A Unique Application of LIBS in the Smithsonian’s Belt Hook Study

The SciAps Z-300 (now the Z-903) LIBS analyzer was recently lent to Ariel O’Connor, an Objects Conservator in the Department of Conservation and Scientific Research; Dr. Blythe McCarthy, Andrew W. Mellon Senior Scientist; and Donna Strahan, Head of the Department of Conservation and Scientific Research from the Smithsonian Institution National Museum of Asian Art to conduct a study on their Chinese belt hook collection. The study will eventually be published in a book on their typology, history, materials, and fabrication technology.

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 ranges from 190nm 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 the2-5 ppm range and potassium without the interference of heavy iron. The Z-903 is most widely used for mineral exploration, forensics, authentication, and archeology due to the wide elemental range.

The Analytical Process

Prior to commencing the study, the team had to figure out how to analyze over 400 belt hooks from the Han Dynasty (approximately the 5^thcentury BCE to the 2^nd century CE) with varying sizes, material, and stages of corrosion with the aim of comparing the results to a 1970’s study conducted by the previous head of the department, W. Thomas Chase. However, the 1970’s analysis was performed by drilling small metal powdered samples (2 to 3 mm in diameter) and performing wet chemistry on them. Unlike the 1970’s study, this study’s goal was to use nondestructive testing, but micro destructive was the second option.

“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 and McCarthy 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 an analytical method that analyzed the bulk alloy.

McCarthy began looking around for techniques. “We originally thought of Inductively Coupled Plasma Mass Spectrometry (ICP-MS). But getting access to an instrument as well as getting 400 objects, some of which are quite large, inside a chamber was just not realistic for this project,” says McCarthy.

They were eager for an approach that would give them results on the metal itself, not on the corrosion layers. “We found people at Yale who were using your LIBS, so I called Dr. Richard Hark [Conservation Scientist, Yale Institute for the Preservation of Cultural Heritage], and he introduced me to Morgan [Jennings],” says McCarthy. “That’s where the SciAps LIBS comes in.”

O’Connor and McCarthy were able to take advantage of the Academic Loaner Program at SciAps and had the top product specialists in the company train them on the LIBS. Morgan Jennings and Jonathan Moller worked with the team throughout their loan, answering questions and coming up with solutions to their unique problems.

“I'm an objects conservator, not a scientist,” says O’Connor, “so I really appreciated the amount of time Morgan and Jonathan spent with us. Trying to analyze ancient alloys is a challenge in the museum world because every methodology has its challenges, and we have limitations on whether we can or can't sample the artifacts. Having Morgan and Jonathan help us think through how to get the best data we possibly could within the constraints of corrosion and sampling was invaluable.”

The Challenges

Before they started analyzing the belt hooks, the team developed new calibrations for the major elements in the alloys. Most commercial alloys today tend to be brasses with zinc in them. They aren’t the same compositions that one finds in the ancient Chinese objects.

They created a new calibration based on the standards they had at the museum. O’Connor also borrowed standards from colleagues at other museums. “For high tin in particular, since we didn't have those in our reference collection,” says O’Connor.

It was a lengthy process to calibrate due to the numerous trace elements in ancient Chinese bronze alloys, as well as the wide range of concentrations of many elements. “We did 41 trial calibrations,” says O’Connor.

Analyzing the Belt Hooks

Morgan’s recommendation for an accurate analysis was to capture 3-5 spots. Consequently, O’Connor and McCarthy opted to examine 3 spots on each belt hook. The challenge was selecting where to select those 3 spots. They would not analyze the front or any part that had decoration. Even though the LIBS is a micro-destructive technique, the spot, “which is about the size of a period at the end of the sentence in 10- or 12-point font, was still visible,” says O’Connor. O’Connor’s plan was to analyze one spot on the button and 2 spots on the back of the belt hooks. “I tried to pick an area that looked the least corroded.” She wore a magnifying optimizer, which had a 7x magnification to help her select a spot.

“I built a platform with a V-shaped indentation on a lift table so that it was movable,” says O’Connor. The analyzer fit nicely into the platform, which was weighted to support the belt hook. “The camera is a little challenging because of the angle, and where the laser hits the artifact is a little off from the camera [in the Z-300]. I learned the nuances of the placement so I could get it accurate to under a millimeter,” says O’Connor. The issue has since been corrected in the Z-900 series analyzers.

The next challenge in analyzing the belt hooks was how to use the analyzer and the calibrations to find the percentages of each element in the artifacts. “Because developing a calibration can take a lot of time, we wanted to be able to tweak the calibration,” says McCarthy.

“We also wanted to be able to run the data through future calibrations,” says O’Connor. “Initially, we had to acquire the spectrum from the analyzer, but then it exported it in a way that wouldn’t allow us to run it through a different calibration. When we acquired the spectrum through the Profile Builder software, we were only getting our intensity ratio numbers, but we wanted the percentages of each element, which we could only get from the analyzer.”

Once again, the team reached out to Morgan. He introduced them to the software feature, known as Spectrum Cal Analysis, for conducting their testing. “And that's how we could test multiple calibrations without additional analysis on the belt hooks,” says O’Connor. “We had the flexibility to rerun all of our object data through every new calibration. That tip from Morgan was what really changed it all for us,” says O’Connor.

“In fact, it was better for us that it wasn't easy off the shelf. We learned more about the technique, which, in the long run, gave a better result,” says McCarthy.

Next Steps

“Our next step is for Ariel to process all the data. And then to do some statistics and groupings to see how they match with the ones that are coming from comparative archaeology and art history,” says McCarthy. “We’ll also look at some X-ray fluorescence for comparison, since XRF is the only option for many museums. Then, we’ll put all that data together with the results of all the other analyses and create a typology,” says McCarthy.

Additionally, they dedicated a substantial amount of time meticulously examining each object under the microscope while capturing detailed photographs of the fabrication and decoration techniques and measuring the width and depth of toolmarks. “We're also looking at production, use, and repairs on the belt hooks. It's a lot of data gathering on each object before we can start to look at trends,” says O’Connor. Eventually they will also look at bulk alloy composition and the use of tin and lead in the belt hooks. “We're also X-raying every single one of the belt hooks,” says O’Connor. “For solid belt hooks, the porosity in certain areas can tell us casting direction. With the ones that are hammered or manufactured in other ways, the X-rays give us further technical information.”

“Having the SciAps LIBS for this project was really critical,” says McCarthy. “I could see the LIBS being useful in multiple ways beyond the belt hooks, for example, in studies of our ceramics and our large collection of Chinese bronzes.”

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.