Complex sentence: The centre spot and the rim of a chau gong are left coated on both sides with the black copper oxide that forms during the manufacture of the gong, the rest of the gong is polished to remove this coating.
Split into simpler sentences: The centre spot and the rim of a chau gong are left coated on both sides with the black copper oxide that forms during the manufacture of the gong. The rest of the gong is polished to remove this coating.