Cast Iron from the US Capitol Dome
Chris McCowan (NIST)

Microscopic cross-section
A light micrograph of a gray cast iron sample from the dome of the U.S. Capitol building, cast about 140 years ago. The sample is polished and etched in acid to show the complex structure of this common material. Gray iron is characterized by a microstructure containing large graphite flakes in a ferrous matrix. The graphite flakes shown here (dark gray color) are present in a pattern known as a “rosette”. The matrix is ferrite crystals that have more or less uniform coloring. The regions having a two-tone (brown and blue) color are another component referred to as pearlite, a fine layered structure of ferrite and iron-carbides. The white lacy phase is steatite, a low melting iron-phosphide. Contrast the structural simplicity of very-low-carbon steel.

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