By the mid-second millennium BCE, the large Neolithic settlements were in trading contact with each other. Marine shells, ornaments, and stone tools were extensively traded, and along these routes passed knowledge of techniques such as copper smelting, bronze alloying, and pottery production. Bronze metallurgy is known from western China in the third millennium BCE. From there, knowledge of early techniques probably diffused east to give rise to the wonderful Chinese Shang bronzes, and could well have also percolated south to the northern part of the Khorat Plateau. There, several important excavations have revealed a flourishing bronze-age civilization dating from between 1,500 and 500 BCE, when bronze began to give way to iron.