Plastromancy

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Replica of an oracle turtle shell

Plastromancy (from plastron + Greek μαντεία, "divination") is a form of pyromantic divination using the plastron, or undershell of a turtle. It was mainly used in ancient China, especially the Shāng dynasty. This gradually fell out of use during the Later or Eastern Hàn dynasty.

The plastrons were first prepared through the boring of shallow pits partway through the interior (flesh-side) surface. During the divination session, the diviner then applied a heat source to the pits, causing cracks to appear on the exterior surface. These cracks were then read as portents or answers to the topic of divination at hand. During the Shāng dynasty, the date, diviner's name and topic of divination were generally written on the plastron, sometimes along with prognostications. Occasionally, verifications about the event or day in question were later added as well. This record was then usually incised into the plastron's surface.

The scapulae of oxen and other animals, and sometimes other bones, and even animal or human skulls were used for such osteomancy (divination using bones). Over a hundred thousand fragments of such oracle bones have been found in archaeological sites in China, particularly from the Shāng dynasty. The inscriptions on the fragments have taught scholars much about the early development of Chinese writing.