Source : Webster's Revised Unabridged Dictionary (1913)

   {Melting pot}, a vessel in which anything is melted; a
      crucible.

Source : WordNet®

melting pot
     n 1: an environment in which many ideas and races are socially
          assimilated
     2: a vessel made of material that does not melt easily; used
        for high temperature chemical reactions [syn: {crucible}]