Purity as little as 0.04 Fe, 0.05 Al, 0.01 Ca, 0.3 C, 0.002 Halides, and 0.7 Oxygen. Particle sizes available down to 20 microns.
Parmascand, the manufacturer, has a booklet on processing of Si3N4 materials. It covers slip preparation, casing, pressing, tape casting, binders, and sintering.