|Monthly Tech-Tip |
Alternate Names: Neph Sy
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.
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
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).
These were applied to the bisque as a slurry (suspended by gelling with powdered or dissolved epsom salts). Notice the crazing (feldspars always craze because they are high in K2O and Na2O, these oxides have by far the highest thermal expansions).
Plainsman 3D! White cone 04 bodies are not vitreous and strong and neither is this. But it is plastic, smooth and fits common low fire glazes. How? 15% Nepheline Syenite (also 50% Plainsman 3D, 35% ball clay and 3% bentonite). The unmelted nepheline particles impose their higher thermal expansion on the fired ceramic. Spectrum 700 clear glaze does not craze and does not permit the entry of water (the mug is glazed across the bottom and fired on a stilt). The mug on the right is made from the same clay, it has been fired ten cones higher, cone 6! Here the nepheline is acting as a flux, producing a dense and very strong stoneware (with G2926B, GA6-B glazes). This is incredible! One note: This cannot be deflocculated and used for casting, soluble salts in the 3D gel the slurry.
Nepheline Syenite is a type of feldspar. We use it in porcelain bodies. The nepheline content determines the temperature at which the body is vitreous. We use this product in preference, where possible over soda or potash feldspar. There are several reasons: It is mined and processed in Canada and is renowned around the world for it's quality. If has a very low iron content, this makes for whiter-burning bodies. It is a more powerful flux than common feldspars. It has been very consistent over many decades.
Nepheline Syenite is similar to a feldspar. I have them open side-by-side here in my Insight-live.com account (the blue panels). In the "Analysis" column notice that Minspar has 9% more SiO2 and 5% less Al2O3. Minspar has 12% fluxes and Nepheline has more than 15%. If a recipe does not contain a significant percentage of Nepheline these differences might be tolerated but what if there is 30% or more? There is no combination of materials that has the chemistry of Nepheline (there is no way to take 10% of the SiO2 out of this feldspar, for example). But it is possible to take SiO2 out of a glaze containing Nepheline Syenite. Notice the two green recipe panels below: The changes made to the one on the right harmonize the oxide chemistry with the original on the left. Those changes were significant: 15% more feldspar, 12% less silica and 2% less kaolin. Notice how easy this was using a little chemistry!
|Materials||Covia Nepheline Syenite|
|Materials||Nepheline Syenite Norwegian|
Substituting Nepheline Syenite for Soda Feldspar
Learn to substitute Nepheline Syenite for Soda Feldspar (and vice versa) using the KNaO concept in Insight. You will see the benefit of in-recipe substitution calculation rather than making general substitution rules.
Nepheline at Wikipedia
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
The most common source of fluxes for high and medium temperature glazes and bodies.
Demonstrating Glaze Fit Issues to Students
Glaze and body can both be adjusted to solve crazing and shivering problems. This describes a simple project to create body glaze combinations guaranteed to craze and shiver to demonstrate the principles involved.
|Oxides||K2O - Potassium Oxide|
|Oxides||Na2O - Sodium Oxide, Soda|
|Oxides||SiO2 - Silicon Dioxide, Silica|
|Frit Melting Range (C)||1100C|
|Body Maturity||This material is generally fluxes better than feldspars and produces whiter burning bodies.|