Warping occurs when different parts of a tile vitrify at different rates — one surface densifies and shrinks before the other, creating curvature.
Understanding This Defect
For single-fired tiles, the most common cause is differential temperature between the top surface (exposed to radiant heat) and the bottom surface (resting on the roller). The top vitrifies first, shrinks, and pulls the tile into a concave shape. Nepheline syenite helps prevent warping through its wide firing range: the broad temperature window between initial vitrification and full densification reduces the sensitivity to top-bottom temperature differences. When both surfaces are within the vitrification range simultaneously (even if at slightly different temperatures), they shrink at similar rates and the tile stays flat. Feldspar has a narrower firing range, making tiles more susceptible to warping from kiln temperature gradients.
Solution & Prevention
For large-format tiles and slabs where flatness is critical, K200 at 25-30% in the body provides the widest firing range. For standard tiles, K100 at 18-25% delivers adequate flatness control at lower cost..
