This brush washer is made of 'marbled' or xipi (rhinoceros hide) lacquer. This technique involved the application in layers of different colors of lacquer over an uneven ground. It is then polished so that the different colors appear in a marbled pattern over the surface of the vessel.
Compare, a cushion shaped circular box and cover made in the same xipi technique, dated 16th-17th century from the Jaehne Collection in the Newark Museum, and illustrated by V. Reynolds and Y. Pei in Chinese Art from the Newark Museum, China House Gallery/China Institute in America, New York, 1980, p. 45, no. 24. Compare, also, another similarly decorated flat marbled lacquer round box, 16th century, illustrated by Sir Harry Garner in Chinese Lacquer, London, 1979, p. 119, pl. 58.