zinc-scumhttp://www.oed.com/dictionary/zinc-scum_n%3Ftab%3Dmeaning_and_usezinc scum, n.
First published 1921; not fully revised

zinc scumnoun

  1. 1881–
    1. 1881
      Zinc-scum, the zinc-silver alloy skimmed from the surface of the bath in the process of desilverization of lead by zinc.
      Transactions of American Institute of Mining Engineers 1880–1 vol. 9 192

Originally published as part of the entry for zinc, n.

zinc, n. was first published in 1921; not fully revised.