Capitol Hill Restoration Society

Ironwork on the Capitol Dome – Preservation Cafe

Posted on May 14th, 2017

Seth Baum with Historical Arts and Casting, Inc. was the May 2017 Preservation Café speaker.  The presentation focused on the firm’s work in the recent restoration/reconstruction of the Capitol Dome.

Cast iron first appeared in a Shropshire, England bridge built in 1770, eventually being used as the core building structure of the Ditherington Flax Mill for the first time in 1797. By 1850, it became a more commonly used construction material, as seen in Joseph Paxton’s Crystal Palace and in the first US Patent by James Bogardus of Cast Iron Facades in the US.

The US Capitol dome we see today was designed by Thomas U. Walter and constructed by Montgomery Meigs between 1855-9.  Set on the existing Dr. William Thornton-designed Capitol structure, the new 287 foot tall dome needed to be lightweight and strong enough to support both the interior and exterior domes.  Building it out of heavy stone to match the existing was not practical, so the whole of the new dome structure (structural ribs, exterior columns and cladding, interior coffers) was constructed with cast iron.  Almost 9,000,000 lbs. of cast iron, produced in Bronx, NY was shipped to DC and assembled with a crane located at the center of the dome, without the availability of electricity. Built during the Civil War, it was considered important to not delay construction of the dome since it was considered a national symbol and since most of the iron already delivered would have rusted if left on site.

The dome was in major need of restoration after years of deferred maintenance and rust issues created from previous non-successful renovation projects.  Historical Arts and Castings, Inc. was a sub-contractor on the large project, tasked specifically with restoration of cast iron decorative elements.  Typical work was reconstruction of corroded/broken castings and fastners and removal of fractures and rust jacking in salvaged materials.

Original material was salvaged and repaired as much as possible before new parts were inserted.  The original material was sandblasted to remove existing paints and rust, then restored either via Dutchman replacements, lock and stitch, brazed and mechanical repairs and filling and patching.  The cast iron was then finished with a Tunemic primer to protect the material.  When replicating missing or broken parts, the originals were documented via sketches, images and digital scans or with plaster molds.  The information was drafted as a shop drawing, then fabricated in the Salt Lake City shop or at the on-site work-shop where an oven was available for braising-type repairs.

One example of the restoration was presented with the Boiler Plate Balustrades (near the top of the

dome).  Existing materials were sand-blasted and documented in place, then disassembled and shipped to Salt Lake City for reconstruction of the system in the shop.  Once all existing and new parts were confirmed to fit correctly, the assembly was shipped back to DC and re-assembled on site.

Historical Arts and Castings, Inc. is a designer, fabricator, and installer of custom, architectural, ornamental-metal products based in Salt Lake Cit andis made up of artisans who restore life to the classical and bring new meaning to America’s architectural heritage. Seth is available to answer questions about the renovation at

The Preservation Café series is a free forum with topics of interest to the greater Capitol Hill community, usually held at the Northeast Branch Library, 330 7th Street, NE (downstairs meeting room).