United States Capitol

Constructed primarily of cast iron, the Dome was highly susceptible to corrosion from exposure to the elements. Water infiltration through pin holes in the Statue of Freedom and through cracks and open joints in the exterior shell led to leaks in the interstitial space between the inner and outer domes, promoting the rusting of ironwork and failure of protective coatings. Irreplaceable historic artwork in the Rotunda, including the Apotheosis of Washington and the Frieze of American History, were at risk of damage due to leaks. At the exterior, decorative elements had succumbed to such severe corrosion that many had fallen from the Dome to the roof of the Capitol building, posing a serious safety concern.

Several condition surveys were undertaken to assess the condition of the Dome and to document cracks and other deficiencies. With each passing year, studies found that items identified as in need of repair in earlier reports had further deteriorated, and the requisite repairs would continue to become more complex and costly if restoration were postponed. Some cracks were several feet long, in cast iron plates about three-eighths of an inch thick, a significant threat to the integrity of the structure. Using both innovative technology and historical tradecrafts, the project team repaired more than 1,300 cracks and deficiencies in the cast iron, repaired or recast intricate ornaments, gutters, and balustrades, removed hazardous materials, upgraded mechanical and electrical systems, and repainted the entire Dome, inside and out, according to a historical color scheme.

As a highly trafficked, active government building and visitor site, as well as home to invaluable art and statuary, the Dome required extensive measures to protect the public and the building interior during restoration, while maintaining full accessibility and operation throughout the project. As such, design of the scaffolding and the protective canopy in the Rotunda were a critical part of the project.

Cracked cast iron was repaired using a proprietary metal stitching process, by which a series of steel pins were installed by drilling and tapping to draw the sides of the crack together, resulting in a continuous row of interlocking pins that creates a strong, pressure-tight repair. Then, locks were driven in across the joint line of the pins, pulling the repair together. The stitch repairs are watertight, absorb compression stresses, and spread tensile strains away from the original failure point, for a long-lasting solution.

At the tholos level, just below the Statue of Freedom, the balustrade was in such poor condition as to merit complete removal to a foundry, where historical pieces that were beyond repair could be salvaged and recycled. Original elements were melted down, re-formed, and reintroduced into the balustrade. At the cupola, some windows dating from the original 1850s construction were too damaged to repair. The original slump pour glass was replicated using historically accurate methods, whereby 2,000-degree molten glass was poured between rollers and cut to shape.

Some of the worst damage to the Dome was at the internal gutter system, which was clogged with rust sloughed off from cast iron elements. Many new parts were forged in the foundry, where the entire gutter system was assembled and tested before being shipped back to the Dome, hoisted up through 21 levels of scaffolding, and reassembled in place.