Spurious marks hamper accurate dating of blue and white porcelains; a specific example is given of two pieces with the same nien-hao, but technical variations indicate the spurious plate dates between 1436-1464. Textural traits of the unglazed base identify Yung-lo (1403-1424) products. The multiple layering of the cobalt pigments was fully refined during the 15th century. Examples of the cobalt process in reverse are very rare. The quality of porcelains intended for export trade indicates less care in the manufacture. -- AATA