noun (n)

• worthless or dangerous material that should be removed(noun.substance)

  Synonym:
  impurity
  

  source: wordnet30
• the scum formed by oxidation at the surface of molten metals(noun.substance)

  Synonym:
  scoria, slag
  

  source: wordnet30
• The scum or refuse matter which is thrown off, or falls from, metals in smelting the ore, or in the process of melting; recrement.(noun)

  source: webster1913