Spectroscopy Around the Globe, Episode 4: Angkor Wat

In Episode 4 of Spectroscopy Around the Globe, we shift to a different continent. Over the first three episodes, we explored the role that spectroscopy is playing in helping us learn more about the Giza Pyramids, Stonehenge, and the Lascaux Caves.

This month, we venture to southeast Asia and examine how spectroscopy is being used to study one of the most famous temple complexes in the world. In Siem Reap, Cambodia, an old historical temple complex, called Angkor Wat, sits at the heart of the Khmer empire. It is the largest religious complex in the world and also one of the most popular tourist sites.

In this episode of “Spectroscopy Around the Globe,” we’ll explore how spectroscopy is revealing the history and condition of the twelfth-century temple without damaging it.

I had the opportunity to visit Angkor Wat back in 2017 with my brother and a friend from high school. It is hard to gain a sense of its size until you are actually on the ground. Exploring the complex took several days, and we didn’t even see everything there.

This vodcast episode opens with a puzzle about where the stone came from. Angkor’s temples were built from sandstone, laterite, and brick. Thousands of nearly identical sandstone blocks carry no record of which quarry they came from or when they were laid. For more than two decades, a Waseda University team has tackled this with portable X-ray fluorescence (pXRF) and a handheld magnetic susceptibility meter.^1–3 Readings of rubidium, strontium, and yttrium distinguished three stone types. Magnetic susceptibility then separated the chemically uniform gray sandstone used in most major temples.^1–3 The work identified seven Angkor-period quarries and helped establish the order in which the monuments were built. pXRF proved especially useful for laterite, distinguishing its sources and helping estimate construction sequences at Koh Ker and Preah Vihear.^1–3

A 2025 study in Geoarchaeology explained why magnetism varies from block to block.⁴ Researchers combined microscopy, Raman spectroscopy, XRF, and rock magnetic experiments. Micro-Raman analysis identified the iron titanium oxides responsible, linking the variation to different igneous sources for the sand.⁴

What this episode shows is the power and value of using handheld instruments out in the field. By using these instruments, researchers are uncovering more about the complex while helping preserve a historic site that has stood for nearly 900 years.

References

1. Uchida, E.; Shimoda, I. Quarries and Transportation Routes of Angkor Monument Sandstone Blocks. J. Archaeol. Sci. 2013, 40 (2), 1158–1164. DOI: 10.1016/j.jas.2012.09.036
2. Uchida, E.; et al. Non-Destructive In-Situ Classification of Sandstones Used in the Angkor Monuments of Cambodia Using a Portable XRF Analyzer and Magnetic Susceptibility Meter. J. Archaeol. Sci.: Rep. 2021, 39, 103137. DOI: 10.1016/j.jasrep.2021.103137
3. Uchida, E.; et al. The Construction Process of the Angkor Monuments Elucidated by the Magnetic Susceptibility of Sandstone. Archaeometry 2003, 45 (2), 221–232. DOI: 10.1111/1475-4754.00105
4. Dellefant, F.; et al. Petrography and Geochemistry of Feldspathic Angkorean Arenites Are Linked to Magnetic Susceptibility and Suggest Different Igneous Sources. Geoarchaeology 2025, 40 (6), e70034. DOI: 10.1002/gea.70034.