Slag Heaps

The mounds around the furnace are slag, the waste product left when iron ore was superheated and fluxed with limestone so that molten iron sank and impurities rose. Skimmed off and discarded, the slag hardened into a glassy or airy rock like volcanic glass or lava rock. It was removed from the furnace about every six hours and allowed to run onto the ground to harden, then the large pieces were broken and loaded into wagons for removal. Disposal became such a problem that slag was used widely to fill holes and was eventually piled in heaps near the furnace.

Maryland · United States