A name for the pattern.
Tortoiseshell glaze takes its name from the mottled appearance of dark glaze and golden-brown markings. The term describes a ceramic surface, not the use of animal shell.
Variation in the kiln.
Glazes with different iron contents interact during firing, creating flowing patches of colour. The pattern is part of the character of Jizhou ceramic decoration.
Look closely at a cup.
Follow the transition from a dark area to a lighter marking, then consider the form of the cup. Some Nanyao pieces combine this surface with painted motifs.
Further reading: The Palace Museum . Product photographs are from Nanyao.