What is sodium feldspar and its specific use as a flux in ceramic glazes

As an important silicate mineral raw material in the ceramic industry, sodium feldspar (NaAlSi₃O₈) plays a key role in glaze formulations due to its unique chemical composition and physical properties. Its rich Na₂O (about 11%) content makes it an efficient flux, significantly reducing the melting temperature of glazes, promoting the formation of glass phases, and optimizing glaze performance.

What is sodium feldspar?

Sodium Feldspar (NaAlSi₃O₈) is a widely distributed silicate mineral with a chemical composition of Na₂O·Al₂O₃·6SiO₂. In nature, sodium feldspar often exists in the form of crystals or blocks, mostly white or off-white in color, with a glassy luster and a Mohs hardness of 6-6.5.

The crystal structure of sodium feldspar is a triclinic system with a tetrahedral skeleton, which is composed of [SiO₄]⁴⁻ and [AlO₄]⁵⁻ tetrahedra connected by shared oxygen atoms, and Na⁺ is filled in the structural gaps as a cation to balance the charge. It has good chemical stability, but it can react with oxides at high temperatures to form a low-melting glass phase, which makes it of great application value in the ceramic industry.

The specific effect of sodium feldspar as a flux in ceramic glazes

As the main flux in ceramic glazes, sodium feldspar plays a key role in the glaze firing process due to its high Na₂O content and suitable SiO₂/Al₂O₃ ratio. Its specific effects include the following aspects:

Reducing the melting temperature

Na₂O (about 11%) in sodium feldspar is a strong alkaline oxide that can significantly reduce the melting temperature of glazes. Na₂O forms a low eutectic system with SiO₂ and Al₂O₃. For example, the eutectic point of the Na₂O-Al₂O₃-SiO₂ system can be as low as about 1050°C (compared to the 1713°C melting point of pure SiO₂). Experimental data show that adding 20-30% sodium feldspar can reduce the firing temperature of the glaze by 100-200°C, thereby reducing energy consumption. For example, in a typical daily ceramic glaze formula, when sodium feldspar accounts for 25%, the firing temperature can be reduced from 1300°C to 1200°C.

Promote glass phase formation

Sodium Feldspar decomposes at high temperature, releasing Na⁺, Si⁴⁺ and Al³⁺, which react with other oxides in the glaze to form a uniform glass phase. SiO₂ (about 65-70%) provides the glass network skeleton, Al₂O₃ (about 18-20%) enhances the network stability, and Na₂O acts as a network modifier to reduce the viscosity of the glass. Studies have shown that the SiO₂/Al₂O₃ molar ratio of sodium feldspar (about 6:1) is close to the ideal glass formation range, which helps to form a smooth and transparent glaze.

Improve glaze fluidity

The strong fluxing effect of Na₂O makes the glaze have good fluidity during the firing process, which helps to fill the tiny defects on the surface of the body and form a smooth glaze layer. According to rheological tests, the viscosity of glazes containing 25% sodium feldspar at 1250°C is about 10³-10⁴ Pa·s, which is suitable for most ceramic firing processes. Appropriate fluidity can also reduce pinholes and bubbles in the glaze, and improve the density and gloss of the glaze.

Optimizing the thermal properties of glazes

The addition of sodium feldspar can adjust the thermal expansion coefficient of the glaze to match it with the body and avoid glaze cracking or peeling. The thermal expansion coefficient of sodium feldspar is about 5.5-6.5×10⁻⁶/°C, which is lower than that of potassium feldspar (about 7.0×10⁻⁶/°C) and is more suitable for high-temperature fired porcelain. Experimental data show that the thermal expansion coefficient of glazes containing 30% sodium feldspar can be controlled at 6.0-6.5×10⁻⁶/°C, which matches well with typical porcelain bodies (6.0-7.0×10⁻⁶/°C).

Reduce the impact of impurities

The low iron (Fe₂O₃<0.17%) and low titanium characteristics of high-quality sodium feldspar can reduce the pollution of glaze color, especially suitable for white or light-colored glazes. Compared with other fluxes (such as natural borax, which may introduce trace amounts of heavy metals), sodium feldspar has higher chemical purity and can ensure the optical properties of the glaze.

Conclusion

With its high Na₂O content and excellent chemical composition, sodium feldspar has become the core flux in ceramic glazes. It can reduce the firing temperature by 100-200°C, promote the formation of glass phase, optimize the glaze fluidity and thermal expansion coefficient, and ensure the purity of the glaze. With the growing demand for high-quality glazes in the ceramic industry, the research on the beneficiation technology and application of sodium feldspar will continue to deepen. If you want to know more, please contact Henan Ankai New Materials Co., Ltd.!