harden

v.

1. To make less pervious

   steel, temper, anneal, solidify, mineralize, vitrify, precipitate, hornify, cornify, crystallize, freeze, amalgamate, coagulate, clot, granulate, make callous, make firm, make compact, make tight, make hard, convert to stone, congeal, ossify, petrify, starch, cure, bake, dry, flatten, cement, compact, consolidate, concentrate, sun, fire, fossilize, desiccate, braze, vulcanize, Harveyize, callous, incrassate, indurate, deposit, candy, toughen, fix, concrete, inspissate, encrust, lapidify; see also compress, press 1, stiffen 2, thicken 2.

   Antonyms soften*, unloose*, melt. *

2. To become less penetrable

   solidify, congeal, jell, become dense, fix, settle, become fast, petrify, ossify, freeze over, fossilize, clot, cake, set, firm, curdle, coagulate, close, contract; see also freeze 1, stiffen 1, thicken 1.

   Antonyms liquefy, thaw*, soften. *

3. To toughen

   acclimate, acclimatize, indurate, habituate, inure, season, make callous, make unfeeling, make obdurate, discipline, accustom, train, coarsen, roughen, sear, embitter, develop, strengthen, brutalize, render insensitive, render insensible, render unimpressible, blunt, dull, envenom, numb, stupefy, benumb, stun, paralyze, steel; see also deaden 1, stiffen 2, teach 2.

   Antonyms weaken*, debilitate*, soften.