Bricks were the primary building material used in the construction of the Great Wall during the Ming Dynasty. The bricks used for the Great Wall were predominantly in a balsam green color and made from sandy clay. Due to variations in the soil composition at different kilns, some bricks contained noticeable impurities. The Great Wall bricks can be categorized based on their shapes and usages. These include long bricks, square bricks, battlement bricks, lookout hole bricks, shooting hole bricks, flagpole bricks, and water trough bricks, which were designed to drain water from the wall. Among the long bricks and square bricks, there are two types: those with inscriptions and those without.
Long bricks: This type of brick is the most commonly used in the construction of the Great Wall and its related structures. It is made of clay, has a balsam green color and a hard texture. Some bricks may contain larger grains of coarse sand. The front surface is relatively smooth, while the back surface is rough. The dimensions of most long bricks are approximately 37 centimeters in length, 15 centimeters in width, and 9 centimeters in thickness. Unutilized well-made long bricks were discovered in the Dalingzhai Brick Kiln in Qianxi County, with dimensions of approximately 41 centimeters in length, 20 centimeters in width, and 10 centimeters in thickness. In "Huangcheng" (deceptive city) near Baiyangyu in Qian'an County, the bricks used for the city gate are also long bricks, measuring approximately 47 centimeters in length, 21 centimeters in width, and 12 centimeters in thickness.
Square Bricks: These bricks are made of clay and have a sturdy texture with a smooth front surface and a rough back surface. They are balsam green in color and measure 37 centimeters in length, 37 centimeters in width, and 9 centimeters in thickness. Square bricks are commonly used for water drainage on the top of the city walls. In some cases, they are also used to pave the surface of the internal base of defensive towers, such as in the case of Defensive Tower No. 1 at Xuliukou in Qian'an County.
Battlement Bricks: These bricks can be divided into two types: battlement bricks and battlement top bricks. The bricks used for building the battlement are generally triangular on one side and rectangular on the other side. The triangular side is positioned at the battlement opening to create a chamfered angle, which expands the field of view, rather than a square-shaped battlement. These bricks have a width equal to the thickness of the wall, approximately 49 centimeters. Due to the construction requirements, the wall joints must be compressed, so another type of bricks, called blue bricks, are produced by splitting the battlement bricks in half vertically. The blue bricks have a width equal to half of the full battlement brick. The battlement top bricks have sloping sides and a ridge in the middle. The surface is smooth, while the bottom is rough, and the cross-section forms a triangular shape. However, there are also battlement top bricks with different triangular shapes on each side, where one end has a larger angle and the other end has a sharp angle. The dimensions of these bricks are 21 centimeters for the side length and 37 centimeters for the ridge length. The triangular face with a larger angle has side lengths of 25 centimeters, 36 centimeters, and 16 centimeters, while the triangular face with a sharp angle has side lengths of 28 centimeters and 27 centimeters.
Lookout hole Bricks: These bricks are square-shaped and have a nearly circular through-hole in the center. The surface is smooth and adorned with raised curling floral patterns, while the back is unadorned and rough. They are made of clay, with a balsam green color and a hard texture. The dimensions of these bricks are approximately 20 centimeters in length, 20 centimeters in width, and 9 centimeters in thickness, with a central hole diameter of 15 centimeters.
Shooting hole bricks: They are prefabricated and generally square-shaped, but with a perforation from the middle to the bottom. The upper half of the hole is arched, while the lower half is rectangular. They are made of clay, with a balsam green color and a hard texture. Some shooting hole bricks feature raised floral patterns on the surface. They have dimensions of approximately 25 cm in length, 9 cm in width, and 9 cm in thickness. The hole measures 16 cm in height and 9 cm in width. However, in certain sections of the wall, many shooting holes are square openings left during the construction of the wall. Only the long bricks above the shooting holes are engraved with decorative patterns, creating unique brick designs. These shooting hole bricks are generally rectangular and have similar dimensions to regular bricks.
Flagpole bricks: Similar to flagpole stones, they are placed at the battlements and serve as the base for the battlement structure. They have a generally similar shape. In the investigation of the Great Wall in Tangshan, flagpole bricks were only found on the city walls of the Xiaoguan north section in Qian'an County. Furthermore, all the flagpole bricks were located on the inner side of the city walls, which were fired using the same process as the regular bricks of the Great Wall. They are predominantly rectangular in shape, with sloping sides on the front of the brick and a central ridge. The sides are concave, forming a triangular structure when stacked on the wall. They have a total length of 80 cm, width of 44 cm, and a central hole with a diameter of 5 cm and a depth of 2.5 cm.
Water trough bricks: Approximately every 23 meters along the enclosed city walls, drainage holes are installed to allow water on the surface of the walls to flow out. This requires the construction of a water-blocking facility on top of the water-filled section of the walls, which is higher than the water level. This facility directs the water to flow towards the drainage hole through a channel, and the drainage hole is equipped with a water spout trough. The water-blocking facility is constructed using rectangular-shaped bricks placed horizontally above the water-filled section. The protruding surface of the bricks is rounded by grinding the square edges.
Single-character bricks: These make up the majority among the character bricks. On the surface of the bricks, there is a single character such as "left,"(左) "center," (中) or "right." (右) The characters are written in relief and regular script style, and most of them do not have a border. The characters are printed in the middle of the brick's surface. However, on the walls within the Zunhua area, bricks with a framed "center" character have been discovered. These characters are larger in size and also written in relief and regular script style, but they are slightly off-centered on the surface of the brick.
Two-character bricks: Currently, five types have been discovered: "left one," (左一) "left two," (左二) "left three," (左三) "right one," (右一) and "right two." (右二) In the Daolingzhai kiln in Qianxi County, a batch of "left three" bricks, which were already fired but unused, was unearthed. The "left three" bricks can be classified into four main types: 1) Engraved in intaglio, small-sized characters of regular script with a border, inscribed on the side of the brick; 2) Engraved in intaglio, larger-sized characters of regular script without a border, inscribed on the surface of the brick; 3) Engraved in relief, larger-sized characters of regular script without a border, inscribed on the surface of the brick; 4) "Left three" characters engraved on both the side and the surface of the brick. In the same village of Dalingzhai, "left one," "left two," "right one," and "right two" bricks of similar style were also discovered, which are currently used in local villagers' houses. These bricks are all engraved in intaglio and regular script, with small-sized characters and a border, and they are consistent in style with the "left three" bricks.
Production of ancient Great Wall bricks and tiles
As a World Heritage Site, the Great Wall is the longest and largest construction project in human history. The main components of the Great Wall are bricks, followed by tiles, wood, and stones. Bricks can be further divided into several styles, including bricks used for wall construction, square bricks used for paving the pathway, and specialized bricks for making stone troughs, shotting holes, drainage holes, and roof capping. Tiles can be classified as flat tiles and tubular tiles. The production of bricks and tiles mainly consists of two major parts: the preparation of clay molds and the firing process.
Production of clay molds
The production of clay molds includes processes such as soil selection, clay preparation, site leveling, and mold making.
1. Soil selection: High-quality soil is the foundation for ensuring the solidity of various brick and tile components. To make bricks and tiles, it is necessary to choose loess soil, as loess has high plasticity and is less likely to crack after drying. Loess also comes in different types, such as bean cake soil, clay, and white sandy soil, among which only white sandy loess is suitable for brick and tile production.
The soil materials for making bricks and tiles also differ. Since the Great Wall bricks have larger dimensions, white sandy loess with sand and stones below 3 centimeters in diameter can be used for brick-making, with a sand content of around 30-40%. Tiles, being smaller in size, require soil with smaller sand and stone particles, with a sand content of around 20-30% being suitable.
2. Clay preparation: When preparing clay for brick-making, the selected loess is piled up, with a hollow space in the middle. Then water is poured into the hollow, and after several hours, the clay mixing process begins. During mixing, a pickaxe is used to smash the soil, increasing its stickiness. Generally, after being struck twice, the clay is left to dry until it reaches a consistency before making clay bricks.
The process of preparing clay for tiles is similar to that of clay bricks, with the main difference being the clay smashing. For tile clay, the clay material needs to be struck more evenly and with greater adhesive force, requiring at least three or more strikes.
3. Leveling the ground: When leveling the ground, a layer of fine white sand is spread over the surface to prevent the newly made clay bricks or tiles from sticking to the ground.
4. Making molds: Dried wood is selected for making molds. Typically, for making molds of rectangular bricks, wooden boards with dimensions of approximately 40 cm in length, 20 cm in width, and 1 cm in height are used. They are paired up and arranged in a rectangular shape, with the bottom part assembled using one to three wooden boards measuring about 45 cm in length.
For making molds of square bricks, four wooden boards with dimensions of approximately 40 cm in length, 10 cm in width, and 1 cm in height are used to form a square shape, with a bottom part assembled using wooden boards measuring 45 cm in length and width.
The molds for making tiles consist of several components: the base plate, a wooden core, the clay cylinder that fits onto the core, a fabric sleeve that goes over the clay cylinder, and a wooden block that matches the height and width of the tiles with a thickness of 5 cm. The wooden block has a small iron nail on the upper side measuring 1 cm in height and 2-3 grooves on the lower side.
5. Making adobe bricks: The process of making adobe bricks is simple. Fill the mold with clay, compact it, and place it on the ground. Open the mold, and the adobe brick is complete. Let the adobe bricks sit for 2 to 3 days until the surface turns white. Then, stand the bricks upright to dry. Once they are about 70-80% dry, they can be stored on shelves in preparation for firing.
The process of making tiles is relatively complex. Firstly, the tile mold is placed on a rotating disc. The clay is evenly cut with a wire, forming clay blanks of the same length as the rotating disc. The clay blanks are placed on the tile mold and pressed and beaten by hand until the tile mold and clay stick together. A wooden block of the same height and width as the tile is pressed against the clay surface, and the rotating disc is turned to smooth the surface of the clay blank. The tile mold is then removed from the rotating disc and placed on a leveled area. The mold is removed, and a knife is used to divide the formed clay blank into four parts, cutting about two-thirds of the way through. Once the clay blanks are about 90% dry, they are gently separated by hand and can be placed on shelves for firing.
The firing process of bricks and tiles
Before firing bricks and tiles, the first step is to select a suitable kiln site. A good kiln site should be close to the source of clay to minimize transportation distance. It should also be located on a soil bank, making it convenient for kiln construction. The firing process of bricks and tiles involves several key steps.
1. Loading the kiln: Bricks can be fired separately, but tiles can only be fired together with bricks because they cannot be loaded into the kiln individually.
2. Firing process: The firing process can be divided into four main stages.
Dihuo (low firing): During the initial stage of firing, the kiln is heated with a low fire to drive out the moisture inside the kiln. This stage is considered a drying phase and requires the use of low heat.
Dahuo (high firing): After one day of firing and driving out the moisture inside the kiln, the firing process continues with high heat. The fire should exceed the peepholes, aiming to quickly raise the temperature. This process continues until the surface of the kiln glows brightly and evenly.
Qihuo (uniform firing): During the firing process, when smaller pieces of bricks and tiles have reached the desired temperature while larger pieces are not yet fully fired, adjustments are made to achieve uniform firing. This involves carefully managing the fire to ensure that all the fired products reach the desired level of firing.
Xiahuo (firing the lower part): Also known as "shaking the fire" or "venting," this stage involves using leg-shaped bricks to fire the lower part of the kiln, ensuring proper firing of the products placed at the bottom.
3. Kiln sealing: After the high-temperature firing in the kiln, the bricks and tiles of the Great Wall can exhibit different colors, such as red and black-blue, which are closely related to the firing process.
To achieve a blue color, the temperature during the firing of the clay must reach above 800°C. In ancient times, only dry or wet straw was used for firing bricks and tiles. During combustion, straw requires a large amount of oxygen and produces carbon dioxide, which then forms carbon trioxide. The high-temperature bricks and tiles absorb the carbon trioxide, resulting in a blue color. This process involves the reaction between carbonates and metal oxides. To achieve a pigeon blue color, it is crucial to carefully control the uniform firing process and avoid excessive heating. On the other hand, if the firing process is too intense or prolonged during the transition from uniform firing to lower firing, it may result in a black-blue color. Additionally, factors such as wet materials, damp straw, or inadequate firing can also contribute to the appearance of a black-blue color in the bricks and tiles.
After the kiln firing is completed, the bricks or tiles need to be soaked in water to prevent color fading. If they are not soaked, they are prone to discoloration and can turn red.
Water trough bricks: Approximately every 23 meters along the enclosed city walls, drainage holes are installed to allow water on the surface of the walls to flow out. This requires the construction of a water-blocking facility on top of the water-filled section of the walls, which is higher than the water level. This facility directs the water to flow towards the drainage hole through a channel, and the drainage hole is equipped with a water spout trough. The water-blocking facility is constructed using rectangular-shaped bricks placed horizontally above the water-filled section. The protruding surface of the bricks is rounded by grinding the square edges.
Single-character bricks: These make up the majority among the character bricks. On the surface of the bricks, there is a single character such as "left,"(左) "center," (中) or "right." (右) The characters are written in relief and regular script style, and most of them do not have a border. The characters are printed in the middle of the brick's surface. However, on the walls within the Zunhua area, bricks with a framed "center" character have been discovered. These characters are larger in size and also written in relief and regular script style, but they are slightly off-centered on the surface of the brick.
Two-character bricks: Currently, five types have been discovered: "left one," (左一) "left two," (左二) "left three," (左三) "right one," (右一) and "right two." (右二) In the Daolingzhai kiln in Qianxi County, a batch of "left three" bricks, which were already fired but unused, was unearthed. The "left three" bricks can be classified into four main types: 1) Engraved in intaglio, small-sized characters of regular script with a border, inscribed on the side of the brick; 2) Engraved in intaglio, larger-sized characters of regular script without a border, inscribed on the surface of the brick; 3) Engraved in relief, larger-sized characters of regular script without a border, inscribed on the surface of the brick; 4) "Left three" characters engraved on both the side and the surface of the brick. In the same village of Dalingzhai, "left one," "left two," "right one," and "right two" bricks of similar style were also discovered, which are currently used in local villagers' houses. These bricks are all engraved in intaglio and regular script, with small-sized characters and a border, and they are consistent in style with the "left three" bricks.
Production of ancient Great Wall bricks and tiles
As a World Heritage Site, the Great Wall is the longest and largest construction project in human history. The main components of the Great Wall are bricks, followed by tiles, wood, and stones. Bricks can be further divided into several styles, including bricks used for wall construction, square bricks used for paving the pathway, and specialized bricks for making stone troughs, shotting holes, drainage holes, and roof capping. Tiles can be classified as flat tiles and tubular tiles. The production of bricks and tiles mainly consists of two major parts: the preparation of clay molds and the firing process.
Production of clay molds
The production of clay molds includes processes such as soil selection, clay preparation, site leveling, and mold making.
1. Soil selection: High-quality soil is the foundation for ensuring the solidity of various brick and tile components. To make bricks and tiles, it is necessary to choose loess soil, as loess has high plasticity and is less likely to crack after drying. Loess also comes in different types, such as bean cake soil, clay, and white sandy soil, among which only white sandy loess is suitable for brick and tile production.
The soil materials for making bricks and tiles also differ. Since the Great Wall bricks have larger dimensions, white sandy loess with sand and stones below 3 centimeters in diameter can be used for brick-making, with a sand content of around 30-40%. Tiles, being smaller in size, require soil with smaller sand and stone particles, with a sand content of around 20-30% being suitable.
2. Clay preparation: When preparing clay for brick-making, the selected loess is piled up, with a hollow space in the middle. Then water is poured into the hollow, and after several hours, the clay mixing process begins. During mixing, a pickaxe is used to smash the soil, increasing its stickiness. Generally, after being struck twice, the clay is left to dry until it reaches a consistency before making clay bricks.
The process of preparing clay for tiles is similar to that of clay bricks, with the main difference being the clay smashing. For tile clay, the clay material needs to be struck more evenly and with greater adhesive force, requiring at least three or more strikes.
3. Leveling the ground: When leveling the ground, a layer of fine white sand is spread over the surface to prevent the newly made clay bricks or tiles from sticking to the ground.
4. Making molds: Dried wood is selected for making molds. Typically, for making molds of rectangular bricks, wooden boards with dimensions of approximately 40 cm in length, 20 cm in width, and 1 cm in height are used. They are paired up and arranged in a rectangular shape, with the bottom part assembled using one to three wooden boards measuring about 45 cm in length.
For making molds of square bricks, four wooden boards with dimensions of approximately 40 cm in length, 10 cm in width, and 1 cm in height are used to form a square shape, with a bottom part assembled using wooden boards measuring 45 cm in length and width.
The molds for making tiles consist of several components: the base plate, a wooden core, the clay cylinder that fits onto the core, a fabric sleeve that goes over the clay cylinder, and a wooden block that matches the height and width of the tiles with a thickness of 5 cm. The wooden block has a small iron nail on the upper side measuring 1 cm in height and 2-3 grooves on the lower side.
5. Making adobe bricks: The process of making adobe bricks is simple. Fill the mold with clay, compact it, and place it on the ground. Open the mold, and the adobe brick is complete. Let the adobe bricks sit for 2 to 3 days until the surface turns white. Then, stand the bricks upright to dry. Once they are about 70-80% dry, they can be stored on shelves in preparation for firing.
The process of making tiles is relatively complex. Firstly, the tile mold is placed on a rotating disc. The clay is evenly cut with a wire, forming clay blanks of the same length as the rotating disc. The clay blanks are placed on the tile mold and pressed and beaten by hand until the tile mold and clay stick together. A wooden block of the same height and width as the tile is pressed against the clay surface, and the rotating disc is turned to smooth the surface of the clay blank. The tile mold is then removed from the rotating disc and placed on a leveled area. The mold is removed, and a knife is used to divide the formed clay blank into four parts, cutting about two-thirds of the way through. Once the clay blanks are about 90% dry, they are gently separated by hand and can be placed on shelves for firing.
The firing process of bricks and tiles
Before firing bricks and tiles, the first step is to select a suitable kiln site. A good kiln site should be close to the source of clay to minimize transportation distance. It should also be located on a soil bank, making it convenient for kiln construction. The firing process of bricks and tiles involves several key steps.
1. Loading the kiln: Bricks can be fired separately, but tiles can only be fired together with bricks because they cannot be loaded into the kiln individually.
2. Firing process: The firing process can be divided into four main stages.
Dihuo (low firing): During the initial stage of firing, the kiln is heated with a low fire to drive out the moisture inside the kiln. This stage is considered a drying phase and requires the use of low heat.
Dahuo (high firing): After one day of firing and driving out the moisture inside the kiln, the firing process continues with high heat. The fire should exceed the peepholes, aiming to quickly raise the temperature. This process continues until the surface of the kiln glows brightly and evenly.
Qihuo (uniform firing): During the firing process, when smaller pieces of bricks and tiles have reached the desired temperature while larger pieces are not yet fully fired, adjustments are made to achieve uniform firing. This involves carefully managing the fire to ensure that all the fired products reach the desired level of firing.
Xiahuo (firing the lower part): Also known as "shaking the fire" or "venting," this stage involves using leg-shaped bricks to fire the lower part of the kiln, ensuring proper firing of the products placed at the bottom.
3. Kiln sealing: After the high-temperature firing in the kiln, the bricks and tiles of the Great Wall can exhibit different colors, such as red and black-blue, which are closely related to the firing process.
To achieve a blue color, the temperature during the firing of the clay must reach above 800°C. In ancient times, only dry or wet straw was used for firing bricks and tiles. During combustion, straw requires a large amount of oxygen and produces carbon dioxide, which then forms carbon trioxide. The high-temperature bricks and tiles absorb the carbon trioxide, resulting in a blue color. This process involves the reaction between carbonates and metal oxides. To achieve a pigeon blue color, it is crucial to carefully control the uniform firing process and avoid excessive heating. On the other hand, if the firing process is too intense or prolonged during the transition from uniform firing to lower firing, it may result in a black-blue color. Additionally, factors such as wet materials, damp straw, or inadequate firing can also contribute to the appearance of a black-blue color in the bricks and tiles.
After the kiln firing is completed, the bricks or tiles need to be soaked in water to prevent color fading. If they are not soaked, they are prone to discoloration and can turn red.
