Potash feldspar and sodium feldspar do the same basic job in a ceramic body or a glass batch: they are fluxes. They melt during firing, form a glassy phase, and bind the quartz and clay particles together. The difference lies in which alkali does the work, and that changes how the material behaves in the kiln.
What each one is
Potash feldspar, also called potassium feldspar or K-feldspar, is the potassium-rich member of the feldspar group. Its main alkali is potassium oxide (K2O).
Sodium feldspar, also called soda feldspar, Na-feldspar or albite, is the sodium-rich member. Its main alkali is sodium oxide (Na2O).
Natural feldspar is never purely one or the other. A potash grade always carries some soda, and a soda grade always carries some potash, which is why a specification sheet lists both oxides.
How they behave in firing
Sodium feldspar starts to melt at a lower temperature and gives a more fluid melt. It is the stronger flux of the two, so a body reaches vitrification faster.
Potash feldspar melts to a stiffer, more viscous glass. The body softens more gradually, holds its shape better at peak temperature, and tolerates a wider firing range.
The two also differ in thermal expansion. Soda raises the expansion of a glaze more than potash does, which matters when a glaze has to fit its body without crazing.
Where each is normally used
- Vitrified and porcelain tiles: sodium feldspar is the usual main flux, because fast-firing cycles need a material that melts early.
- Sanitaryware, tableware and electrical porcelain: potash feldspar is preferred, because large or thin pieces must not deform during firing.
- Glazes and frits: both are used. The choice depends on the firing temperature and on the expansion the glaze needs.
- Glass: feldspar supplies alumina together with alkali. Either type can be used, depending on the batch calculation.
Many producers blend the two to balance firing speed against deformation.
What to check on a specification sheet
- K2O and Na2O: the alkali content sets the fluxing power, and the ratio tells you which type you are really buying.
- Fe2O3 and TiO2: iron and titanium colour the fired product. Lower values give a whiter body.
- Al2O3 and SiO2: these show how much free quartz comes with the feldspar.
- Loss on ignition (LOI): a low figure points to little clay or mica contamination.
- Particle size: lumps, grains or powder, and the mesh size if milled.
Ask for a fired test button as well. Chemistry tells you what is in the material, and the fired colour tells you how it will look in your product.
Our grades
Sun Minerals manufactures both types in Rajasthan, India. Potassium feldspar is available as grades POTASH-01 and POTASH-02, and sodium feldspar as grades SODIUM-01 and SODIUM-02. Each product page carries the full chemical specification and a technical data sheet.
If you are not sure which grade suits your body or glaze, send us your requirement and we will suggest the matching grade.