548 METALLURGY OF CAST IRON. FIG. 135. The core mixture mentioned possesses very little body to stand up in a green state; so little that, in making the larger cores, rodding was very necessary, in order to hold, the cores together. When this mixture is dry the cores are exceptionally strong to handle, but crush very easily when the castings commence to contract. To form the small neck in the green sand tensile test bars as at D, Fig. 133, cores made of the above mixture were used as at F above, Fig. 136. This division in the tensile test bars was made for the purpose of giving a long and very short test specimen. To obtain the contraction, a device, Fig. 144, page 557, was arranged so as to punch 3/£-inch holes in the cores and green sand molds. These formed pins in the mould that were exactly 12 inches apart, so that when the castings were cold the contraction could be accurately r ~ ° : FIG. 136.