Why do ceramic powders often look white and opaque even when the same material can be transparent as a crystal?

Ceramic powders and porous compacts appear white and opaque because they scatter light, even if the underlying material is inherently transparent as a single crystal.1

In their pure state, many ceramics are wide-band-gap insulators that allow light to pass through them. However, when these materials are in powder form or contain residual porosity, the numerous interfaces between the particles or pores and the surrounding material cause light to scatter in many directions. This scattering prevents the transmission of light, resulting in an opaque, white appearance.12

To achieve transparency in polycrystalline ceramics, it is necessary to eliminate this porosity. This is typically accomplished through sintering, a process where particles are bonded and densified under heat. For example, while pure alumina does not naturally sinter to translucency due to light-scattering pores, the addition of magnesia as a sintering aid allows the material to reach the extremely low porosities required for optical transparency.23