braze vs toughen

braze

noun
  • A kind of small charcoal used for roasting ore. 

verb
  • To cover with brass, or as with brass. 

  • To join two metal pieces, without melting them, using heat and diffusion of a jointing alloy of capillary thickness. 

toughen

verb
  • To become tough. 

  • To make tough. 

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