molten metal 42 for best performance the impeller 56 should be disposed within the range of about 4 12 inches beneath the surface of the molten metal 42 the shaft 54 and the impeller 56 usually will be made of graphite particularly if the molten metal being treated is aluminum other materials such as ceramics or castable refractory compositions could be employed if desired if graphite is used it preferably should be coated or otherwise treated to resist oxidation and erosion oxidation and erosion treatments for graphite parts are practiced commercially and can be obtained from sources such as the carborundum company metaullics system division 31935 aurora road solon ohio 44139 referring now to fig5 and 6 the impeller 56 includes a relatively thin rectangular prism having an upper face 58 a lower face 60 and sidewalls 62 64 66 68 the faces 58 60 are parallel with each other as are the sidewalls 62 66 and the sidewalls 64 68 the faces 58 60 and the sidewalls 62 64 66 68 are planar surfaces which define sharp right angled corners 70 the sidewalls 62 66 have a width identified by the letter a while the sidewalls 64 68 have a depth indicated by the letter b the height of the impeller 56 that is the distance 10 between the upper and lower faces 58 60 is indicated by the letter c preferably dimension a is.