dross vs scoria

dross

noun
  • Residue that forms on the surface of molten metal from oxidation. 

  • Worthless or trivial matter. 

  • The impurities in metal. 

  • Waste or impure matter. 

  • A waste product from working with metal. 

verb
  • To remove dross from. 

scoria

noun
  • The slag or dross that remains after the smelting of metal from an ore. 

  • Rough masses of rock formed by solidified lava, and which can be found around a volcano's crater. 

How often have the words dross and scoria occurred in a corpus of books? (source: Google Ngram Viewer )