This rare brush washer was made for the Qianlong emperor in deliberate imitation of Imperial Ru wares produced in Northern Song dynasty. Song Ru wares have captured the imagination of collectors ever since they were first made, but surviving examples from the Northern Song period are very rare.
Compare a Ru-type washer of this form, but with a four-character Yongzheng seal mark, included in the Min Chiu Society exhibition, Monochrome Ceramics, 1977, Catalogue no. III. An unmarked example is illustrated in the Kempe Collection, Catalogue, no. 202.