Gresto abrasive block
Gresto is a rectangular abrasive block (8 x 5 x 2cm) that cleans, polishes, smooths, or causes copper and zinc plates to become matte, but it can also be used on such materials as wood, ceramic, and stone. Adding water, soap, or a turpentine-based product strengthens its effects.
3 grit sizes are available:
Coarse - 70 / Medium - 140 / Fine - 240