While the Ming dynasty is most famous for its blue-and-white ware, this ware was produced primarily for trade with countries in East and Southeast Asia, as well as in Islamic countries to the West and Europe. This trade in blue-and-white porcelain first developed during the Yuan period, and once the Ming emperor Hongwu gained control of China, he took efforts to forbid foreign trade. While these efforts were never entirely successful, it did disrupt the import of cobalt. For this reason, the production of blue-and-white wares temporily decreased in the Flate fourteenth century, while the production of wares decorated with underglaze red, which used copper oxide rather than cobalt, saw a substantial increase. The copper oxide glaze, however, was much more difficult to control than the blue. The red color often bled in the heat of the kiln, and thick applications of glaze sometimes turned varying shades of gray.¹  The best ceramicists, however, often took advantage of these effects, such as we see in this piece, where the dark gray spots are used to add interesting accent to the leaves.

(1) From the British Museum web page dedicated to this porcelain jar.