The mechanical properties of ductile iron (DI) and compacted graphite iron (CGI) iron are measured and reported on standard machined specimens (as per ASTM). However, most castings retain most of the as-cast surface. This surface layer (the casting skin) includes both surface and subsurface features. Because of the casting skin, the mechanical properties of the part are in most cases significantly lower than those found on the standard ASTM machined specimens. After reviewing the current understanding of the current understanding of the mechanism of formation of the casting skin in cast iron, the authors propose a magnesium diffusion theory of skin formation in compacted graphite cast iron that includes the role of natural convection during solidification. The theory is supported by a computational model and experimental measurements of compositional changes at the casting surface.