How did Robert Coble improve alumina for use in sodium-vapour lamps in 1961?

In 1961, Robert Coble of the General Electric Company achieved a major breakthrough in materials science by demonstrating that alumina could be sintered to optical density and translucency.1

While alumina possesses the necessary refractory properties to contain the intense heat of a sodium-vapour lamp, it typically does not sinter to translucency because residual pores trapped within the grains scatter light. Coble discovered that by using magnesia (magnesium oxide) as a sintering aid, he could effectively slow the migration of grain boundaries during the sintering process. This allowed pores to remain on the grain boundaries, where they could be eliminated through grain boundary diffusion, resulting in extremely low porosity and a translucent material.2

This innovation enabled the creation of lamp envelopes capable of withstanding the 1,200 °C (2,200 °F) temperatures of a high-pressure sodium discharge while simultaneously transmitting the resulting light.1