R. J. Anderson 
Inclusions in Aluminum-Alloy Sand Castings [PDF ebook] 

Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. In regard to hard spots in aluminum-alloy castings, our experience with this trouble is that it usually comes from using remelted aluminum which has been run down from borings, where the iron content has not been eliminated before melting. Iron chips in aluminum borings will not melt, but they ball-up with aluminum on the outside of each chip and are suspended in the metal. This iron chip or boring (hard spot) oxidizes and becomes very hard after melting once or twice. These hard spots will not segregate, but will be held in suspension throughout the casting in different parts. At least this is our theory of this trouble, be cause for several years we were bothered very much, and found out that it was due to using remelted aluminum made from aluminum chips containing iron borings. After we stopped using such aluminum, we have had no further trouble.