Nassawango Iron Furnace was structurally typical of its period while distinctive in several ways: it smelted bog ore, it is principally of brick rather than stone, and, of greatest significance, it probably is the earliest surviving American furnace that employed the “hot blast.” By this means, now universally used, the production of pig iron was greatly increased. The cast-iron blast-air “stove” at the top of the furnace stack was installed here circa 1835, less than a decade after introduction of the process in England.