The 20-inch Rodman was the largest muzzle loading cannon ever forged in the United States. The ability to forge cannon of that size required new technology that was developed by Thomas Jackson Rodman. This new casting process came about because of Rodman's interest in cannon advancement as acquired during his training at the U.S. Army's Military Academy at West Point New York, and a particular tragic event that occurred on the USS Princeton in 1844.
Second Lieutenant Thomas Jackson Rodman was assigned to the Army Ordnance Department at the Allegheny Arsenal in Pittsburgh, Pennsylvania after graduating seventh in his class from West Point in 1841. In 1844, the USS Princeton was one of the most advanced ships of her time - becoming the first screw steam warship fielded by the United States Navy. She was fast-equipped with a 14 foot, 6-blade screw powered by a coal burning boiler. She needed guns that projected power so the Princeton sailed for New York in January of 1844 to receive her two big cannon - named "Peacemaker" and "Oregon".
The Oregon was a 12-inch (300mm) smooth-bore, muzzle-loaded cannon made of wrought iron and could fire a 225-pound (100kg) shot some 5 miles (8 km) using a 50-pound charge of gun powder. Made in England by the Mersey Iron Works, the weapon was sold to the US Navy in 1841. A new built-up construction process was used, this accomplished by arranging iron hoops delivered hot from the forge, around the back of a casted cannon. The process was to allow higher amounts of gun powder to be used to prevent an explosion when firing. The second cannon - the Peacemaker - was made in the United States by Hogg and Delamater of New York and was also a 12-inch muzzle-loader able to match the shot and range of the English-made Oregon. The construction process was different, however, as the Peacemaker did not make use of additional bands for support. The breech end was increased in metal thickness during the casting process itself. The weight of the gun was greatly increased without reducing the flaws experienced in wrought iron cannon construction prior.
The USS Princeton arrived in Washington D. C. to board a distinguished group scheduled to observe the firing of the new heavy cannon. She sailed out to sea near the capital and the demonstration began with the first test cannon - the Peacemaker. The first shot proved successful. However, the second shot exploded the cannon, killing several nearby observers including Secretary of State Abel P. Upshur, Secretary of the Navy Thomas Gilmer and several other members of the distinguished party. The Navy investigated the incident and reviewed the manufacturing process, thereby coming to the conclusion that a serious flaw was created during such casting of the gun. This incident was the military talk of the day and Lieutenant Rodman, stationed at the Allegheny Arsenal, began to look at the metallurgy properties of gunmetal during the casting of cannon.
Rodman was an inherently talented inventor with the means to experiment at now personal cost. Being an Army officer assigned to an arsenal gave him a workshop and competent staff for trial-and-error testing of various theories and processes. Cannon casting was a long and difficult process and, perhaps to some, even an art form. Rodman concluded the progression of molding, heating, cooling and reheating created stress and strains on the metal thereby producing critical weak points that revealed themselves only later on. Even during the last stage the cannon was bored, this alone placed even more stress on the metal. Rodman agreed with the Navy's investigative findings that stress fractures are the cause of the Peacemaker's explosion.
The method Rodman invented was to reduce the temperature flux during the casting process by having water continuously flowing through a hollow casting tube. Now the metal cooled inside-out so each layer of metal could compress together without stress and create a gun able to be fired with less failure. Rodman reviewed the new process with the Army department but was it was initially rejected. However, he requested and was granted an application for a US patent to produce his new cannon at the Fort Pitt Foundry. By 1849 the cannon Rodman produced using his new process proved to be safer and could fire more consecutive shells without incident than any others built in the United States before.
Major Rodman was now the superintendent of Watertown Arsenal in Massachusetts and oversaw operations of the foundry at Fort Pitt, Pennsylvania. He knew the 20-inch gun would weigh over 100,000lbs and would prove heavier than the capacity of the armory. New machinery had to be designed and made to support the massive project including six furnaces. On February 11th, 1864, the 20-inch cannon was cast using a four-piece mold. The process made use of running water, this passing through the barrel. The manufacturing of the weapon took over a week to complete. At the conclusion of the process, the gun was inscribed "No. 1 116,497 lb / Fort Pitt Pa 1864". She was then assigned to Fort Hamilton, located in New York Harbor, where she still resides even today. A second Rodman gun was situated across the harbor in New Jersey at Fort Hancock. The third was sold to Peru. The carriage for the 20-inch Rodman itself weighed in at an impressive 36,000lbs.
Confederate arsenals completed local production of Rodman Gun copies during the American Civil War as well. However, their quality was not as good as those originating from Northern factories.