About my Sterling Silver and Lapidary Work
Iron Slag 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
Fordite
The Origin
Also known as "Detroit Agate" or "Motor City Agate", Fordite was created in Ford factories in the mid to late 1900s, when car parts and exteriors were spray painted by hand with enamel paint in chambers, tracks, and skids, creating thin layers of different paint colors. These layers of paint were baked after each application and thus hardened. The paint overspray eventually became thick from buildup and had to be removed and discarded. A few creative individuals discovered that if you shaped these discarded pieces, it exposed the colorful layers of paint creating beautiful pieces, each with unique patterns and colors!
More Fordite!
Fordite's beauty and intriguing history inspired others to seek similar materials, transforming them into beautiful pieces that could be incorporated into jewelry. These Fordite relatives include, but are not limited to Jeep Fordite, Corvette Fordite, Speed Boat Fordite, Graffiti Fordite made from compressed layers of graffiti, recycled materials such as Bowlerite from vintage bowling balls, and Marble Fordite made from marbles in the early to mid 1900s.
Recommended Further Reading:
Fordite:
The following articles outline the historical timeline of the types of paints used in the automotive process, and highlight the significant role paint had in the industry.
"Watching Paint Dry", Material World, by Ed Conway
https://edconway.substack.com/p/watching-paint-dry
"A Breif History of Automotic Coatings Technology", American Coatings Association, by Douglas M. Lamb
https://www.paint.org/coatingstech-magazine/articles/brief-history-automotive-coatings-technology/
"A Brief History Of Car Colors — And Why Are We So Boring Now?", Consumer Reports, by Mary Beth Quirk
Historic PA Iron Slag:
* https://www.curtinvillage.com/
(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 www.centrehistory.org. Both Centre Furnace Mansion in State College, PA, and Curtin Village, in Milesburg, PA, offer tours!
