This is History: Microprobe Analyses Decipher Identities of Metallurgical Artifacts

“Indiana, we are simply passing through history. This, this is history.” Belloq’s line referring to the Ark of the Covenant from Raiders of the Lost Ark highlights the role of artifacts in understanding historical truth. Technologies such as element mapping and microanalysis techniques enable archaeologists to play historical detectives, deciphering artifacts to contribute to our knowledge of the past. According to Dr. Robert Gordon, a Yale professor in Geology and Geophysics, “The intellectual challenge of examining the artifacts transmits a historical record and enlightens our understanding of people’s interaction with materials and the natural world.” Artifacts stand as silent and irrefutable witnesses to history and the technology of their eras.

Encountering the Artifact

The work of Gordon and Colin Thomas, a graduate student in Yale’s Anthropology department, exemplifies the archaeological approach to discovering historical truth. Archaeologist Dr. Richard Hunter, Founder and President of Hunter Research Historical Resource Consultants, was commissioned to work on an excavation behind the state house in New Jersey. There, under centuries of debris, he found an 18th century metallurgical site, which he believed was a furnace for converting iron into steel. During the excavation, the foundations of a furnace house were revealed, but the most interesting artifacts were what appeared to be iron bars. These iron bars were sent to Thomas and Gordon for identification and confirmation.

The iron bar artifact received at Gordon’s lab. Courtesy of Dr. Robert Gordon.

Gordon’s work began in a “typical” way: “I have something that looks unpromising like bits of iron or slag, and I ask how old? What does it tell of the technological ability or scientific understanding of its time?” The multistep process of extracting data from the artifact begins with an examination of its size and shape. Then the microstructure is examined under a light microscope and probe. Based on knowledge of the technology of its era of origin, the artifact can be identified by comparing it to a reference collection of metal artifacts, the only one of which in the United States is housed at Yale.

Microstructure Analysis

The metal bar artifact was examined by a microstructure analysis technique that maps the elements found in the microstructure and identifies the type of iron contained in the artifact. For example, sulfur, silicon, and carbon are expected to appear in cast iron. Mapping for oxygen shows the amount of oxidative corrosion that has taken place in the iron. In tandem with scanning electron microscopy, energy dispersive X-ray spectrometry (EDS) creates elemental maps by focusing a beam which consequently releases an x-ray spectrum. The spectrum lines can be identified and their intensity measured to determine the concentration of the elements in the sample. Here, EDS analysis determined the composition of the external oxidation products, and wavelength dispersive analysis (which measures the concentration of selected elements) quantified the metal’s constituents, enabling Gordon and Thomas to decipher the metal’s microstructure.

After analzing both the internal metal and the external oxidation product, the microstructure of the metal itself proved that it was not wrought (pure) iron as suspected. Instead, carbon graphite flakes suggested the artifact was cast iron. The microstructure had a consistent iron nitride precipitate along its metal matrix, a product formed during the cooling of metal. This indicated that the piece of cast iron had been subjected to repetitive heating and cooling, which suggests that the artifact was part of the furnace itself. Non-metallic elements were also identified in the sample, the most abundant being iron oxide. Iron oxide is a product of an internal oxidation reaction that occurs over time, replacing the graphite flakes of cast iron.

Standard cast iron image with graphite flakes. Photo courtesy of Robert Gordon.

The artifact was then compared with a reference specimen, which came from a pair of andirons used to hold wood fuel in a fireplace. X-ray mapping and imaging showed that the reference and artifact had similar structures. While both had graphite flakes, those in the artifact had been replaced mostly with iron oxide. Iron oxide has a low uniform iron concentration, and the artifact did not show any concentration gradients in iron or oxygen at the border between the metal and the reaction product. In contrast, iron in the reference specimen was diluted in the areas surrounding the graphite flakes by the increased oxygen that was contributing to the reaction. The comparison showed that both the artifact and reference were undergoing the same internal oxidation reaction. However, this reaction was still underway in the reference specimen whereas it had already reached equilibrium in the artifact.

Final Identity

All these analyses identified the artifact as a grate bar from the firebox in the furnace. The cast iron of the furnace bars is particularly susceptible to internal oxidation because of oxygen solubility in the metal, elements with oxygen affinity (silicon, manganese, and phosphorus), and the high diffusion rate of oxygen. The fire of the furnace provides oxygen, and the high temperature increases the speed of oxygen diffusion. Internal oxidation degraded the grate bar, but technology recovered its original composition and identity.

History of the Cementation Furnace

The knowledge extracted from the artifact can be placed in a historical context, enriching our understanding of colonial America. The excavation site was confirmed as a steel cementation furnace built in the 1740s used to convert iron into steel, a complex process that was being developed in Germany and Great Britain at the time.

A schematic of a steel cementation furnace that would have been similar to the one discovered in Trenton. Photo courtesy of Robert Gordon.

The excavation site belonged to the Trenton Steel Works, which was built on Petty’s Run, a tributary of the Delaware River. Between 1745 and 1750, Benjamin Yard built the cementation furnace — one of four of its kind built in the U.S — marking a radical move for colonial economic independence. Great Britain responded by passing the Iron Act in 1750, which attempted to limit the development of iron manufacturing in the U.S. The colonists wanted cheap sources of steel for cheaper production for agricultural and military purposes; Great Britain’s effort to stifle economic progress, including iron manufacturing, in the colonies was a large factor leading to the Declaration of Independence.

Through their use of scientific techniques, Gordon and Thomas were able to enhance our understanding of 18th century America. Artifacts are the doors to historical truth, and scientists like Gordon and Thomas hold the key to unlocking this potential.

Photo of excavation site in Trenton, New Jersey. Photo courtesy of Robert Gordon.

About the Author
Theresa Oei is a sophomore Molecular Biophysics and Biochemistry major in Pierson college. She is on the board of Synapse, Yale Scientific Magazine’s Outreach Program, and works in Professor Steitz’s lab studying target genes of the viral miRNAs HSUR4 and HSUR5 for their role in tumorigenesis

The author would like to thank Doctor Gordon for his time and inspiring dedication to archaeological discovery and research.

Further Reading
Gordon, Robert. “Process Deduced from Ironmaking Wastes and Artifacts.” Journal of Archaeological Science. Vol 24 (1997): 9-18.