Well developed mullite whiskers were obtained by heating compacts of kaolin and 2 wt.% sodium pyrophosphate powder at 1350°C for 10 h, with or without a small addition of aluminium sulphate. Without aluminium sulphate, mullite whiskers were obtained with an aspect ratio of >8
and reasonably uniform diameter (∼0.05–0.3 μm). When aluminium sulphate additions were employed a mass of relatively well developed and interlocked whisker-shaped crystals were produced, with an aspect ratio of >10 (∼0.05–0.3 μm in diameter). Larger whiskers were
also observed, with aspect ratio of >25 (∼0.2–0.5 μm in diameter).