When Mount Vesuvius buried Pompeii under a torrent of ash in A.D. 79, it also preserved one of the ancient world’s most remarkable artifacts: a massive mosaic depicting Alexander the Great’s triumph over the Persian king Darius III at the epic Battle of Issus. For nearly two centuries, this intricate artwork of over 1.9 million tesserae (the tiny stones that form the mosaic), unearthed in 1831 at the House of the Faun, has captivated historians and archaeologists alike.

But a recent study has uncovered a new dimension to its story — its tesserae come from an extraordinary range of quarries spanning Europe and North Africa.

Alexander’s victories

The Alexander Mosaic, often hailed as “the most important mosaic of the Roman age,” depicts the Macedonian army’s victory at the Battle of Issus in 333 B.C., now near the Turkish-Syrian border. Among the surviving battle scenes is perhaps Alexander’s most iconic portrait, rendered in delicate shades of pink.

The mosaic, thought to be a Roman copy of an earlier Hellenistic painting by Philoxenus of Eretria, portrays a pivotal moment in the battle. At its center is Alexander, young and determined, driving his forces into the Persian ranks. Darius, overwhelmed by the chaos, is shown in retreat, his outstretched hand a mixture of desperation and command. Darius would leave behind his mother, wife, and children, which were captured by Alexander.

Following Issus, Alexander moved south to secure key cities along the Mediterranean coast, including Tyre and Gaza. By gaining control of these coastal hubs, he cut off Persian naval power and ensured his supply lines were secure for future campaigns. After Issus, Alexander’s campaign gained unstoppable momentum. He turned his attention to Egypt, where he was welcomed as a liberator from Persian rule and crowned Pharaoh. The victory at Issus also paved the way for his legendary triumph at Gaugamela in 331 BCE, which effectively sealed the fate of the Persian Empire.

And this piece of artwork captures not just history but, like Alexander, covers a range of geographies.

A historical puzzle

In a recent restoration initiative, researchers deployed a suite of non-invasive technologies. They used portable X-ray fluorescence (pXRF) to identify chemical compositions, infrared thermography (IRT) to detect hidden thermal anomalies, and multispectral imaging to study surface treatments and tesserae compositions. Techniques like Raman and Fourier-transform infrared (FTIR) spectroscopy rounded off the battery of cutting-edge analytical tests. Remarkably, all these techniques allow an unprecedented study of The Alexander Mosaic without ever touching any of the delicate tesserae or disturbing the artwork in any way.

Scientists classified the tesserae into four groups based on their composition: carbonate-bearing, silicate-bearing, intermediate (natural and vitreous), and iron-rich. These classifications reflect the materials’ distinct geological origins.

For instance, white tesserae may hail from Italy’s Apuan Alps, while the pink and red tiles suggest connections to Mediterranean marbles and breccias. Green tesserae hint at serpentinites or other stones sourced from Roman-era quarries across the empire.

This logistical chain reflects the impressive reach of the Roman Empire across the entire Mediterranean basin. By the time of its installation in the House of the Faun, artisans were accessing materials from distant regions.

The ultimate goal: better conservation

Still, time has not been kind to the Alexander Mosaic. Past restorations left their mark, with protective wax coatings and gypsum layers applied during its 19th-century transport to the National Archaeological Museum of Naples, where it can still be admired. These materials, while initially protective, have aged poorly, creating thin gypsum layers and thermal inconsistencies across the surface.

Infrared thermography pinpointed areas where tesserae had shifted due to environmental stresses. While alarming, these insights will go a long way in improving ongoing conservation strategies, helping restorers mitigate risks while maintaining the artwork’s integrity.

For visitors at the National Archaeological Museum of Naples, where the mosaic resides today, these findings offer a new appreciation of what lies beneath its surface — not just the artistry, but the far-reaching networks that made it possible.

The findings appeared in the journal PLOS ONE.