Origins of the Rodman – Early Experiments in the Rodman Method, Part 2

In part one, I discussed two 8-inch Columbiads cast in 1849 and used to test the Rodman method of casting large guns.  The conclusion of that experiment indicated a slight improvement to the gun, but not great enough to prove the hollow-core, water-cooled method was greatly superior.  Gunfounder William Wade suggested more tests with better quality metal.

Lieutenant Thomas J. Rodman, working closely with Wade, waited until 1851 for the next full-scale tests.  Fort Pitt Foundry cast another pair of 8-inch Columbiads – one solid and one hollow-core – on July 30, 1851.   Then on August 21 of that year, Fort Pitt cast a similar pair of 10-inch Columbiads.  Wade selected what was considered high quality iron from Greenwood Furnace of New York.

Rodman revised the cooling technique after the first experiment.  For this 8-inch hollow-core columbiad, the gunmakers left the insert in the gun bore for the first twenty-five hours of cooling.  The insert (just as with the earlier experiment) consisted of a tube, closed at the bottom.  Inside that tube, a smaller diameter tube, open at the bottom, allowed water to flow down into the bore.  The water then ran off from the top of the casting mold into a trough.  All the while, a fire burned under the casting mold in the open pit to slow the overall cooling of the gun.

Diagram of Rodman Casting Arrangements

After that initial cooling phase, the foundrymen removed the core and let water flow directly into the now hardened gun bore.  Excess water continued to run out from the top of the casting mold into the trough.  This continued for forty hours.   About halfway through that period, the gunmakers extinguished the fire in the pit, allowing the gun to cool completely in the last twenty hours.  Wade indicated that for the 8-inch Columbiad 10,000 cubic feet (300 tons) of water circulated through the gun during the entire sixty-five hours of cooling.

As seen on the fracture diagrams supplied by Wade, the guns conformed to the Model 1844 pattern.

Fracture diagram of 8-inch solid cast Columbiad No. 3

Casting of the 10-inch hollow core Columbiad differed in both measures and technique.  Foundrymen left the core insert in the bore of the 10-inch Columbiad for ninety-four hours.  At that time extraction failed, as the bore had contracted around the core.  With the interior tube removed, water continued to flow at a reduced rate into the 10-inch Columbiad for an additional forty-eight hours.  Although Wade does not state such, presumably the core insert was drilled or wedged out after the final cooling cycle.

There was one other critical difference in the casting of the 10-inch hollow-core Columbiad.  Because the casting mold (both solid and hollow castings) for the 10-inch Columbiad were not large enough to allow what Wade considered sufficient clay, the foundrymen placed both 10-inch Columbiads in a pit filled with “green sand” to ensure the hot metal did not melt the casket of the casting mold.  This, as we shall see, changed the cooling rates and performance of the gun.

Fracture diagram of 10-inch solid cast Columbiad No. 5

With the guns cooled, all four went to the range.  The 8-inch Columbiads went through test firings between August 28 and October 2 of that year.  The 10-inch Columbiads followed in October 7 to October 18.  Just as with the 1849 tests, all four guns fired from a test mounting.  Gunners supplied proofing charges for the first two fires.  The 8-inchers fired a 12 pound charge for the first shot then a 15 pound charge for the second, after which the guns fired a 10 pound standard service charge.  Likewise the 10-inch Columbiads first fired a 20 pound charge, followed by a 24 pound, then settling on the 18 pound service charge.  The 8-inch Columbiads shots alternated between a 63 ½ pound solid shot or a 49 pound shell.  The 10-inch Columbiads fired 124 pound solid shot and 91 pound shells.

Wade reported a mixed bag of results:

  • 8-inch Columbiad, No. 3, cast solid – burst after 73 fires
  • 8-inch Columbiad, No. 4, cast hollow – survived 1500 fires
  • 10-inch Columbiad, No. 5, cast solid – burst after 20 fires
  • 10-inch Columbiad, No. 6, cast hollow – burst after 249 fires

The description of the bursting of 10-inch No.5 speaks to the power of black powder.  Wade wrote, “…the upper part, weighing 4400 pounds, was thrown upward, and fell in the rear, about 80 feet distant.  In its flight, it broke a limb of a tree, on a hill side, sixty feet above the level of the gun when fired.”

While the 8-inch No. 4 proved an excellent argument in favor of Rodman’s method, the 10-inch No. 6 dampened that success.  Upon examination of the fragments, Wade noted several cavities and fissures in the metal even describing some sections as “sponge-like”.  Wade attributed these flaws to the use of sand for exterior cooling, estimating that seven-tenths of the heat dissipated through the exterior as opposed to the water in the interior.  Most of these fissures appeared on the front part of the gun.  One large fissure was noted on the fracture diagram, along the chase of the Columbiad.

Fracture diagram of 10-inch hollow core Columbiad No.6

However, Wade insisted the fissures did not cause the gun’s failure.  As he noted, the crack in the gun started over the chamber and proceeded forward along the barrel, crossing over the fissure.  Instead, Wade attributed the failure of the gun to the brittle nature of the metal overall, a property caused by the rate of cooling of the gun.

Examination of the gun metal at the conclusion of these tests indicated, just as with the 1849 tests, no significant difference in the tensile strength of the metal of the different guns.  After assuring the paired castings took place under identical circumstances with the same batch of raw metal, Wade proclaimed, “The great difference of endurance must therefore be ascribed, to the different methods by which the castings were cooled; and to them alone.” The method, as Wade and Rodman would explain in detail, allowed the gun to contract upon itself while cooling, thus changing the way the metal reacted to the stress of firing.

In his report Wade made the conclusion that casting large caliber guns required even more fastidious selection of metal for gunmaking, calling for softer and weaker iron that underwent a managed cooling process.  He further added,

The method devised by Lieutenant Rodman, for accelerating the cooling of the interior of the guns, by passing a stream of water through them, and for retarding, at the same time, the cooling of the exterior, by surrounding it with heated air, appears to lead in the right direction, even if it does not fully accomplish the purpose designed.

Wade recommended further tests, with 32-pdr caliber guns in addition to the large caliber columbiads, with the aim to refine the process further.  (To my knowledge, the 32-pdr experiments were never conducted.)  CORRECTION: I overlooked the mention of tests in 1852.  Details in the next post in this series.

Rodman’s technique required even more refinement and some concurrent experiments with metal composition before it would bear fruit at the end of the 1850s.  As with many successful weapon systems throughout military history, the Rodman gun was not so much “revolutionary” but rather “evolutionary.”

————————————————————

References:

William Wade.  “Report on the Manufacture and the Extreme Proof of two 8 inch Columbiads and of two 10 inch Columbiads….” dated January 24, 1852.  Reports of experiments on the strength and other properties of metals for Cannon. Philadelphia: Henry Carey Baird, 1856,  Pages 181-204.

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, Appendicies C115 and C142, pages 237 and 253.

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

3 thoughts on “Origins of the Rodman – Early Experiments in the Rodman Method, Part 2

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