Smithsonian Utilizes LIBS Academic Loaner for Chinese Belt Hooks Study

The Smithsonian Institution’s National Museum of Asian Art recently utilized a SciAps LIBS analyzer through the Academic Loaner Program to conduct a study on their Chinese Belt Hook collection. Ariel O’Connor, an Objects Conservator in the Department of Conservation and Scientific Research, and Dr. Blythe McCarthy, Andrew W. Mellon Senior Scientist, completed Laser Induced Breakdown Spectroscopy LIBS analysis on 401 of the 423 belt hooks using the Z-300 (now the Z-903). The majority of the collection originates from the Han Dynasty era, spanning from approximately the 5th century BCE to the 2nd century CE, and all have various degrees of burial corrosion.

Like its predecessor the Z-300, but with updated form and software, SciAps Z-903 handheld analyzer measures every element in the periodic table of the elements – from H to U. The extended spectrometer range from 190 nm to 950 nm, allows measurement of longer wavelength emission lines from elements like H, F, N, O, Br, Cl, Rb, Cs, and S. Other benefits include a more sensitive line for lithium near 675 nm to achieve limits of detection in the 2-5 ppm range and potassium without the interference of heavy iron. The Z-903 is most widely used for mineral exploration, forensics, authentication, and archaeology due to the wide elemental range.

The Chinese Belt Hook Study

This research project started in the 1970s with a previous head of the department, W. Thomas Chase. Chase wrote his master’s thesis¹ on the Freer belt hooks. He continued his research with a technical study at the museum, analyzing 157 belt hooks by drilling small holes and collecting powdered metal samples on which he did wet chemistry.

Over the years, the museum added to the collection. They now have 423 belt hooks, most of which are made of copper alloy and have an average length of approximately 6”. Although this study is only looking at bronze belt hooks, some in their collection are of other metals like iron, are gilded, or have inlays with silver, gold, jade, and glass. It's a unique collection, both in terms of materials and size.

“When we started this project, we were trying to figure out how we could do copper alloy analysis in a way that would compare Tom's data from the 1970s with our new pieces and look at the collection as a whole,” says O’Connor.

At first, O’Connor used handheld X-ray fluorescence (XRF) but realized that since XRF is a surface technique, they were getting very different numbers when compared to the wet chemistry analysis. They needed to find a technique to look below the surface corrosion.

The 1970s drilled samples are 2 to 3 mm in diameter, so the team wanted a method that wouldn't be as noticeable, which is why Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was their first thought; however, the size of the collection as well as some of the belt hooks made ICP-MS problematic. “I did a broad search on the Internet to try to find another technique that might give us results that would meet our criteria of not doing major damage to an object,” says McCarthy. Nondestructive testing was their first option, but micro destructive was the second.

“Dr. Richard Hark [Conservation Scientist, Yale Institute for the Preservation of Cultural Heritage], was using your LIBS for his work, so I called and talked to him. He then introduced me to Morgan [Jennings],” says McCarthy, “which led to SciAps very generously loaning us the instrument.”

The team was able to analyze most of the belt hooks. The analysis excluded the solid gold, silver, or iron belt hooks, focusing exclusively on the copper alloy artifacts that exhibited suitable analytical areas.

“Now we have a group of objects with both wet chemistry from the 1970s and the 2023 LIBS analysis. We have a good data set to compare both techniques,” says O’Connor.

“We’ll also look at some X-ray fluorescence for comparison because so many people in our field use XRF, even though it's a surface technique,” says McCarthy. Unlike the XRF, the LIBS laser can ablate through the outermost surface of the artifact with the cleaning feature and get a more representative analysis of the underlying material. Many museums prohibit the conservator from doing any sort of destructive technique, even if the sample is as small as that of the LIBS sample site, so XRF is the sole option. “You can't directly compare results from multiple techniques, but I think we have a body of data we can use.”

The Goal

Ariel O’Connor is working with Donna Strahan, Head of the Department of Conservation and Scientific Research, on a book of the rare collection.

“All the belt hooks originate from sites around China. The book will discuss belt hook typology and history from the viewpoint of the materials and technology, with the LIBS content in the appendix. We're also working with other conservators and curators and conservation scientists,” says O’Connor.

“Our next step is for Ariel to process all the data. And then to do some statistics and groupings and see how they match with the ones that are coming from comparative archaeology and art history,” says McCarthy.

A major research question O’Connor and Strahan are hoping to answer is how the belt hooks were fabricated and how the bulk alloy composition may provide information on workshop and date. They hope correlating the alloy groups with decorative techniques will allow further refinements. The most common method of decoration involves application of gold inlay or gilding to the surface. “We'll be using the LIBS datapoints to see if there is a trend of higher copper content in the alloys of gilded objects versus undecorated belt hooks. We're not quite there yet with our conclusions, but those are things we're looking for,” says O’Connor.

The belt hooks most likely came into China from the northwest or northeast and were then incorporated into Chinese clothing. “The terracotta warriors have belt hooks that are depicted as part of the uniform that they wear. So, we know that they were functional, but we also have ones that are decorated with turquoise, gold and jade, and glass, so they were also ornamental. I think of them as similar to the belt buckles that you see in rodeo culture, which are status symbols you can see from across the room,” says O’Connor. “Like the buckles, belt hooks were probably status symbols.”

Academic Loaner

O’Connor and McCarthy both talked about the benefits of the Academic Loaner Program.

“I would say, it's not just the instrument. It's all the help that we got from Morgan and Jonathan,” says McCarthy. Morgan Jennings is SciAps Product Manager and Jonathan Moeller is SciAps Product Specialist. They met with O’Connor and McCarthy via Zoom to walk them through the software.

“But there's no world in which this complicated software could be understood in a 1-hour Zoom Meeting. We took a lot of notes, and then, as we started to use the analyzer and then started our calibration, questions came up, and it wasn't just that we had questions all at once. It was many, many sessions with them on zoom again or through email. They were always very generous with their time,” says O’Connor.

“Testing the LIBS analyzer on this project was really critical, because corroded bronzes are such a large part of our collection,” says McCarthy.

SciAps Academic Loaner Program provides both handheld XRF and LIBS instruments for lab work with students or research in the field. Our academic partners are always coming up with novel applications for the instruments and giving us invaluable feedback.

How can we support your work with our Academic Loaner Program? Reach out to us at academic@sciaps.com. University researchers can borrow a unit for a month provided they give us a summary publication of their research, that we can post, without impacting any of their publishing rights.

SciAps, Inc., is a Boston-based instrumentation company specializing in handheld portable analytical instruments to measure any element, any place on the planet. Their industry-leading X-ray fluorescence (XRF) and laser-based (LIBS) analyzers are at work across every major industry, including oil/gas, metals and mining, aerospace, battery and strategic metals (lithium, rare-earth elements), scrap metal recycling, chemical and petrochemical, military, forensics and law enforcement. SciAps instruments are configured to measure elements in all types of materials, so applications are always expanding, recently including space research, pandemic anti-viral coatings, agriculture, and environmental contaminants.

¹W. Thomas Chase’s 1967 M.A. Thesis for New York University’s Institute of Fine Arts “Chinese belt-hooks in the Freer Gallery of Art” (739.151.C5 in FS main library)

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