Origins of the Rodman – Early Experiments in the Rodman Method, Part 4

Previous posts in this set have covered experiments of the Rodman Method conducted in 1849, 1851, and 1856.  I’ll turn next to the next set of experiments conducted in 1857 and 1858.  I know the tally of these experiments is somewhat mundane, but I’ve not seen these discussed at length outside Rodman’s official reports.  Most accounts point to decade long experiments, leaving the details below the surface.  In addition, my thesis looking through all this is the hollow-core, water-cooled method was only a part of the solution.  Further there is an “administrative” story ranging from Rodman’s patent award, royalty payments, Ordnance Department policies, and congressional inquiries that runs parallel to that of the gunmaking evolutions.

In 1857, Captain Thomas J. Rodman received permission to continue tests.  For this round of tests Rodman cast three 10-inch Columbiads.  As with earlier tests, Fort Pitt Foundry produced castings, one solid and one hollow.  Added to the sample set was a solid cast gun from West Point.  Working with Lieutenant F. I. Shunk, Rodman graded and selected iron for these castings from stocks at West Point.  Each casting consumed equal portions of the various gradings, to ensure near identical mix.

Rodman intended to cast all three guns on the same day, but “owing to mismanagement in the Post Office Department” was unable to coordinate operations.  Fort Pitt poured metal for its castings on August 13, 1857.  West Point cast its gun on the following day.  The West Point gun cooled in an enclosed pit, packed with “green” or moist sand.  The Fort Pitt guns cooled in open pits as with the previous experiment.   Of note, Rodman reported the Fort Pitt castings filled in eight (hollow) and nine and a half minutes (solid) of pouring molten metal.

The West Point gun, cast solid, received foundry number 983 and cooled within eight days. Likewise Fort Pitt gun removed its solid cast number 335 after eight days of cooling in the pit.  According to Rodman’s observations, the solid cast guns had a cloudy and somewhat dappled appearance.

Fort Pitt foundry number 334, cast hollow, cooled in the same process as earlier hollow core guns.  For the first sixteen hours of cooling, water circulated through the core insert.  Water entered at 75° and exited at 95°.  After removing the core, water circulated directly through the bore, entering at 75° and exited at 136°.  After 19 ½ hours, the crew increased water flow and exit temperatures gradually decreased.  After five days of water cooling, number 334 came out of the pit and the casting flask on August 21.  Rodman noted the hollow core gun’s metal had a “much finer, and more uniformly mottled appearance.”

In terms of pattern, fracture diagrams indicate both foundries used the Pattern of 1844, but with some minor changes.  The form omitted the base ring for instance.  So these are “transitional” guns to the “new Columbiad” pattern.

Fracture Diagram of Fort Pitt 335

These columbiads retained the sub-caliber chamber of the older Model 1844 pattern – a twelve inch deep, eight inch diameter chamber.

After the West Point gun arrived in Pittsburgh, all guns went to the proofing range on October 22.  The first fire of all guns was with a solid shot and 20 pounds of powder.  The second fire consisted of a shell propelled by 24 pounds of powder.  At that point, shots used the standard 14 pound service charge, but alternating shot and shell.  Test firings used fresh powder from 1857 batches directly from DuPont, described as uniform in grain and free of dust.

After 88 fires, both solid cast guns exhibited cracks in the second reinforces.  The West Point gun completely failed on the 169th fire.  The Fort Pitt solid gun survived until the 399th fire.  Upon examination of the fragments, the Fort Pitt gun exhibited some small cavities left from the casting, which likely aided the failure.  Although not referenced in the report from 1857, Rodman later noted the West Point gun was heavier, denser, and in general stronger in almost every possible measure.  Yet it failed first.

Fracture Diagram of West Point 983

But the hollow core gun continued to fire through tests unfazed until the 600th shot.  Firing continued as crews drilled a second then a third new vent.  Cracks continued to develop through 1000 and even 1600 fires.  But Rodman described these cracks as different, “not the tortuous appearance of those in the other guns, but had more the appearance of having been cut and burned out by the gas.”

In his report to the Ordnance Department, Rodman provided the usual detailed measurements of bore, chamber, and vent enlargements taken after each firing.  He also provided meteorological data for each day of firing.   Tests of the metal showed the hollow core number 334 between the two solid cast guns in terms of density and tenacity.  The West Point gun actually used more dense and stronger metal.

Of the 1857 tests, Rodman offered very few conclusions.  The pattern of fracture and cracks did provide support for Rodman’s theories about gas expansion within the gun’s chambers. Concurrent with the big gun test of 1856 and 1857, Rodman also conducted experiments on a smaller scale.  Some of these focused on the behavior of iron cylinders containing the force of powder ignition.

One of Many Diagrams from Rodman's Cylinder Tests

Other tests assessed the metal’s resistance to force, or what Rodman called “potential for work.”  As Rodman noted, the performance of the gun had much to do with the elasticity and compressibility of the metal, perhaps more than with its strength.  More importantly, Rodman felt there was predictability to the performance of the metal.  Much of the data gathered from the 1857 tests provided support for a more substantial report the following year, which I’ll look at next.

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The 1857 report of Thomas J. Rodman appears in Reports of Experiments on the Properties of Metals for Cannon, and the Qualities of Cannon Powder; with an Account of the Fabrication and Trial of a 15-inch Gun (Boston: Charles H. Crosby, 1861), pages 59-89.

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

2 thoughts on “Origins of the Rodman – Early Experiments in the Rodman Method, Part 4

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