Built in 1911 and enlarged in 1914, the assay office was 40 ft. by 60 ft. with a 25 ft. by 25 ft. furnace room on the uphill side. Located directly above the store, it housed a laboratory plus sampling, fluxing, and balance rooms. Four assay office workers tested an average of 88 samples of ore daily for both the commercial mines and independent prospectors. Ore testing from the four Treadwell mines before, during and after milling tracked the efficiency of the process and determined the value of the ore.