Introduction to Iron Smelting

At the very front of the gathering crowd, Fang Xuanling, Wang Gui, and Li Jing exchanged glances. In their eyes flickered a complex emotion—a mix of admiration for Li Zhi’s extraordinary talent and a deepening concern for the shifting tides of the court. Thus far, each of the three legitimate princes had proven himself remarkably gifted. The Crown Prince, Li Chengqian, was gentle and wise; time and again he had ably managed state affairs in the emperor’s stead. The second son, Li Tai, stood unrivaled among the princes for his learning and shared many political views with the emperor, earning him particular favor. Now, the Prince of Jin, Li Zhi, not only displayed precocious intelligence, but his aptitude in the study of natural philosophy bordered on the uncanny.

Though this abundance of talent seemed a blessing for the royal house, to the ministers at court it instead heightened the uncertainty surrounding the succession. Fang Xuanling and his peers could well imagine that before long, a faction in support of the Prince of Jin would soon emerge at court. For now, the trust controversy regarding Li Zhi subsided with the debut of the new carriage, and Li Zhi devoted himself ever more to his studies.

Emperor Taizong had originally intended to rescind Li Zhi’s appointment at the Directorate of Works to allow him to focus on his education, but Li Zhi refused. “Father,” he said, “many of my ideas require the help of the craftsmen at the Directorate. I humbly ask you to let me remain in my post.” After some thought, Emperor Taizong agreed.

Alongside his medical studies with Sun Huipu, Li Zhi frequently sought advice from Sun Simiao as well. During this period, he shared his knowledge of the medicinal uses of lime with both men, giving them opportunities to further explore its applications as they treated their numerous patients. Li Zhi also learned various health practices from Sun Simiao, and would often insist that Emperor Taizong, Empress Zhangsun, and Li Lizhi join him in exercise whenever he had free time.

Emperor Taizong, a veteran of countless battles, bore more than a dozen wounds from years of warfare. While still young enough that these injuries scarcely troubled him, Li Zhi knew well that if his father’s health was not carefully managed now, those hidden ailments would take their toll in the future. As for Empress Zhangsun, her constitution was delicate to begin with, and repeated childbirth had left her even more in need of strengthening her body.

To encourage them to take exercise more willingly, Li Zhi decided to finance the construction of a large swimming pool within the palace at his own expense. Not only would it be built with cement, but Li Zhi also intended to have special ceramic tiles fired for decoration. Together with Yan Lide and others, he designed a water supply system for the pool, complete with intake and drainage pipes.

To improve the efficiency of heating water, Li Zhi unveiled his metallurgical talents for the first time, introducing to the world—centuries ahead of schedule—the boiler, an invention destined to become essential in later ages. When he laid out the blueprint before Yan Lide and the other master craftsmen of the Tang, they were left speechless with astonishment, unable to fathom what kind of prodigious intellect could conceive such a marvel.

After a long silence, Yan Lide, his voice trembling with excitement, said, “Master, your gifts must be heaven-sent. To witness such a thing in this lifetime—I could die without regret.” Li Zhi waved his hand dismissively. “It isn’t as miraculous as you say, simply a larger copper kettle, nothing more.” Yan Lide replied, “Master, you are too modest. We spend our days working with copper kettles, yet none of us ever conceived such an innovation. Truly, we are humbled.” He was not wrong: with the rise of tea culture in the Tang, copper kettles were commonplace, yet their uses were only just being explored, and few imagined other possibilities.

Li Zhi had no wish to dwell on the subject and changed the topic. “This device is large; we must study carefully to find the best way to manufacture it.” The others nodded in agreement. Since copper was prohibitively expensive, Li Zhi chose iron as the material for the boiler. However, neither copper nor iron could yet be produced in large sections with the available techniques, and even if they could, the necessary processing methods were lacking. Thus, they would have to solve each problem step by step—an opportunity for Li Zhi to become acquainted with Tang ironworking.

As the ironworks were under the jurisdiction of the Imperial Household Department, Li Zhi brought Yan Lide to visit Pei Feishu. Pei Feishu, upon hearing Li Zhi’s wish to tour the ironworks, readily agreed—after all, Li Zhi’s reputation for technical brilliance was already well known at court. If Li Zhi could offer insights into ironworking, it would be a tremendous boon.

In Da’an Ward stood Chang’an’s largest ironworks, conveniently near Anhua Gate for the transport of materials and adjacent to the Yong’an Canal for ample water supply. When Li Zhi and his party arrived, they discovered that charcoal had already been supplanted by coke as fuel, and several water-powered bellows were steadily pumping air into the furnaces.

The foreman, Baken, was a Turkic man who had come to Chang’an after the fall of his homeland and swiftly earned Emperor Taizong’s favor with his superb ironworking skills, becoming head of the workshop. Touring the facility under Baken’s guidance, Li Zhi noted that the bloomery furnace remained the standard equipment. Such furnaces worked too slowly for steelmaking, and their low output made casting large objects impractical.

Surveying the scene, Li Zhi immediately conceived of building a blast furnace, which would allow for the mass production of molten iron—and thus, larger castings. As construction on the swimming pool could not commence until spring, this interval was perfectly suited for transforming the ironworks.

He turned to Baken and asked, “Master Ba of Iron, have you ever considered suspending the furnace for smelting?” Baken looked puzzled. “May I ask, Your Highness, what leads you to such an idea? What benefits might it bring?” Li Zhi replied, “This is my reasoning: with the earthen furnace on the ground, heat can only rise from below and melt the ore gradually.” He pointed to one of the open-hearth furnaces. “In this way, the materials inside do not mix well, which prevents the molten iron from achieving uniform composition.”

At this, Baken’s eyes lit up. He exclaimed, “Your Highness, are you suggesting that the furnace should be set in motion to achieve a stirring effect?”