PRELIMINARY ANALYSIS OF HISTORIC CHARCOAL BLAST FURNACE SLAGS FROM NORTHWESTERN PENNSYLVANIA

The charcoal iron industry was at its peak in western Pennsylvania in the 1840s and 1850s, when hundreds of furnaces were active. Today, while the physical remains of some of these furnaces can still be found, at other sites only metallurgical slag remains as evidence of past iron production activities. It is of archaeological interest to know if the physical and chemical signatures of iron furnace slags are reflective of regional differences in iron ore and limestone flux material used during the furnace operation. Iron ore in this region occurred principally as siderite found in association with numerous Pennsylvanian-age limestone and shale beds, such as the widespread “buhrstone” ore associated with the Vanport limestone member of the Allegheny Formation. Bog ore also served as a source of iron at several furnaces. Trace element analysis, specific gravity, color, magnetic susceptibility, and fluorescence were used to characterize over 65 slag samples collected from over 30 different furnace sites. Additionally, when available, local siderite ores and limestone samples were collected and similarly analyzed. Principal component analysis (PCA) was used to preliminarily determine similarities among all ore and slag samples. This showed the high-silicate northwestern Pennsylvania furnace slags to be relatively similar to each other in composition and enriched in major elements concentrated from ores and flux materials, but also characterized by unique trace element signatures (e.g. Co, Cr, Cu, Ni, V and Zn) specific to site location or ore/flux sources. A PCA analysis best separated ore and slag into two distinct groupings based on potassium and vanadium, which appear to concentrate in the slag during the furnace blast. Slag characteristics such as color, fluorescence, magnetic susceptibility and specific gravity all appeared to be products of furnace operating conditions, more so than the use of specific source materials.