How is a brick of the Great Wall fired?

Starting from Shanhaiguan in Hebei in the east and ending at Jiayuguan in Gansu in the west, the majestic Great Wall was built from the late Western Zhou Dynasty to the Ming and Qing Dynasties, with a total length of 21,196.18 kilometers. Such a magnificent world wonder was built by the hardworking and wise ancient Chinese working people using earth, stone and bricks. The most well-preserved Great Wall that everyone sees now is the Ming Great Wall. The important sections of the Ming Great Wall that guard the capital mostly use hard stones to lay the foundation and use fired blue bricks to build the walls. The blue bricks of the Ming Dynasty Great Wall have survived for hundreds of years. Their volume and weight are much larger than the red bricks used to build houses today, and they are used on a large scale. So how were the blue bricks used to build the Great Wall fired?

Image source: Xinhua News Agency

In late autumn of 2002, a brick kiln with Great Wall bricks was discovered in Banchangyu Village, about 29 kilometers north of Qinhuangdao City, only 500 meters away from the well-preserved Ming Dynasty Great Wall. Banchangyu was originally part of Jizhen. Qi Jiguang, a famous anti-Japanese general in the Ming Dynasty, once sent people to build the Great Wall here. As of August 2003, a total of 66 brick kilns with Great Wall bricks were discovered here. On the basis of the ancient Great Wall and brick kiln sites, the Banchangyu Great Wall Museum was built. The museum displays Great Wall bricks of various specifications and sizes, which can be divided into dozens of types such as long bricks, square bricks and flagpole bricks according to their shapes and uses. When you come to the Great Wall brick kiln site, you can see that these brick kilns are all in the shape of horseshoe kilns, with a diameter of about five meters and a depth of about 3.5 meters. Some of these brick kilns still preserve the Great Wall bricks fired at that time.

Banchangyu Ming Dynasty Great Wall Ruins Photo source: Banchangyu Scenic Area

Bricks and stones of various sizes and specifications of the Great Wall. Image source: Banchangyu Scenic Area

To make a good brick for the Great Wall, the first thing to do is to solve the material problem. Modern brick-making raw materials are mainly industrial waste such as coal gangue and fly ash, while in the Ming Dynasty, all brick and tile raw materials required a lot of clay. The construction of the ancient Great Wall emphasized the use of local materials. A large number of ancient brick kilns were discovered in Banchangyu Village, and there must be clay suitable for brick-making nearby. After searching, modern people found reddish high-iron soil in Changgukou, 5 kilometers away from Banchangyu Village. The blue bricks made from this high-iron soil are dense in texture, resistant to high pressure and erosion, and are likely to be the raw materials for the Great Wall bricks at that time.

Xu Guohua, the guardian of the Great Wall restoration in Banchangyu Village, imitated the shape of an ancient brick kiln and built a firing site. He then took high-iron soil from Changgukou and sifted the soil through several coarse and fine sieves to finally get the raw materials for making Great Wall bricks. With the raw materials, the second step is to mix the mud. The water and clay are mixed in a certain proportion. After the workers stir the mixture constantly, they need manpower or cattle to step on it. This process is similar to kneading dough with hands. The air is squeezed out by external force to allow the water and mud to be fully mixed, so that the soil is more solid and the bricks are more resilient. After the mud is activated, the activated mud can be put into the brick mold, formed and dried. If you want to make text bricks, you only need to stamp the side with a mark indicating the time and production unit before the bricks are dried.

Ming Great Wall Brick Kiln Site Image source: Thepaper.cn

After that, the bricks were stacked at 90-degree right angles according to the pattern of the bamboo mat to ensure uniform heating. After that, raw materials with high calorific value were selected to ensure sufficient and vigorous firepower, and the bricks were continuously roasted in the brick kiln at a high temperature of 800 degrees Celsius for three days and three nights. Finally, a "magic trick" was performed to make the red bricks turn green. The secret of this "magic trick" is water. When white smoke comes out of the flue on the top of the kiln, cold water is poured into the arc-shaped waterway on the top of the kiln. After the water penetrates, it turns into water mist in the high-temperature kiln and adheres evenly to the surface of the bricks. Under the high-temperature continuous roasting of the bricks in the kiln, the iron-rich elements in them turn into iron oxide, that is, ferric oxide, which appears red. The addition of water vapor consumes more oxygen. When there is not enough oxygen, the ferric oxide is reduced and becomes ferrous oxide, and the color of ferrous oxide is blue-gray. The bricks fired in this way not only change color, but also become harder and less likely to crack.

By comparison, the density of modern antique-style Great Wall bricks is almost the same as that of Ming Dynasty bricks. These antique-style bricks are used to repair the Ming Dynasty Great Wall, so that the wonders of the ancient Chinese Great Wall can continue as long as possible, so that future generations can continue to witness the wisdom of their ancestors in science and technology, and do a good job in the intergenerational transmission of ancient Chinese civilization.