Color micrograph of cast iron from the U.S. Capitol Dome, showing a blue background (a ferrite structure) with brown regions (pearlite structures) that have white lacy features in them (phosphorus rich regions). Scattered throughout are black rosette shaped structures that are made up of graphite.
This micrograph came out of a study I did in support of a restoration project for the dome. Cracks in the dome were in need of a repair procedure.
The dome at the United States Capitol was designed in the 1850s by Thomas U. Walter. It was the second largest cast iron dome in the world when it was built and is currently the world's largest cast iron dome. It replaced an earlier wooden dome that was no longer in scale with the expansions needed for the House and Senate wings. A cast iron dome was chosen because it could be cast with cutouts in areas where material was not required; it was fire resistant; it could be formed in complex shapes; and it could be erected with pieces of convenient sizes.
The dome itself was cast and installed y Janes, Fowler,
Kirtland and Company (Bronx, NY) for seven cents per pound. The project was awarded in 1860. Construction continued following the outbreak of the Civil War in 1861. Two thirds of the work was completed when the Secretary of War called a halt to the work in 1861, citing the government's need to finance the efforts of the War of the Rebellion rather than works of art. However, the Janes, Fowler, Kirtland and Company had 1.3 million pounds of iron on the site when the Secretary of War advised them not to expect payment. They decided to continue work. The authority for construction of the dome was later transferred to the Department of the Interior and work resumed in earnest (with funding). The installation of the dome was completed in 1865 at a reported loss of $125,000, mostly due to increases in cost associated with the one year delay ($60,000 of this loss was reimbursed by the government).
A historical photo of mine. An optical micrograph of cast iron from the U.S. Capitol Dome. The dome construction was started prior to the civil war. The effort to continue work during the war is a history worth reading. (see ALT text for a bit of the history)
#history
#microscopy
#cast_iron