1872 US Treasury Assay Office, Boise
Boise, Ada County
- Grants: 2019, 2024 – Repair and repaint the outbuilding’s doors, windows, and wood trim
Location: 210 Main Street, Boise, ID 83702
During the 1860s, Idaho was the 3rd largest producer of gold in the United States. The closest mint, however, was in San Francisco, so there was great demand for an assaying office nearer to the mines. Ground was broken in 1870 for this 46×48′ sandstone structure design by Treasury Department architect Alfred Mullet. The first floor and basement contained the assayer’s office, vaults, and safes along with assaying and melting rooms, furnaces and a lab; the upper level was entirely devoted to living quarters for the chief assayer. The U.S. Treasury and Assay Office opened in 1872 and served its intended function until the 1930s, its windows protecting the gold and valuable contents inside with heavy iron bars.
In the 1930s it was then handed over to the US Forest Service, where it was the headquarters of the Boise National Forest chapter. In the early 1970s, the Idaho State Historic Society took control of the building, and it is the current home of the State Historic Preservation Office.
The corresponding outbuilding, sometimes referenced as the “Carriage House” is the only additional structure on the site of the US Assay Office Building, one of Idaho’s four National Historic Landmarks. While the Assay Office itself was completed in 1871 to designs provided by the US Treasury Department, no architect or specific date of construction has been identified for this smaller structure. An 1890 lithograph depicting a bird’s eye view of Boise provides the first documentation of the building. Sanborn maps suggest that the original footprint of the building may have been expanded between 1903 and 1912. These early maps simply refer to the structure as a “shed.” No evidence has been found to confirm use as a carriage house. A portion of the structure closer to the Assay Office is built of local sandstone while the remainder is frame on a concrete foundation. The entire exterior is finished in a rough, unpainted stucco installed after 1932 and the building is protected by a wood shingled, hipped roof.
Although relatively small, this outbuilding is an integral part of the Assay Office site and has been used during the three eras of ownership – US Treasury Department (1869-1932), United States Forest Service (1932-1972), and Idaho State Historical Society (1972-present).
All told, the Assay Office and its accompanying ‘shed’ are one of the oldest monumental structures in the Northwest, the first major federal building in the Idaho Territory, and a prominent fixture downtown Boise to this day. The Assay Office is one of only three National Historic Landmark buildings in the State of Idaho.
