Dry ash is usually made from burnt coal ash or ash from a wood-fired stove. A large amount of ash is needed. Gu Qingcheng instructed every household to stockpile the dry ash produced during cooking. For every basket of ash they collected, they were to deliver it to the brick and tile factory. This solved the issue of dry ash.
The collected ash had to be sifted through a sieve to remove pebbles and impurities before it could be used.
With everything ready, they began mixing the mud.
First, pebbles were picked out of the mud. Then, water was sprinkled to soak it. This is where the cattle from the pasture came in handy. Several cows were brought daily to tread on the mud in circles. After the soil was softened, the mud was formed into large chunks and placed next to the workbench, ready for making red brick embryos.
During brick making, the brick mold was placed on the workbench. The mold was a wooden frame. Prepared wooden boards for placing the brick embryos were ready, as was the ash. Using a saw, a piece of mud was cut from the mixed soil. The craftsman would then slam the mud onto the workbench a few more times, shaping it to fit the mold.
If the mold stuck to the mud, ash would be smeared inside the mold as well. The shaped mud block was slammed into the mold, and then the saw was used to cut it from the mold. With the right hand, a board for holding the brick embryo was inserted. The mold was quickly flipped over with the left hand, and the board was pushed in with the right. This way, the brick embryo was produced.
Gu Qingcheng saw that Master Zhuang was indeed an experienced hand. If the brick embryo sometimes got stuck, Master Zhuang would skillfully press on the side of the mud brick with his thumbs, and the embryo would fall out.
It was only when Gu Qingcheng observed this detail that she realized this was why she had often seen two finger holes on some red bricks.
Through the repeated operations of the brick-making masters, red brick embryos were made one by one.
After four or five were made, they were stacked into a tower. Another person would then carry the brick embryos to the drying ground.
There were also specific ways to stack the bricks. They couldn't be placed too close together, and the bottom bricks shouldn't bear too much weight. They also needed to be stacked so they wouldn't easily fall. Generally, they were staggered, leaving a gap roughly the thickness of a brick.
Once sun-dried, the mud embryos were completed, awaiting firing in the kiln. If it rained, the finished bricks had to be covered with film or straw. If the sun was too strong, they couldn't be exposed to direct sunlight and needed to be covered with straw accordingly.
It turned out that before being fired, brick embryos were such "delicate" things.
At this time, the kiln for firing was also urgently dug during the brick-making process.
Once the brick embryos were completely dry, they could be placed in the kiln for firing, which was the most crucial step. Master Zhuang explained that ordinary bricks were fired using coal as fuel, while denser filter-slurry bricks were slowly fired using fuels like wheat straw and pine branches.
After more than ten days of firing, the bodies were sintered.
Through Master Zhuang's explanation, Gu Qingcheng learned that even though clay was the raw material for both, fired blue bricks would be even stronger and more durable.
Blue bricks are made by pouring water directly into the kiln after the brick embryos are fully fired. Because the temperature in the kiln is very high at this point, the water immediately turns into steam upon contact, effectively blocking air. In this oxygen-deficient environment, the ferric oxide in the bricks forms ferrous oxide, ultimately turning them into blue bricks.
Simply put, adding water during the firing process produces blue bricks, while not adding water produces red bricks. However, the raw materials are basically the same.
Nevertheless, blue bricks outperform red bricks in terms of resistance to oxidation, hydration, and atmospheric erosion. The ancient "Qin bricks and Han tiles" that have remained intact for over a thousand years are a testament to the excellent performance of blue bricks.
Blue bricks are stronger than red bricks, have better alkali resistance, and are more durable. However, the firing process for blue bricks is complex, consumes more energy, has a lower yield, and is more expensive, thus their price is 2 to 3 times that of red bricks.
Master Zhuang said that he primarily fired blue bricks before, which commanded higher wages.
Seeing his confidence, Gu Qingcheng naturally let him focus on firing blue bricks.
Making tiles, compared to making bricks, was relatively more complicated, but the initial process of making mud embryos was the same, only the subsequent steps differed.
Many tools are needed to make tiles. Brothers Da Lü and Xiao Lü introduced to Gu Qingcheng tools such as tile tubes, tile sleeves, polishing plates, wire bows, and rotating platforms.
First, the kneaded mud embryo was formed into a "tile embryo" measuring 120 centimeters long, 30 centimeters wide, and 150 centimeters high, and placed in a cool place next to the rotating platform.
Next came making the tiles. The tile tube was placed on the circular rotating platform, covered with a tile sleeve made of cotton cloth. Using the wire bow, a tile embryo piece about two centimeters thick was cut from the tile embryo. The craftsman had to evenly lift the tile embryo piece with both hands and wrap it around the tile tube. While rotating the platform with the left hand, the right hand held the polishing plate, dipped in water, to evenly pat the tile embryo piece until the surface was smooth and even.
Once these steps were completed, the handle of the tile tube was gripped, and the tube was lifted and placed into a bamboo basket filled with sawdust, allowing the bottom of the tile tube to be coated with sawdust. Then, the tile tube was placed on a flat surface. The locking buckle on the tile tube was removed by hand, and it was pulled upwards with force, separating the tile tube from the tile embryo piece, forming a cylindrical tile embryo.
Because there was a layer of gauze tile sleeve between the tile tube and the tile embryo piece, they did not stick together and separated relatively easily.
The tile tube was designed with five subtle dividing lines. After the cylindrical tile embryo piece was air-dried, a gentle tap on the sides of the cylinder with the hand would cause the cylindrical tile embryo piece to split into four tiles. However, this tested the experience and skill of the craftsman. If one was not an expert, a gentle tap with the hand could easily turn it into a pile of crushed mud.
Da Lü said that one tile tube could make four tiles at a time, and one person could make over a thousand tiles in a day.
But despite the large number of tiles made, when all the processes were combined, they used to be paid by the day, with an estimated daily wage of less than ten copper coins. It was hard and tiring work.
After the tiles were shaped, they were stacked neatly, placed vertically on the embankment to air dry. After about a month of drying, they could be fired in the kiln.
The firing process for bricks and tiles was similar; they could be fired separately or mixed together.
During mixed firing, bricks were heavier and had better load-bearing capacity, so they were placed at the bottom. Tiles were fragile and were placed at the top.
During firing, the stacking technique for bricks and tiles required experienced masters.
The stacking required a mix of horizontal and vertical arrangements, with gaps left for ventilation at certain intervals. Charcoal was used to fill these gaps. A certain proportion of bricks and tiles was matched with a certain amount of charcoal to ensure the charcoal could fire the bricks and tiles thoroughly.
A traditional earth brick kiln could fire 30,000 bricks and 50,000 tiles at once. Loading the kiln took an entire day. After loading, the kiln door, skylight, and ground windows were sealed. The kiln door was designed to resemble a stove, with a fire pit inside and an observation window.