Here’s the real reason why Terracotta Army weapons are so well-preserved

Enlarge / View of Pit 1 of the Terracotta Army showing the hundreds of warriors once armed with bronze weapons. (credit: Xia Juxian)

Ever since archaeologists excavated the first figures of the famous Terracotta Army of Xi'an in the 1970s, they have marveled at the pristine condition of bronze weapons accompanying the figures. Scholars suggested that this was evidence of one of the earliest known anti-rust technologies, and over time, this hypothesis took on the veneer of fact, at least in popular accounts. But according to a new study in Nature: Scientific Reports, it's the unique chemical composition of the surrounding soil that is responsible for the exceptional preservation.

The Terracotta Army is composed of thousands of life-sized ceramic figures dating back to the late third century BCE, housed in three large pits inside the mausoleum of the first emperor of China, Qin Shihuang (259-210 BCE). Essentially a form of funerary art, these warrior figures were meant to accompany the emperor to the afterlife. They once held fully functional bronze weapons: spears, lances, swords, crossbows, and so forth. Over the decades, archaeologists have excavated tens of thousands of valuable weapon artifacts from the site, many of which were in nearly pristine condition, even though handles, scabbards, and similar organic pieces had long since rotted away.

Early tests showed traces of chromium on the bronze weapon surfaces, a metal element found in stainless steel that is resistant to tarnishing. Those traces suggested that the Qin artisans who made the weapons might have employed an early forerunner to the chromate conversion coatings invented in the 20th century, which are still used for preservation today.