Yes, 3-4% VeeGum or Bentone (smectite, hectorite) would make it the most plastic of all of these (at a high cost of course).
The two mugs on the left: Traditional Grolleg porcelain using Nepheline and bentonite (fired to cone 10R). The right: Using New Zealand kaolin, Nepheline Syenite and VeeGum.
Three Cornwall Stone shipments fired at cone 8 in melt flow testers and compared to Nepheline Syenite. Each contains 10% Ferro Frit 3134.
These were applied to the bisque as a slurry (suspended by gelling with epsom salts). The nepheline is thicker. Notice the crazing. This is what feldspars do. Why? Because they are high in K2O and Na2O, these oxides have by far the highest thermal expansions. So if a glaze is high in feldspar it should be no surprise that it is going to craze also.
Out Bound Links
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Glaze and body can both be adjusted to solve crazing and shivering problems. This describes a simple...
In Bound Links
Na-Feldspar, Na Feldspar, Albite
Issues surrounding substitution with complex mater...
North Cape Nepheline Syenite