The field of archeology has faced a tricky dilemma since its inception in the 1800s: how to unearth and examine valuable historical artifacts and structures without damaging them. Inevitably, non-destructive testing (NDT) methods and investigative techniques proliferated in the field. This article presents key examples of NDT used for archaeological analyses.
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The importance of NDT survey techniques is widely recognized in the fields of archaeological research, monument preservation and restoration, and antiquity.
NDT methods are used to understand the extent of archaeological deposits, assess the state of preservation of artifacts without damaging them, and plan excavation or restoration operations that will allow archaeologists to better target and understand features of interest on the prospected site.
Any method of inspection that does not destroy or irrevocably damage the test subject falls under the CND banner. A wide range of techniques are used in archaeology, including satellite image processing, Global Positioning System (GPS) surveys, X-ray fluorescence spectrography and radiography, mass spectrometry and isotope analysis of lead.
CND for operations planning at the Tutbury Castle site
Tutbury Castle in the UK has recently been the subject of extensive CND investigations which included geospatial technologies such as light detection and ranging (LiDAR) sensors, geographic information system computing ( GIS) and GPS data. The CND surveys program enabled researchers to explore the castle without undertaking potentially damaging excavation work
Tutbury Castle is a ruined medieval castle located in Staffordshire, England. It belongs to the Duchy of Lancaster, but it is a British government recognized “Scheduled Ancient Monument” that welcomes visitors. Mary Queen of Scots was briefly imprisoned there.
The castle archaeological project only used NDT methods due to the historical significance of the site, which made excavation impossible. A conventional digital survey allowed the team to achieve a common spatial framework based on differential GPS. The team carried out a ground LiDAR analysis of the castle’s interior and earthworks.
CND for the conservation of ancient works of art
The conservation of ancient works of art is always important, as they give us insight into the dreams and fears of human societies and cultures that would otherwise be forgotten. Historians use NDT methods to study ancient works of art while protecting them for future generations.
Active infrared (IR) thermography applies numerical simulations of heat transfer from source to target to understand the properties of the artwork, but this can be a destructive method as it requires gentle heating of the surface of the room.
To avoid damage such as color shifts or shrinking and warping effects, researchers need to understand the nature of the pigments used as well as the frame, backing, and any foreign material.
To do this, non-destructive techniques of X-ray fluorescence spectroscopy and radiography can be applied. This demonstrates the importance of adjusting, combining and carefully monitoring different inspection techniques to ensure minimal or no damage to the artwork under consideration.
CND for determining the elemental composition of ancient metal coins
A team of chemists from the National Institute of Standards and Technology (NIST), Colorado College, and Mount Saint Mary’s University in Maryland demonstrated how CND methods can help determine the elemental composition of ancient coins. The technique has even worked on parts that were previously considered too corroded for this type of inspection.
The coins were minted in the ancient Roman colony of Judea in the first century AD. The researchers’ analysis provided insight into the region’s business patterns and local economy, while raising new questions about who the region’s ruler was at the time.
NDT methods employed included elemental and isotopic analysis of metal parts. These analyzes can reveal a geographic footprint due to the unique composition of ores from different regions. Combined with historical records of local mining activity, this analysis can determine when the coin was minted.
However, researchers wanted to determine precisely whether the coins were minted between 41 and 45 CE by Agrippa I or after 61 CE by Agrippa II. To do this, they used X-ray fluorescence with lead isotope analysis to precisely identify the minerals used to make the parts.
Corrosion generally affects the results of these inspection techniques, and so they are not commonly used on antique coins. However, the team used polarizing optics with a powerful new software method to analyze X-ray fluorescence results to overcome these challenges, combining this with careful calibration of the mass spectrometer to maximize their results.
The NIST facility was used for lead isotope analysis, and there it was shown that coins attributed to Agrippa I were in fact minted at that time. However, analysis also revealed that the copper in the coins likely came from mines that were previously thought not to have opened for at least another century.
The fact that this ore came from local mines in Arabia may show that the Romans reached the region earlier than previously thought, or that the mines were already open when the Romans arrived. Either way, the new NDT findings suggest that the relationship between ancient Rome and the region was closer than researchers previously thought.
References and further reading
Brown, R A (1959). A List of Castles, 1154–1216. The English Historical Review. https://academic.oup.com/ehr/article-abstract/LXXIV/291/249/441569?redirectedFrom=fulltext
Epstein, M. et al. (2010) Chemical attribution of corroded coins using X-ray fluorescence and lead isotope ratios: a case study from first-century Judea. Applied spectroscopy. https://journals.sagepub.com/doi/10.1366/000370210791114211
NIST (2010). Non-destructive methods for valuing ancient coins could be worth their weight in gold. [Online] ScienceDaily. Available at: https://www.sciencedaily.com/releases/2010/05/100527124053.htm
Shrestha, R. et al. (2021) A digital-thermal-thermographic CND evaluation of an ancient marquetry integrated with X-ray and XRF surveys. Thermal Analysis and Colorimetry Log. link.springer.com/article/10.1007/s10973-021-10571-2