In the extensions, it was used for roof trusses, plumbing and gas lines, and decorative window and door trim. The Capitol dome is made of 8,909,200 pounds of cast-iron girders, plates, columns and ornaments. A popular construction material at the time, it was used for bridges, pipes, machinery and many other objects until it was widely supplanted by steel.
Cast iron is an alloy of the elements iron, carbon and silicon that is formed into shapes by pouring the molten metal into a mold. Invented by the Chinese more than 2,000 years ago, it was for centuries so expensive to produce that it was used almost exclusively for small objects or for tools that had to be extremely durable (for example, cooking pots and plowshares).
In the mid-18th century, English improvements in furnace technology allowed prices to drop dramatically, and cast iron found wide use in manufacturing and construction. The casting process allowed mass production of identical parts that could be assembled quickly in a factory or at a construction site, and cast iron was soon used in railway bridges, boilers, lamp posts, fences, pipes and machine parts. Its ability to take finely molded shapes made it suitable for sculpture and ornamentation as well. Its great strength allowed load-bearing forms to be thinner and lighter than in stone structures, so buildings could be taller and windows could be larger than was previously possible. Its nonflammable nature made it especially desirable in fire-prone buildings, such as textile mills. It was widely used for buildings in the 19th century until steel, an even stronger ferrous material, became more widely produced.
During construction of the U.S. Capitol's House and Senate extensions and new Capitol Dome in the mid-19th century, architect Thomas Ustick Walter and engineer Montgomery C. Meigs used cast iron extensively for both decorative and structural elements. In the extensions, it was used for roof trusses, plumbing and gas lines, and decorative window and door trim. The dome contains 8,909,200 pounds of cast iron girders, plates, columns and ornaments — even the pedestal on which the bronze Statue of Freedom stands. (The Capitol’s cast-iron dome is among the oldest such structures in the world. Only a few domes, such as those on St. Isaac’s Cathedral in St. Petersburg, Russia, and the Old St. Louis County courthouse in Missouri, predate it.) Fireproof cast iron was also used for the no-longer-extant Library of Congress that Walter built in the west front; some lampposts and railings on the grounds are made of cast iron, as is the Bartholdi Fountain in Bartholdi Park.