INVESTIGATING A LINEAR SPATIAL ARRANGEMENT OF HISTORIC IRON FURNACES ACROSS MIDDLE TENNESSEE USING SLAG GLASS AS A PROXY

Iron was critical in early United States history, revolutionizing construction, agriculture, transportation, and other necessary industries. A linear spatial pattern of historical iron smelting operations across Middle Tennessee indicates an underlying structural geologic control having locally concentrated the ores.

A geochemical investigation is challenging due to the overgrowth of the depleted mines; however, the slag by-product remaining from the smelting process may serve as a proxy for investigation. While iron production has significantly decreased in Tennessee since the 1930s, the presence of overlooked critical metals could have lasting impacts on the region's economic development. Furthermore, identifying crystalline phases and the glass' composition may offer insight into the smelting processes for archeological investigators.

Slag glass samples selected from 30 of the 86 sites collected by the Tennessee Division of Archeology were prepared for testing. Crushed material was used in X-ray diffraction (XRD), Fourier transform infrared (FTIR), and Raman spectroscopy. Polished epoxy mounts containing fragments of the samples were used in micro X-ray fluorescence (micro-XRF) and a scanning electron microscope equipped with energy dispersive spectroscopy (SEM-EDS). The XRD determined the amorphous nature of the glass. FTIR and Raman identified separate iron crystals within the glass. Micro-XRF identified the glass's major elements of silica, calcium, manganese, potassium, magnesium, and sulfur. Imaging in the SEM verified the presence of iron crystals; micro-XRF and EDS identified trace elements of titanium, chromium, strontium, zirconium, and copper in the glass. Generally, the glass is homogeneous, but some samples show discrete bands with higher concentrations of elements than others.

While preliminary, the banding is thought to be related to an unmixing phenomenon during slow cooling of the glass. Some chemical variations between samples indicate unique production methods amongst sites or through time. The local presence of elements such as titanium and zirconium may be primary, suggesting local variations in the ores. This may be useful for investigating critical metal potential. For instance, there is an interest in the production of titanium from overburden in areas of Tennessee but further west of these historic iron occurrences.