How does sintering alumina to low porosity make it translucent?

In its natural state, alumina is opaque because residual pores within the material act to scatter light. To achieve translucency, the material must be sintered to an extremely low porosity, as these internal pores are the primary cause of light scattering.12

While solid-state sintering can densify ceramics, it is often difficult to eliminate pores that become trapped inside growing grains. To overcome this, magnesia is added as a sintering aid. The magnesia functions by slowing the migration of grain boundaries during the sintering process. This allows the pores to remain on the boundaries rather than becoming trapped within the grains, where they would otherwise be nearly impossible to remove. By keeping the pores on the grain boundaries, they can be effectively eliminated through grain boundary diffusion, resulting in a dense, translucent ceramic suitable for applications such as high-pressure sodium-vapour lamps.12