A core forming method is provided for forming a core. The core forming method includes forming one or more mold parts of water-soluble polymer (WSP), each mold part including a bond area and a part forming area, moistening the bond areas, bonding the mold parts together with the bond areas moistened and the part forming areas aligned to form a cavity shaped as a negative of the core, injecting a slurry, which is non-reactive with the WSP, into the cavity and removing the WSP with water once the slurry is cured.