|Monthly Tech-Tip |
Alternate Names: Na-Feldspar, Na Feldspar, Albite
Soda Feldspar is generally a clean white mineral that sources sodium, alumina and silica along with some potassium and calcium. No actual materials have the ideal formula shown here, they all have some K2O and small amounts of other contaminants. Manufacturers refer to their materials as 'soda' if Na2O predominates over K2O (often manufacturers refer to their materials at soda-potash feldspars (or vice versa) if neither of the oxides predominates). Nepheline Syenite is often used in similar circumstances, however it has a higher alumina content (often the lower alumina content of soda feldspar provides more room in glaze recipes for alumina sourcing ball clay or kaolin to help suspend the slurry).
Albite mineral, from which soda feldspar is derived, often tends to be white in appearance compared with the pinkish hue of orthoclase mineral from which potash feldspar is ground.
In many cases soda and potash feldspar are interchangeable in glaze recipes. However, some differences will be apparent. Soda feldspar normally melts better. Also, slightly different color responses will be encountered (i.e. cobalt tends toward purple copper toward blue with soda spar). Sometimes colors will be brighter with soda feldspar. In addition, soda spar has a higher coefficient of expansion and melts earlier and has a shorter range before it begins to volatilize.
Because sodium has the highest expansion of all oxides and because it is high in this material, glazes with a lot of soda spar will almost always craze. Thus, if any glaze recipe has more than 30% soda feldspar, it is wise to test carefully to assure the glaze is not in tension (use glaze calculation software to reduce the Na2O and replace it with CaO, MgO or another fluxing oxide if possible). It is important to remember that if a glaze is high in Na2O, diluting it with other oxides to try to reduce crazing is a losing battle, it is the cause of the problem, it needs to be reduced.
A cone 8 comparative flow tests of Custer, G-200 and i-minerals high soda and high potassium feldspars. Notice how little the pure materials are moving (bottom), even though they are fired to cone 11. In addition, the sodium feldspars move better than the potassium ones. But feldspars do their real fluxing work when they can interact with other materials. Notice how well they flow with only 10% frit added (top), even though they are being fired three cones lower.
|Materials||MC Na-200 Feldspar|
The most common source of fluxes for high and medium temperature glazes and bodies.
Materials used in Denmark
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.
Study on melting of Albite - Institute of Geophysics and Planetary Physics
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.
|Oxides||Na2O - Sodium Oxide, Soda|
|Density (Specific Gravity)||2.60|
|Frit Melting Range (C)||1100-1150C|