What material was used for the new dome built in 1855?

The new dome built in 1855 for the Capitol was primarily made of cast-iron. This material was chosen due to its strength and durability, which were essential for supporting the dome's large structure. Cast-iron allowed architects and builders to create the intricate details and reliefs that adorn the dome, enhancing its aesthetic appeal.

Additionally, the properties of cast-iron made it a revolutionary material during the 19th century, as it had the ability to withstand significant weight while also being relatively lightweight compared to solid stone. The success of the dome, which still stands today, is a testament to both the material used and the engineering advances of that period. Other materials like steel and concrete were either not as widely used for such large structures at the time or didn't possess the same decorative qualities that cast-iron offered. Brick, while strong, does not have the same architectural integrity required for dome construction as cast-iron.