Real Reasons Why Terracotta Army Weapons are Well-preserved

After being buried for more than 2,200 years, the unearthed Terracotta Army weapons are found still in good condition. In order to figure it out why these ancient swords, crossbows and spears are well-preserved, researches have examined the weapons’ chemical composition, metallurgic techniques, and surrounding environment. Below are the four likely reasons.

As we know, the Terracotta Army weapons were made of bronze, which is an alloy mixed with copper and tin. Pure copper is kind of soft, so ancient craftsmen added much tin to improve the bronze’s hardness. To be specific, tested by Brinell Scale, the hardness of pure copper is 35HB. Once it is mixed with five percent tin, the hardness will increase to 65HB. Mixed with ten percent of tin, the bronze’s hardness will be 165HB. Apart from improving hardness, the high tin content can also make the finished bronze bigger and have less air holes after it cools down.

However, the higher tin content the bronze has, the easier it is for it to be broken. As a result, this kind of bronze weapon can be rather brittle and easily fracture. In order to get rid of this shortcoming, Qin craftsmen mastered the quenching technique. With this technique, the tempered martensite structure in the bronze is well preserved, which helps to increase the bronze weapon’s plasticity.

Another striking feature of bronze weapons in Terracotta Army is its composite casting technique. During the Eastern Zhou Dynasty (770BC-256BC), in order to improve both the plasticity and the hardness, the ancient craftsmen had started to combine two bronzes of different tin contents to cast weapons. Take the bronze sword in Terracotta Army as an example, the sword’s very central part is a rectangular piece made of hard bronze with more than 19 percent tin content. And this central piece is completely wrapped by the blade made of resilient bronze with tin content of 8 to 14 percent. The finished bronze sword is very stiff, sharp and not easy to fracture.

Some people think that the good preservation of Terracotta Army weapons should be attributed to their surrounding soil. Specifically, the pH level of the soil can be a very important indicator, because it directly affects the oxidation-reduction reaction, water drainage, biological activity and ventilation at that place. The Terracotta Army is buried by yellow soil, whose moderate alkalinity and little organic matter can effectively prevent the acid erosion. Indeed, the test result shows that the pH of sample soil in Pit 1 and Pit 2 is between 8.1 and 8.5, which is a relatively high. Moreover, this type of yellow soil is made up of very tiny granules, and this feature can also slow down the exchange of air and water underground and help prevent erosion.

In order to prove the function of yellow soil, British researchers conducted a contrast experiment. They simulated the weathering process outdoors, and placed the replicas of bronze weapons under extreme temperature and humidity. After four months, the replicas buried in British soil were severely damaged, whereas those buried in yellow soil were still in good condition.

Though there is still no historical record about the anti-erosion techniques of Terracotta Army weapons, it does not mean there were none. We’d better not look down upon the ancient craftsmen’s wisdom, who created the majesty Terracotta Army.

At the time, ancient craftsmen already knew how to smelt iron. And this metal material is much harder than the bronze made of tin and copper. So, why didn’t Emperor Qin Shi Huang choose iron weapons for his Terracotta Army? Two guesses are plausible. The first one is that the quenched bronze swords would present beautiful golden color. And this appearance serves a perfect foil to Emperor Qin Shi Huang’s majesty. In contrast, the iron swords fail to meet this need. The second guess believes that the quenching technique will make the bronze swords withstand erosion, thus are more suitable to be buried as funeral objects.

 See also Where are the weapons used to be in Terracotta Warriors' hands?​