PURPOSE: To discriminate easily fresh and old materials and to determine exactly the timing for replacing the old material by making the color of the fresh material to be installed different from the color of the mold material in the stage of applying additionally castable refractories to a blast furnace tapping spout lined with castable refractories.

CONSTITUTION: The lining 2 applied initially to a steel shell 1 in the rectangular sectional view of the blast furnace tapping spout in the longitudinal direction is worn by the local erosion which is called slag line 3 and the metal line 4 after use. The fresh material 2b is additionally applied on such old material 2a. The castable refractories contg. pure white sintered alumina as aggregate are used for, for example, the lining 2, i.e., the old material 2a and the castable refractories contg. the aggregate consisting partly or fully or brown electrofused alumina are used for the material 2b. The material 2a and the material 2b are then easily discriminatable and the sure sampling of the old material is possible. The easy determination of the timing for replacing the old material is thus made possible.