The high artistic and technical qualities of Shang dynasty bronze vessels reflect the sophistication of the piece mold casting technology employed. The microstructures and elemental compositions of a group of mold fragments said to come from Anyang together with a range of pottery and potential raw materials have been examined using optical and scanning electron microscopy (SEM) with energy dispersive x-ray analysis. The molds are made from loess, a fine-grained wind-blown sediment which is abundant in north China. While some of the pots are also of loess, they contain a much higher clay fraction than do the molds. The loess used for the molds was selected to have a low clay, low lime, and low coarse-particle content to give the high porosity and dimensional stability required of casting molds. Mold technology evolved away from other ceramic technologies, as an integral part of the bronze working tradition.