Although the density of solid alumina is similar to that of aluminum melt, it takes a long enough time to sink to the bottom of the crucible after entering the inner part of the aluminum melt. However, the alumina film formed by oxidation of aluminium melt is compact only on the one side which contacts with aluminium melt. The one side which contacts with air is loose and has a large number of pores with diameters of 60-100A. Its surface area is large and its adsorption is strong. It is easy to adsorb water vapor and has the tendency to float. Therefore, the specific gravity difference between the oxide film and the aluminum melt is small, and when the oxide film is mixed into the melt, the floating and sinking speed is very slow, so it is difficult to remove from the melt, and the porosity is too inclusion in the casting. Therefore, the key to transfer the aluminum melt is to minimize the stirring of the molten metal and to minimize the contact between the melt and the air.