If your production is highly sensitive to kiln temperature variations — good tiles at 1195 degrees C but bloated or warped at 1200 degrees C and under-fired at 1190 degrees C — you have a narrow firing range problem.
Understanding This Defect
This is most commonly caused by using feldspar as the sole flux. Standard feldspar (whether K-feldspar or Na-feldspar) produces a narrower vitrification range than nepheline syenite because it relies on a single alkali type for fluxing. Nepheline syenite contains both Na2O AND K2O plus significant Al2O3, creating a broader glass formation range. The mixed-alkali effect widens the temperature window between initial vitrification and over-firing by 20-40 degrees C compared to single-feldspar formulations. This wider window means your kiln can tolerate the normal temperature variations across its width (typically 10-20 degrees C) without producing defects on either side.
Solution & Prevention
Switch to nepheline — use our conversion guide to calculate the right percentage — and you will immediately notice the reduction in temperature-related rejects..