Accurate process control of single-crystal and directionally-solidified castings requires knowledge of the exact location of the solidifying front. If the front advances too rapidly, single crystal growth in a preferred orientation degenerates into the formation of polycrystals. A solidification front which moves more slowly than necessary is wasteful of the casting resources. A sensing technology is being developed which determines the location of the boundary between a solidifying crystal and liquid metal. The sensing method utilizes the ordered pattern of x-rays diffracted from the solid as an absolute indicator of the liquid-crystal interface.