Pennsylvania Slag Stones

The History

The Pennsylvania Slag Stone, or Slag Glass, is a beautiful historical artifact of the famed iron furnace era of the 18th and 19th centuries. There were a number of furnaces in Centre County, including Centre Furnace (Centre Furnace Mansion) in State College and Eagle Iron Works (Curtain Village) in Milesburg. Both are excellent examples of preserved furnaces and museums in PA that are worth visiting! *


Throughout the state, iron ore was mined and converted into pig iron through the process of smelting in massive stone furnaces. The pig iron was then sold to forges in locations such as Pittsburgh for further refinement into steel, and often hammered into wrought iron in local forges prior to shipment. The byproduct of the iron ore smelting process was known as “slag” which furnace workers simply discarded as waste. Once cooled and solidified, it had a blue/green glassy or stone-like appearance.

Pennsylvania is not the only location to have slag stones with similar history. In fact, one of the first known origins and uses of slag was in Greece during the Bronze Age (3300-1200 BC) where slag from copper foundries was used in jewelry and ceramic pieces 4,5. Slag from past iron ore charcoal blast furnaces in Michigan, Sweden, and Tennessee are well known for their local historical significance and beauty, and are celebrated as semi-precious gemstones often featured in jewelry pieces.

The Process of Iron Ore Smelting

The basic process of iron ore smelting included the use of charcoal as the furnace heat source, brought up to temperatures of 2200-2700° F using blasts of cold air pumped through the furnace by huge bellows powered by a waterwheel. Limestone was added as a flux to aid the separation of impurities (slag) from the iron. Coke, which was mostly carbon, reacted with the air to form carbon monoxide. The carbon monoxide acted as a reducing agent with the iron oxides in iron ore (mainly magnetite and hematite), resulting in molten iron. During this process, the limestone broke down from the heat and gasses within the furnace into calcium oxide. This reacted with acidic impurities in the iron, primarily silica, resulting in calcium silicate, the main component of slag.3 At peak temperature, the heavier iron settled to the bottom of the crucible in the furnace and the lighter slag remained above the molten iron which was later discarded.

PA Slag Stones vary greatly in color including solid to variegated rich blue and green, black, gray, and even purple. Although PA Slag Stones are considered glass due to their composition, often possessing a dense transparent appearance, some slag is opaque and porous with a sponge-like texture. Each stone’s unique color and texture is a result of the heating and cooling processes, along with the chemical composition of the slag after separation from the iron.6,9 The composition of PA Slag Stones vary but are primarily composed of silicon, calcium, sulfur, magnesium, aluminum, manganese, iron, and often several other elements.1,2,6,7,8,9




(1)      Bhatt, C. R., Goueguel, C. L., Jain, J. C., Edenborn, H. M., & McIntyre, D. L. (2017). Analysis of charcoal blast furnace slags by laser-induced breakdown spectroscopy. Applied optics, 56(28), 7789-7795.

(2)      Chaouche, Mohend, et al. "On the origin of the blue/green color of blast‐furnace slag‐based materials: Sulfur K‐edge XANES investigation." Journal of the American Ceramic Society 100.4 (2017): 1707-1716.

(3)      Flowers, Paul; Robinson, William R.; Langley, Richard; Theopold, Klaus (2015). “Occurrence, Preparation, and Properties of Transition Metals and Their Compounds”. Chemistry. OpenStax. ISBN 978-1938168390.

(4)      Lilyquist, C. & Brill, R.H. 1993. Studies in early Egyptian glass. New York: Metropolitan Museum of Art.

(5)      Lucas, A. & Harris, J. 1962. Ancient Egyptian materials and industries, 4th edition. London: Edward Arnold.

(6)      Parsons, Michael B., et al. "Geochemical and mineralogical controls on trace element release from the Penn Mine base-metal slag dump, California." Applied Geochemistry 16.14 (2001): 1567-1593.

(7)      Piatak, N. M., Parsons, M. B., & Seal II, R. R. (2015). Characteristics and environmental aspects of slag: A review. Applied Geochemistry, 57, 236-266.

(8)    Sloto, R.A., and Reif, A.G., 2011, Distribution of trace metals at Hopewell Furnace National Historic Site, Berks and Chester Counties, Pennsylvania: U.S. Geological Survey Scientific Investigations Report 2011–5014, 79 p.

(9)      Wallace G. Imhoff , “Gasses Affect Fluidity of Slag”, Iron Trade Review, September 9, 1929, , p. 647-651.


For more information and resources about the history and smelting process, visit the Centre County Historical Society at Both Centre Furnace Mansion in State College, PA, and Curtin Village, in Milesburg, PA, offer tours!