Feldspar functions as a flux in both glass and ceramics, industries that together accounted for essentially all of its 2024 U.S. end use. This page covers that role and what firing research shows about how particle size and feldspar type affect the outcome.
What is it?
Feldspar is used as a flux — a material that lowers the melting or firing temperature of the batch it’s added to. In 2024, glass manufacturing accounted for an estimated 50% of U.S. feldspar and nepheline syenite end use, with ceramic tile, pottery, and other uses making up the other 50%. It’s typically ground to about 20 mesh for glassmaking and to 200 mesh or finer for most ceramic and filler applications.
Why should I use it?
- It’s a flux with well-documented, roughly even demand across both glass and ceramics — see “What is it?” above.
- Its particle size and sodium/potassium content directly affect firing outcomes, not just its presence in the mix — see Limitations.
How it’s used
In Glass, feldspar is ground to about 20 mesh and used as a flux to lower melting temperature. In Ceramics, it’s ground to 200 mesh or finer for the same flux function in tile, pottery, and related applications.
Limitations — when it isn’t the right choice
- In one documented firing study, coarser particle size and potassium-rich feldspar both measurably increased water absorption in fired ceramic bodies — a real quality outcome, not a minor detail. A 2014 peer-reviewed study using a fixed slip formulation (50% clay, 25% quartz, 25% feldspar) fired at 1240°C compared two feldspar particle sizes (45 and 75 microns) and three flux compositions (sodium-based, potassium-based, and a mix). The finer 45-micron particle size reduced water absorption by 0.39–0.8% compared to 75 microns, and increasing the sodium oxide content in the feldspar reduced water absorption from about 1% to near zero compared to the potassium-rich version. This was documented for one specific formulation and firing cycle; it should not be assumed that every commercial feldspar grade and ceramic body responds identically without checking its own firing data.
