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Explanation: I found you the following reference:http://www.oit.doe.gov/e3handbook/s.shtml
Skim (see also Aluminum Salt Cake, Dross).
A by-product of aluminum-melting processes, skim is an accumulation of oxide with entrapped metal formed on the molten surface. These surface oxides are introduced on all charge components as the result of the molten aluminum being exposed to the atmosphere. "Skimming" is the physical act of removing the skim (or dross) from the surface of the molten aluiminum. Skim is generally considered to have a higher aluminum content than dross (The Aluminum Association, undated).