According to Zhou Hui of the Southern Song dynasty in his Qingbo Miscellany: “Ru ware was fired for the palace under strict prohibition, and its glaze contained agate. Apart from pieces intended for the imperial court, no other works could be sold openly. In recent times, they have become especially difficult to obtain.”

This indicates that the Ruzhou region itself was a source of agate, with abundant supplies of unprocessed material. It also clearly shows that agate was indeed used in the glaze of Ru ware celadon.

In fact, resource-rich Ruzhou was a gathering place for merchants during the Song dynasty, with a deep cultural heritage. With the advent of the prosperous Zhenguan era, Ruzhou’s economy flourished as never before, and the mountains along both banks of the Ru River held abundant agate suitable for ceramic production.

The glaze on Ru ware celadon has the gentle warmth of ancient jade, with a soft luster that is never harsh. In practice, the agate in the glaze played an important role in giving the surface its lustrous quality. Variations in agate quality directly affected the texture and color of the Ru ware glaze, lending each piece a subtle, distinctive beauty.

Compositional analysis of Ru ware sherds excavated from Ru kiln sites, together with archaeological literature, provides further evidence that agate was indeed added to the glaze during the firing of Ru ware in the Song dynasty.

Why, then, was agate used in the glaze? In practice, it was found to have two main effects:

1. It helps produce the sky-blue glaze
   Adding agate (silicon dioxide) to the body and glaze can improve a low-silica, high-alumina composition, lowering the glaze’s firing temperature. At the same time, the firing temperature of the body acts as a constraint, helping to produce a sky-blue glaze.

2. It enhances the milky, jade-like appearance
   Agate promotes the formation of abundant anorthite crystals, giving the glaze a milky, jade-like appearance while creating a soft sky-blue hue.

Thus, although Song-dynasty Ru kiln workers lacked modern knowledge of chemistry and physics, through adding agate, controlling kiln temperatures, and conducting extensive experiments and repeated adjustments, they ultimately created a sky-blue glaze with a rich, milky, jade-like quality—giving rise to this unique treasure of Eastern art.