This type of brush washer is described as of 'gong' shape, or tangluo xi, as it has a very compressed body.
It belongs to a group of eight vessel shapes referred to as the ba da ma, 'Eight Great Numbers', all covered in a peachbloom glaze and thought to have been devised to serve as requisite appointments for the Emperor's writing table. However, according to J. Ayers, 'The Peachbloom Wares of the Kangxi Period', TOCS, vol. 64, 1999-2000, pp. 31-50, they may have been made as presents to be presented at court. To see a similar washer within the context of a complete set, see S. Valenstein, The Handbook of Chinese Ceramics, New York, 1989 (rev. ed.), p. 237. See, also, the brush washer sold in these rooms, 19 March 2008, lot 636.