Metallurgy of the U

This report describes the composition and some properties of the cast iron and wrought iron that form the dome of the U.S. Capitol. It is a compilation of two recent investigations, one by NIST and one by Lucius Pitkin, Inc. These investigations were implemented by the Office of the Architect of the Capitol to determine the dome’s condition and also to predict how the outer shell might respond to weld repair of cracks. However, the studies also provide an interesting glimpse into the technology that was in existence at the time of the dome’s construction, the mid-1800's, and document (perhaps for the first time) the microstructure and properties of materials that form the dome. Knowledge of the properties and other characteristics of the dome materials is fundamental to maintaining the dome and to predicting its future performance.