Transparent polycrystalline ceramics present significant economical and functional advantages over single crystal materials for optical, communication, and laser technologies. To date, transparency in these ceramics is ensured either by an optical isotropy (i.e. cubic symmetry) or a nanometric crystallite size, and the main challenge remains to eliminate porosity through complex high pressure-high temperature synthesis. Here we introduce a new concept to achieve ultimate transparency reaching the theoretical limit. We use a controlled degree of chemical disorder in the structure to obtain optical isotropy at the micrometer length scale. This approach can be applied in the case of aniso-tropic structures and micrometer scale crystal size ceramics. We thus report Sr1+x/2Al2+xSi2-xO8 (0<x≤0.4) readily scalable polycrystalline ceramics elaborated by full and congruent crystallization from glass. These materials reach 90% transmittance. This innovative method should drive the development of new highly tra