cast iron vs pig iron

cast iron

noun
  • A hard and brittle, but strong, alloy of iron, carbon, and silicon, formed by casting in a mould. 

adj
  • Inflexible or without exception. 

  • Made of cast iron. 

  • Durable; tough; resilient. 

pig iron

noun
  • A type of crude, brittle iron shaped like a block, unsuitable for working but suitable for casting, commonly used as an industrial raw material. 

  • crude iron 

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