Tom:
               Although one ounce of copper per square foot equates to
          .0014", most fab houses purchase cores with copper cladding
          with an associated tolerance (ex: .0014" +/-.0002").  It
          seems that the copper folks very shrewdly send us copper
          which tends to be on the low end to save money.  Our
          experience has been an average of .0013", but the material
          is still within spec at .0012".  Further board processing
          can remove as much as .0001" during cleaning, so a bare
          minimum (copper at the low end minus processing) would be
          .0011" (1.1 mils).  That is how the terms "one ounce copper"
          and "one mil copper" have become somewhat erroneously
          interchangeable in our industry.
          Regards,
          Tom Coyle
          Field Services Engineer
          HADCO Corporation