Ru porcelain possesses a deep resonance, a green-tinted depth arising from the beautiful, oil-like glaze. These qualities all together give Ru porcelain a multicolored, multilayered appearance. Ru porcelain is highly conductive of heat, thus it is suitable for non-fermented and slightly-fermented teas, such as green tea and high-mountain teas. The Ru kiln was foremost among the five great kiln centers of the Song dynasty. Ru porcelain is famed for the thinness and purity of its clay bodies, the fine workmanship in shaping, and the pure sky-blue color of its glaze. The latter was praised in the classic phrase, like the blue of the sky in a clearing amongst the clouds after rain. The glazed surface always featured very fine, complex crackelure that suggested fish scales or the fine cracks in the ice.