Basics of Mirror Grinding

Glass mirrors are typically ground from a blank, or cylindrical piece of glass, using the abrasion created by moving the mirror across a second glass blank (the tool) which is covered with silicon carbide or aluminum oxide grits. The initial rough grinding process uses grit sizes of 60, 120, and 180 to carve out the basic spherical cavity. The mirror is then fine ground with grits of sizes ranging from 220-900. In both grinding processes, the grit is harder than the mirror blank and tool, resulting in a concave mirror and complementary convex tool. The primary action of the grit is to fracture the glass. Once the mirror is fine ground, it is then polished (or figured) into a parabaloid using rouge or cerium oxide and a pitch lap, which is typically made by adding a layer of pitch carved with channels to the surface of the tool. The pitch is a semi-hard but smooth surface. The primary action of the pitch lap and cerium oxide is to create enough friction that the glass surface of the mirror melts and undergoes liquid flow, afterwards re-solidifying in the shape of the pitch lap (or the shape created by various different strokes with the pitch lap).¹

The goal for my composite mirror is that it have a surface that can be fine ground and polished using the same methods as those for glass. In order for this to happen, the material at the surface must have a hardness similar to glass - if the hardness is greater, the grit won't be able to abrade the surface, and if less, the surface will be to easily abraded by the grit or otherwise easily damaged.

Footnotes

... lap).¹

Texereau, How to Make A Telescope