The water continued flowing as the metal cooled. The water rose through the space between the two tubes and flowed out at the top. As the molten iron was poured into the mold, water was pumped through the smaller tube to the bottom of the larger tube. A second, smaller tube, open at the bottom was inserted into the first. This core consisted of a watertight cast-iron tube, closed at the lower end. With Rodman's method of casting, a cooling core was placed in the mold before casting. Rodman devised a method of hollow casting where the gun cooled from the inside out, so that as cooling occurred, it created compression rather than tension. Drilling out the bore removed the voids, but the tensions in the metal were still toward the outside. As each succeeding layer cooled it contracted, pulling away from the still molten metal in the center, creating voids and tension cracks. With this traditional method, the gun cooled from the outside inward. Guns had been traditionally cast solid and the bore was bored out of the solid metal. The outer iron flask, the fire built outside the flask, and the cooling core are also shown. The engraving shows the gun mold in the casting pit. Engraving showing a gun being cast using Rodman's hollow-casting technique.
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