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U.S. Assay Building

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1875–1876, Alfred B. Mullett. 206 E. Broadway St.
  • (Photograph by Chere Jiusto)
  • (HABS)

Montana Territory was founded in 1862, after gold strikes in the creek valleys of this region proved rich and lasting. The gold rush camps of Bannack and Virginia City each served as territorial capitals, as placer mining drew thousands to Montana in 1862 and 1863, respectively. In 1864, gold was struck in Last Chance Gulch and the town of Helena was born. Ten years later, in May 1874, with more than 50 stamp mills and mining concentrators and over 1,000 placer mining districts in the Territory, Congress appropriated the funds to build a federal assay office in Helena’s Last Chance Gulch, the core of the townsite’s budding commercial district. Helena became the mining and financial center of the Territory, and in 1875, the city’s prominence was reinforced when the territorial capital was relocated to Helena, and that fall, the cornerstone was laid for a two-and-a-half-story brick building that exuded the gravitas of the U.S. Treasury.

Designed by Alfred B. Mullett, Supervising Architect for the Treasury Department, Montana’s U.S. Assay Office stands along the east side of the gulch on Broadway Street, a major thoroughfare in early Helena. The Assay Office is a Second Empire style building with a symmetrical square plan and a small, single-story addition to the rear. The building features red, brick-bearing walls twenty-four inches thick on the ground and first floors, and sixteen inches thick above. Brickwork on the exterior is trimmed with belt courses and pointed arches of light gray granite above window and door openings, as well as corbelled brick dentils at the top of the building that accentuate the flared eaves of the roof. Granite trim also capped a series of gable dormers projecting from the pyramidal mansard roof. Windows throughout are double-hung sash units, and were originally shaded with striped fabric awnings.

According to a 1909 description of the building, the primary entrance led into a hallway on the first floor, where gold was weighed on a large set of bullion scales through an iron-barred window. The melting room where the gold was then assayed was also on the first floor, and had several melting kilns of varying sizes, ranging in capacity from a few ounces to 1,000 ounces. The floor of the ten-by-twelve-foot melting room was “covered with sheet iron to prevent the loss of gold by accident.” An inch-and-a-half layer of shale between the layers of the original wood floor provided an additional level of fire resistance to the building. Originally, there were two walk-in vaults on the first floor (later removed), and one on the ground level.

The Montana Assay Office opened in January 1877, housing assaying operations on the first floor, and offices and living quarters for the Superintendent of the Assay Office on the second. In 1889, “the largest gold bar in the world,” then valued at $100,000, was cast at the U. S. Assay Office for the Montana National Bank in the shape of a pyramid. Weighing 6,945 ounces (434 lbs.), it was displayed first in Helena, then at the Minneapolis Exposition, and finally at the Chase National Bank in New York City.

One of five U.S. assay offices in the country in the early twentieth century, Helena’s output was exceeded in volume only by the Seattle office. Gold and silver mining declined in Montana, however, and in 1934, the Helena Assay Office closed. The government sold the building to private owners, and it subsequently changed hands several times. Assaying equipment was removed from the interior in the mid-twentieth century and the building was later converted into five apartments. On the exterior the Assay Building retains much of its former stately character, reflecting its essential role in the early history of the Montana Territory.


DeHaas, John, “U.S. Assay Office,” Lewis and Clark County, Montana. HABS No. MT-26, Historic American Building Survey, 1967. Prints and Photographs Division, Library of Congress, Washington, D.C.

Writing Credits

Chere Jiusto
Chere Jiusto



  • 1875


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Chere Jiusto, Chere Jiusto, "U.S. Assay Building", [, Montana], SAH Archipedia, eds. Gabrielle Esperdy and Karen Kingsley, Charlottesville: UVaP, 2012—,

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