The fine-grained, plastic sediment — shaped when wet, rigid when dried, fused when fired — that served as humanity's first durable fired vessel, first building block, and first writing surface, and whose abundance in river valleys made it the quiet substrate of the earliest cities, the oldest libraries, and the potter's art.
In the ruins of the ancient city of Nippur, archaeologists found tens of thousands of clay tablets. Many of them are school exercises: a student's hand pressing a stylus into wet clay, making the wedge-shaped marks that the Mesopotamians called cuneiform. The student made a mistake and smoothed the tablet flat and started again. The clay remembered everything. When the city burned — perhaps around 1720 BCE — the fire that destroyed it accidentally baked those tablets hard, preserving them for four millennia. A child's arithmetic lesson became part of one of the great surviving archives of the ancient world. Clay did not preserve these records on purpose. It just did what clay does: it holds the shape of whatever presses into it, then holds that shape forever when the heat comes.