Silicon Carbide 254 micron (60 grit)
Silicon Carbide is a hard, sharp, angular material primarily used for rough grinding. Under pressure, it fractures into smaller angular particles, and hence is an aggressive abrasive that removes a lot of glass quickly. Silicon carbide is a synthetic abrasive which approaches the diamond in hardness. It is made by mixing finely ground petroleum coke with pure glass sand. The mixture is loaded into a crucible and heated by electric arc to temperatures in excess of 2000°C. The intense heat of the arc causes the carbon in the coke and the silicon in the sand to fuse into silicon carbide. By varying the speed of the heating and cooling cycle the final crystal size is determined. A long, slow cooling cycle allows a few crystals to grow to a very large size while quick cooling forms numerous small crystals. The cooled mass is then broken up and rough graded by screening. The individual crystals are treated by impact, crushing or air blasting to remove brittle edges. Finally, it is graded by screening, water sedimentation, hydraulic flotation or air classification, with the latter used for the very fine grades. Because of its extreme hardness, silicon carbide is ideal for grinding Pyrex, which is harder than ordinary glass, or very deep curve generation used for refractor, Maksutov, or richest field objectives.