The FurnaceInformation from The American Society of Mechanical Engineers 1991 publication listing it as A National Historic Mechanical Engineering Landmark.  See also ASME History - Biographies, Landmarks, Patents on line.  See also Bowie, John R. And Dianne M. Pohlsander, “Nassawango Iron Furnace (1830) Recording Project”, Historic American Engineering Record MD-76, 1989.

The Nassawango Iron Furnace is notable for being the earliest surviving hot-blast furnace in the United States, and was recognized October 19, 1991 by The American Society of Mechanical Engineers as A NATIONAL HISTORIC MECHANICAL ENGINEERING LANDMARK.


Bog iron was first discovered in the swamps along Nassawango Creek in the 1780's and in 1828 the Maryland Iron Company was incorporated to extract and process it.  In 1830, the Company constructed a furnace along the creek at a point roughly four miles northwest of its confluence with the Pocomoke River near Snow Hill, MD.  Shortly thereafter, the Nassawango furnace began producing pig iron by the cold-blast process.

In 1836, two of the Company's creditors, Arthur Milby and Joseph Waples, foreclosed on the property; that same year they sold it to Benjamin Jones, a Philadelphia ironmonger.  Jones, who owned other furnaces and had a formidable business that bought and sold iron products, only maintained the furnace at Nassawango a short time.  In 1837, he sold the property and business to Thomas A. Spence of Worcester County.  Spence, a lawyer son of a Snow Hill doctor,  was an energetic and enterprising ironmaster who produced over 700 tons of pig iron per year at Nassawango; Spence was also credited with the installation of the hot-blast stove on top of the furnace.

Iron was produced at Nassawango until 1847 when lack of labor and poor market conditions caused Spence (who fell into financial ruin) to shut down the furnace.  The property sat idle from that time forward and was used by successive owners mostly for the timber rights.  In 1962, the heirs of Georgia Smith Foster donated a portion of the property to the Worcester County Historical Society.  In 1978, the Furnace Town Foundation was organized as a nonprofit group charged with preserving, restoring, interpreting and developing the twenty-five acres of Furnace Town Living Heritage Museum, the 19th century village of the Nassawango Iron Furnace.  Programs in archaeology, history, nature and artisan interpretation were developed and presented to educate visitors.


About 1828 one of the most important developments in blast-furnace technology was introduced in England.  This was the “hot blast”.  The blast air, before entering the furnace's tuyeres (air nozzles), was raised to a very high temperature by being passed through a heat exchanger or “stove” heated by the hot waste gasses as they left the furnace.  This accelerated the combustion process within the furnace and decreased the amount of charcoal fuel necessary to reduce a given quantity of iron ore, but more significantly, it appreciably increased the production of iron.

The Nassawango Furnace responded to this innovation about 1835, by the fitting of a brick-encased cast-iron-pipe stove at the top of the furnace stack.  This functioned by channeling the waste gasses into the hot-blast stove enclosure.  On top of the enclosure two draft stacks with dampers carried off and controlled the flow of the hot waste gasses.  The blast air, compressed by the bellows  was piped through a sheet-iron air main up the south side of the furnace and into a series of  nine cast-iron labyrinth pipes within the stove, which were heated by the gasses surrounding them.  As the pressurized air passed through the stove, its temperature was greatly raised before it descended to the tuyeres through a heavy cast-iron downcomer.  A sheet-iron baffle plate deflected the furnace gasses into the stove enclosure.  It may have contained a charging door or else it was moved aside during charging. 


Raw bog ore was dug from beds along the sides of Nassawango Creek and the adjacent swamps.  It was then hauled to the wooden charging bridge where it was carried up to the top of the furnace stack and dumped into the furnace.  At the same time, pine and other trees in the 5000-acre adjacent area were cut and burned to produce charcoal, which was also carried up the charging bridge and dumped into the furnace.  The third element of the furnace was oyster/clam shells, which were brought to the site by barge up the canal from Crisfield via Pocomoke River and Nassawango Creek.  Note: bog ore, charcoal and shells were stored in warehouses and removed as needed.

Each of these three components was loaded, layer-by-layer, into the furnace until it was full; then the charcoal was ignited.  As it took fire, water was let into the head race from the 300-acre pond upstream; the water then traveled down a timber flume and powered the large wood breast wheel before being discharged into the tail race-canal.  The water wheel provided power for a bellow that probably was housed in a blowing room.  The compressed blast air  from the bellows was piped to the hot-blast stove on top of the stack where it was heated by the furnace exhaust gasses.  The hot blast air was then piped back down to the base of the furnace where it was forced into the hearth, providing the oxygen necessary for the smelting of the iron ore.

Since the molten iron produced by the smelting process was heavier than the impurities released, also in molten form, it collected at the bottom of the furnace hearth.  Two or three times a day it was tapped off, flowing into depressions formed in the sand floor of the casting house forming when cool the iron “pigs” that were the furnace's principal product.  These were sold for remelting in foundries and casting into various products.  A certain amount of the iron also was run directly into sand molds to produce a variety of cast-iron domestic and industrial articles.

After the iron had run out, the non-iron impurities in the ore, combined with the lime from the shells, was tapped off as a molten “slag”.  This waste product was dumped into the nearby swamp after it had solidified.

Meanwhile, the furnace was being charged with fresh ore, charcoal, and oyster shells about every two hours.  The furnace operated twenty-four hours a day from spring thaw to winter freeze.  Finished pigs and castings were loaded onto barges for the trip down the canal to Nassawango creek, where they were transferred to sailing ships bound for Philadelphia and Baltimore.