Titanium Dioxide (TiO2) is the primary white pigment used in paints due to its high refractive index (2.7). However, pigment cost fluctuations force paint chemists to optimize pigment spacing using mineral extenders.

The Calcination Effect Calcining hydrous kaolin at 1000°C changes its structure, removing chemically bound hydroxyl groups (dehydroxylation). This reaction creates a porous, amorphous structure with high brightness (93%+ ISO) and a refractive index of 1.62.

Optical Spacing Mechanism When calcined kaolin particles are dispersed alongside TiO2, they act as mechanical spacers. By keeping TiO2 particles separated, they prevent crowding (which reduces light scattering efficiency). The air voids within the calcined kaolin structure introduce extra light-scattering boundaries, maintaining paint opacity and scrub resistance while cutting chemical formulation costs.