98 METALLURGY OF CAST IRON. especially those using fine grades of ores, as any stoppage of blast is apt to cause a temporary chill and to retard good working of the furnace. Not all grades or kinds of clay are suitable for stopping notches. It must be of a quality to withstand fire to the best possible degree. Some use a good grade of fire clay and others grind up old crucibles to mix with the fire clay in an effort to improve its heat-resisting qualities. The clay is mixed to a consistency about like that found good for cupola stopping clay, and in some places is prepared in pans crushed by heavy rollers. The success of stopping a notch by hand being due to the fact of having the metal lower than the level of the notch, affords the furnace an advantage not permitted to the cupola. Conditions in the latter calling for a ' ' bottom drop, ' ' every heat makes it most desirable that no metal should remain in the bottom of a cupola when a FIG. 23. heat is £^5]^. For this reason the bed of a cupola as seen at Y, Fig. 23, is generally made on a slant, and the tap-hole placed at its lowest level, as seen at R. With such an arrangement, when difficulty in tapping and stopping once commences, it often causes the cupola tender much harassing labor, and the founder loss in casting. Any one desiring further information on tapping out and stopping up cupolas is referred to " American Foundry Practice, " page 331.