Excessive fired shrinkage causes dimensional accuracy problems — tiles come out smaller than the die size, making them incompatible with specification tolerances or mix-and-match requirements.
Understanding This Defect
The primary flux-related cause is over-fluxing: too much nepheline or feldspar creates excessive glass phase that pulls the body together more than intended. If you recently increased nepheline content or switched from feldspar to nepheline without reducing the percentage, the additional flux is causing extra vitrification and therefore extra shrinkage. Fix: reduce nepheline by 2-4% and compensate with quartz (which does not contribute to vitrification and has low shrinkage). Or reduce firing temperature by 10-20 degrees C if your water absorption still meets specification. Note that nepheline requires approximately 15-20% less weight than feldspar for the same flux effect — overshooting the conversion ratio causes the excess shrinkage..
Solution & Prevention
