An optical effect (rather than a pigment) produced the beautiful bright blues of jun ware. The glaze is suffused with minute bubbles which reflect blue light. The unique effect was achieved by a precise combination of factors, including composition of glaze and clay body, thick application of glaze (possibly to prefired vessels), and long kiln cycles for both firing and cooling. Differences in the temperature within the kiln produced variations in the glaze, from pure bright blue, through grayish-blue, to whitish blue.