PURPOSE: To improve quality at top of an ingot and to increase workability by forming double layers consisting of a formed body having at least expansive graphite but no metallic Al and the other formed body having metallic Al.

CONSTITUTION: The 1st layer 1 consists of the metallic Al, Al2O3, iron oxide, silica, binder, etc., as same as usual heat insulating board. The 2nd layer 2 consists of the expansive graphite and the other, Al2O3, SiO2, binder. The expansive graphite, which is obtd. by soaking nitric acid, phosphoric acid, sulfuric acid, hydrochloric acid, etc., into flake graphite and allowing to expand by about 150W200 times by heating. By using the board having such a constitution, as the 2nd layer insulates effectively the heat generated by the 1st layer, the quality at top of the ingot is improved.