#scum  a film of impurities or vegetation that can form on the surface of a liquid
  supertype:  #film  a thin coating or layer; "the table was covered with a film of dust"
  subtype:  #slag__scoria__dross__dros  the scum formed by oxidation at the surface of molten metals
     subtype:  #basic_slag__basicslag  slag produced in making steel; low in silica but having large amounts of calcium phosphate; useful as fertilizer

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