Ru ware belongs to the Five Great Kilns identified by later Chinese writers, but it stands apart from the others in its exclusivity. These pieces were made specifically for the imperial court during the Northern Song dynasty, likely under the personal direction of the Huizong Emperor, who reigned from 1100 to 1125. One contemporary source, Zhou Hui, claimed the glaze contained agate, and indeed the kiln site at Qingliangsi in Baofeng County, Henan, sits near an agate mining area. Modern analysis, however, attributes the distinctive pale blue to dissolved iron oxide with minimal titanium dioxide. The glaze was applied in multiple layers and covered the entire piece -- rims, bases, everything -- creating an effect described as "like lard dissolving, not flowing." This all-over technique, apparently invented at the Ru kilns, made the pieces resemble jade, the most prestigious material in Chinese art.

The pieces are small and intimate: brush-washers for a calligrapher's desk, cups, wine bottles, incense burners, and oval planters for daffodils. Because the glaze covered the entire surface, the potters could not set them directly on kiln shelves. Instead, each piece was balanced on three or five tiny metal spurs during firing, which left small unglazed oval marks on the underside -- marks the Chinese call "sesame seeds." Many pieces display a subtle crackled pattern in the glaze, though evidence suggests the most prized examples were those without cracking, and the effect was not deliberate. The fired clay body beneath is a light grey, sometimes compared to the color of incense ash, and the ware occupies an ambiguous category -- stoneware by Western standards but fired at relatively low temperatures.

Production ended abruptly. In the 1120s, Jurchen invaders from the north overthrew the Northern Song dynasty, capturing the Huizong Emperor himself. His younger son fled south and established the Southern Song court, but the Ru kilns now lay in enemy territory. The workers dispersed. A record from 1151 notes a gift of just 16 Ru pieces to the exiled emperor -- a pitifully small number by imperial standards, already suggesting their extreme rarity. When the aged retired emperor visited a garden in 1179, a single Ru vase was placed there for his admiration. In the south, potters attempted substitutes in Guan ware, producing pieces more green than blue, but nothing matched the originals.

The Qianlong Emperor, who reigned from 1736 to 1795, described Ru ware in a poem as "rare as stars at dawn." He was a passionate collector who likely owned at least half the surviving examples, and he had poems engraved on their bases -- sometimes unknowingly writing on 18th-century imitations that had been made for his father. The 87 complete pieces catalogued in 2017 are scattered across the world's great museums: 21 in the National Palace Museum in Taipei, 17 each in the British Museum and the Palace Museum in Beijing, 9 in the Shanghai Museum, with single pieces in Osaka, Cleveland, Philadelphia, and Oxford. New identifications still occur -- a bowl was recognized in Dresden's porcelain collection as recently as 2021.

The kiln site itself covers 250,000 square meters in the village of Qingliangsi, with kilns densely distributed throughout. First identified in 1950, the site was confirmed in 1977 when ceramic historian Ye Zhemin found a sherd that proved chemically identical to a known Ru piece in Beijing. Excavations beginning in 1987 revealed that the kilns produced far more than imperial ware -- black ceramics, three-color wares, and carved pieces of lesser quality all came from the same complex. The official-quality kiln and workshop were uncovered in 2000. Sherds found there include shapes more elaborate than any surviving whole piece, possibly test pieces for designs never put into production. The ruins preserve the ghost of an industry that, for one brief generation, created objects now worth more per gram than gold.