The Pennsylvania Blue Stone is a beautiful historical artifact of the famed iron furnace era of the 18th and 19th centuries. The furnaces and museums of Curtain Village (formally Eagle Ironworks) in Milesburg and Centre Furnace Mansion (formally Centre Furnace) in State College are two of many examples of preserved furnaces in PA that are worth visiting!*
Throughout the state, iron ore was mined and converted to pig iron through the process of smelting in massive stone furnaces, then shipped to Pittsburgh for further refinement into steel. The byproduct of the smelting process was known as “slag” which furnace workers simply discarded as waste. Once cooled, it had a blue/green glassy appearance, thus the name “Pennsylvania Blue Stone”.
Pennsylvania is not the only location to have Blue 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. In more modern history, slag from 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 as jewelry pieces.
The Process of Iron Ore Smelting
The basic process of iron ore smelting included the use of charcoal made from nearby ash, hickory, or oak 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 2. Limestone, abundant in the state was added as a flux to aid the separation of impurities from the iron. Coke, which is mostly carbon, reacted with the air to form carbon monoxide, acting as a reducing agent with the iron oxides in iron ore (mainly magnetite and hematite) resulting in molten iron. During this process, the limestone breaks down from the heat and gasses within the furnace into calcium oxide. This reacts with acidic impurities in the iron, primarily silica, resulting in calcium silicate, the main component of slag. At peak temperature, the heavier iron settles to the bottom of the crucible in the furnace and the lighter slag remains above the molten iron which is later discarded.
PA Blue Stones vary greatly in color ranging from solid to variegated blues to a rich black-blue or earthy green. Blue Stones also vary in density and texture, from porous to a dense rock-like quality and are often partially to fully transparent. 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 7. The composition of
Blue Stones varies but is primarily composed of silicon, calcium, magnesium, aluminum, manganese, iron, sulfur and often several other elements 1,3,6,7.
Every stone discovered is individually selected and each step of its journey from unearthing to setting into a finished pendant is documented with care. Each stone is then washed with soap and water with a bristle brush to prepare for cutting and shaping.
The original shape and features of each stone are retained as much as possible to preserve and highlight its natural and unique character. Only necessary cutting and shaping are done to ensure proper stone setting.
Each stone is then tumbled for 4-5 weeks to accent the best features by polishing and softening its overall shape. Tumbling simulates the natural process of rock shaping and polishing that occurs over hundreds of years in oceans and other large bodies of water.
Once tumbling is completed, stones are used to create a unique pendant design specific to the individual characteristics of each stone including its shape, color, and particular features which vary greatly among Pennsylvania blue stones!
(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) 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
(3) Piatak, N. M., Parsons, M. B., & Seal II, R. R. (2015). Characteristics and environmental aspects of slag: A review. Applied Geochemistry, 57, 236-266
(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) 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
(7) Wallace G. Imhoff , “Gasses Affect Fluidity of Slag”, Iron Trade Review, September 9, 1929, , p. 647-651