For transparent iron oxide, the key is not so much grinding the primary particles, but fully deagglomerating and dispersing them, since these pigments are usually nano-sized but strongly agglomerated. The most effective technology is a bead (stirred media) mill in wet mode – on lab scale a basket mill or small horizontal bead mill, and on production scale a horizontal bead mill with recirculation and cooling. This allows you to achieve very fine dispersion, typically in the sub-micron range (<1 µm), which is essential for transparency. If you need a dry powder instead of a dispersion, a jet mill is suitable, capable of producing particles in the few-micron range without thermal damage. Key things to focus on: high energy density, fine grinding media (e.g. zirconia beads), good temperature control, and correct dispersants - these often matter more than the mill itself for achieving true transparency.