During the reigns of the Yongzheng and Qianlong emperors there was significant experimentation at the imperial kilns to increase the range of fine monochrome wares. Subsequently, during the early and mid-18th century, the number of different monochrome glazes produced at Jingdezhen expanded considerably. The aim was not only to produce attractive colors, but also to achieve interesting textures. Thus, glazes such as the imaginatively termed 'robin's egg' and 'eel skin' were developed. Often these glazes were refinements of earlier admired glazes. 'Robin's egg', or lu Jun (furnace Jun), is generally viewed as a free interpretation of Song dynasty Jun glazes.