The 7,000 soldiers buried with Qin Shi Huang in 210 B.C. were made of clay. But the bronze weapons the terra cotta army carried into the enormous tomb complex near Xi’an in western China were the real things: tens of thousands of swords, axes, spears, lances and crossbows, all as capable of spilling blood as anything Qin’s real army wielded when they triumphed, ending centuries of war and uniting China under a single rule for the first time.

What has been a puzzle for scientists is how so many weapons could have been made so skillfully, so uniformly and so quickly. (Qin reigned for only 11 years; construction of his mausoleum complex is thought to have started long before his death.) They now have a likely answer. A new study of 40,000 bronze arrowheads suggests they were produced in self-sufficient, autonomous workshops that produced finished items rather than parts that fed into an assembly line of sorts....