THE color of the sky, a jade-like surface, lines like a crab’s claw, and grains like the stars. These are the words that best describe the Chinese ruci, or ru porcelain, which features the tian qing glaze, a name that translates into “pure sky green.”

The subtle and tranquil color is unique in its own way, and looking at it is like peering into clear sky. Ruci was produced for the imperial court of the Song Dynasty (960-1279) and the major kilns were located near the capital of Northern Song, which is today’s Ruzhou City.

Ruci in general has a few variations in color, like the more common pea green or fresh green. But it’s the sky green glaze that’s the most valuable, and many emperors valued as the greatest of treasures the ruci with this special glaze.

The idea of an imperial court-designated kiln, or royal kiln, started with Song Dynasty Emperor Huizong, a ruler who was known for his artistic achievements. The Song Dynasty had five famous royal kilns, the ruci kiln, Northern Song royal kiln, Jun kiln, Ding kiln and Ge kiln.

Emperor Huizong set up the royal ruci kiln for one important reason: To pursue the philosophy and aesthetics of Taoism through creating a sky green glaze. His motivation was Chinese Taoism believes the sky is clear because it’s the beginning of infinity and the origin of all things.

When the Northern Song Dynasty ended with endless wars, the royal kilns were destroyed, making the ruci a rarity ever since the Southern Song Dynasty.

For porcelain makers, ruci is a curious and unpredictable product because the coloring is not based on any chemical colorant or furnace transmutation. In other words, imitating the sky green glaze cannot be done by analyzing the ceramic chips and adding modern colorants.

The sky green glaze of ruci was lost for hundreds of years amid the changing of regimes. But countless porcelain artists dedicated their careers trying to revive the epic color.

Eventually one man did it after the art had been lost for more than 800 years. Zhu Wenli, born in Ruzhou, Henan Province in 1946, is an inheritor of national intangible cultural heritage and a porcelain maker who successfully recreated ruci wares with their distinctive sky green glaze.

Zhu became an apprentice at the No.2 Ruzhou Ruci Factory in 1975 when he retired from the military with no more than high school qualifications at age 29. Zhu was particularly interested in chemistry and became assigned to the team working on recreating the pea green glaze. This was achieved after three months. With a strong interest in ruci as well as history and archaeology, Zhu started his long journey to recreate the sky green glaze.

“Ruci is very mysterious. The color of the glaze changes in accordance with the temperature,” Zhu explained. “At the highest temperature (about 1,300 degrees Celsius) other ceramics don’t change color any more. But ruci will gradually change to the sky green color in the cooling process.”

Ruci has another signature trait which is that cracks develop when the glaze cools and it contracts faster than the body. This leads to the glaze stretching and ultimately splitting. One cannot know if a batch of ruci wares are successful until the kiln is opened. The rate of failure is very high.

In 1984, a royal ru porcelain research project was established to produce the sky green glaze by the Ministry of Light Industry and Henan Province Science and Technology Committee. Zhu participated in the project whose initial goal was to complete the task within six months. But there was no success after two years.

When the project team was disbanded, Zhu kept going. He studied ancient folk legends about famous kilns and found one story about ruci that implied adding bones may be a key. He started experimenting with pig and sheep bones and created 328 formulas, producing each one at least three times. Zhu spent over three years, trying more than 1,500 times. And still he failed.

“I almost lost my confidence. In 1987, I told myself that this is the last possible formula and my last hope,” he recalled. “I was exhausted. I finished the last batch and didn’t have the strength to open the kiln. I slept for a few days and remembered the kiln was still shut.”

What he saw when opening the kiln was like a lightening strike, Zhu said. Four porcelain items in that last batch had the color sky green.

The results of his experiments were affirmed by two porcelain experts in Beijing, Li Guozhen and Feng Xianming. Feng wrote: “The ruci glaze color is the most difficult to imitate. Jingdezhen had tried it in the Ming and Qing dynasties but failed. You’ve made it happen, bringing back the sky green glaze once again.

“The key of firing the sky green glaze is to control the temperature during the cooling process. If it’s too fast, the porcelain would be pea green or grayish green. Only the correct cooling speed can have the right result,” he said.

Zhu’s achievements have not been limited to producing the impossible ruci wares. He’s also a leading expert in ceramic archaeology and has found several sites of ancient ruci kilns.

“Ruzhou is famous for the ruci, and we are proud of it. But are there royal kilns located inside the city was a question I always asked myself after one royal kiln site was discovered in Qingliang Temple, Baofeng County,” Zhu said.

His persistence led to another amazing discovery ­— finding the Zhanggongxiang Kiln.

“At the time, I thought if you are looking for historic sites of ruci kilns, you need to check the foundations people dig when building new houses. It’s a stupid method, but very effective,” he said.

No matter which family was digging the ground to build foundations, Zhu would stay watching from morning to dusk. In March, 1999, he discovered some ceramic relics of the same color as ancient ruci at a residential building site near Wen Temple.

In the following year’s spring, Zhu found dozens of porcelain items at a construction site near Zhanggongxiang and reported them immediately to the cultural affairs bureau. A team of archeologists went to Ruzhou where they dug a 10 meter-long, 2.5 meter-wide pit and recovered some relics.

Zhu took a few of the relics to Shanghai Museum in 2001. Three experts compared the pieces with the only four specimens from ancient royal kilns preserved in the museum. They confirmed the relics were made of the same porcelain and were from the same kiln.

Wang Qingzheng, an ancient ceramics expert, said that the four specimens at Shanghai Museum had finally found their original home and he suggested the Zhanggongxiang kiln site could be the royal kiln of the Northern Song dynasty. The idea was confirmed in 2004 after more evidence was discovered in Ruzhou.

Zhu didn’t stop with his success in finding the Zhanggongxiang kiln site. In 2012, he found some new ceramic relics near Jiangtai in Ruzhou that fit the description of the Chai kiln from the Five Dynasties period (around the 10th century).

The recovered pieces had a glaze as thin as paper. Historically, royal porcelain has been forbidden in folk culture, apart from Ruzhou, and there’s no place to find the right ingredients for the sky green glaze.

Finding historic royal kiln sites has helped Zhu understand how the famous porcelain arts are connected.

“It’s like finding the gene,” he said. “The Northern Song royal kilns and others all have the Chai kiln and Ru ci kiln gene, so it’s necessary to find the kiln sites,” he added.

“Ruci is the greatest treasure our ancestors gifted Ruzhou,” he said. “It’s our responsibility to inherit and carry it forward.”