I’ve mentioned the Rodman technique for manufacture of heavy cannons on a number of occasions. That process, simplified to a thumbnail, involved casting the gun in a hollow mold then cooling the metal from the inside out using a flow of water through a pipe in what would become the bore. This allowed the interior metal to cool faster than the exterior. In the cooling process, the gun compressed itself around the bore, providing the highest strength at the very part of the gun most needed.
Before we get too far though, let’s set the record straight. Thomas J. Rodman did not invent hollow casting. In fact, many of the earliest cast cannon were hollow core. Not until the early 1700s did solid casting become the dominant means of production. By the American Revolution, gunmakers cast weapons as a complete block, then cut out the bore after cooling. But as gun calibers and powder charges grew larger, some began to ponder other ways to construct stronger guns.
In addition to testing different metal composition, construction techniques, and exterior forms (recall the experimental 6-pdrs), gunmakers revisited hollow casting in the late 1840s. In 1849, Fort Pitt Foundry received an order to produce two 8-inch Columbiads for tests – one cast solid and the other cast “… according to the plan invented by Lieutenant Rodman, on a hollow core, through which a stream of water passed while the metal was cooling.”
Civilian ordnance inspector (and later foundry owner), William Wade reported these two guns shared the same metal composition, furnace, and other production details – save of course the casting technique. The hollow Columbiad cooled for forty hours with water flowing through an insert. Then the gunmakers removed the insert and cooled for another twenty hours with water flowing through the empty, recently cooled, bore. All told the process used 6000 cubic feet of water.
Both guns went to a test range (presumably near Pittsburgh). The Columbiads first fired reduced charges for initial tests of both the guns and the support apparatus. Satisfied the arrangements were set, the crews then started firing full service charges – 10 pounds of powder, one sabot, and either a 63 ½ pound solid shot or a 49 pound shell. The guns fired alternatively through the proof firings. The ordnance inspectors made meticulous observations after each firing, particularly noting the level of vent erosion.
The solid cast Columbiad burst on the eighty-fifth shot (firing a shell). Firing the hollow core Columbiad continued until it burst on the 251st shot. Both guns split through the reinforce to the breech, but at different planes of fracture. Wade recorded the general shape of the breech fragment in his report:
Fort Pitt used the Model 1844 Pattern then in production, which featured base ring along with first and second reinforces. Wade reported that metal samples were removed from sections of these fragments for further testing. From those samples, observers concluded the hollow core Columbiad exhibited slightly higher density and tensile strength. However this slight increase in strength was, in the words of Wade, “… not sufficient to account satisfactorily for the strongly marked difference in the endurance of the two guns.”
Concluding his report, Wade submitted:
No other cause for this unequaled endurance can be perceived, but that of the different methods by which the castings were cooled. The precautions taken, to ensure an equality of the material composing the two guns, and to preserve an exact uniformity in their respective proofs, were such, that the different endurance cannot be ascribed to inequities in either of these respects. Neither of the guns, however, endured a sufficient number of fires to be satisfactory.
While promising, clearly the hollow-core, water-cooled process required more refinement. Wade recommended another round of experiments, this time with 10-inch Columbiads cast from higher quality metal. I’ll look at those tests next.
William Wade. “Report on the Manufacture and the Extreme Proof of two 8 inch Columbiads….” dated October 26, 1849. Reports of experiments on the strength and other properties of metals for Cannon. Philadelphia: Henry Carey Baird, 1856, Page 169.
Olmstead, Edwin, Wayne E. Stark, and Spencer C. Tucker. The Big Guns: Civil War Siege, Seacoast and Naval Cannon. Alexandria Bay, NY: Museum Restoration Service, 1997, Appendix C115, page 237.