thomas slag

from The Collaborative International Dictionary of English v.0.48

Thomas phosphate \Thom"as phos"phate\ or Thomas slag \Thom"as
slag\ .
   Same as {Basic slag}, above.
   [Webster 1913 Suppl.]

from The Collaborative International Dictionary of English v.0.48

Basic slag \Basic slag\
   A by-product from the manufacture of steel by the basic
   process, used as a fertilizer. It is rich in lime and
   contains 14 to 20 per cent of phosphoric acid. Called also
   {Thomas slag}, {phosphatic slag}, and {odorless phosphate}.
   [Webster 1913 Suppl.]