Directional solidification is a type of solidification that occurs within castings. In other words, it occurs in such a manner that liquid feed metal is always available for that portion that is just solidifying. Directional solidification can also be used as a purification process. Most impurities are more soluble in the liquid phase than in the solid phase during solidification. As a result, these impurities will be pushed by the solidification. This causes the finished casting product to have a lower concentration of impurities than the feedstock material. The last solidified metal, however, will be filled with impurities. Thus, that last piece can be recycled.