Jizhou Ware Tortoiseshell Tea Bowl, Southern Song-Yuan dynasty, 12th-13th century. Diameter: 11.2 cm. Height: 6.5 cm. © 2024 Kaikodo

The bowl, a product of a kiln in Jizhou, Jiangxi province, is of rounded form resting on a roughly cut foot and slightly recessed base where the pale buff-colored, somewhat course body, not unusual for this ware, is visible. To create the "tortoiseshell" effect the bowl was covered inside and out with a combination of an opaque dark-brown glaze with a lighter caramel-colored glaze of greater translucency, the lighter glaze pulling the darker glaze in drips down the interior walls and more randomly on the exterior.

For more in formation,

https://www.kaikodo.com/wp-content/uploads/2023/09/aaa20-Jizhou-TortoiseTeabowl-2767.pdf