Why is magnesia added when sintering alumina lamp envelopes?

In the production of alumina lamp envelopes, magnesia is added as a sintering aid to achieve the necessary translucency for sodium-vapour street lights.12

Without this additive, alumina does not sinter to a translucent state because residual pores become trapped within the growing grains, where they scatter light and prevent transparency. Magnesia functions by slowing the migration of grain boundaries during the sintering process. By keeping the pores located on these grain boundaries, the material allows them to be effectively eliminated through grain boundary diffusion. This process enables the ceramic to reach extremely low porosities, resulting in the high-quality, translucent envelopes required for high-pressure lighting applications.12