Definition of harden :
1. To accustom by labor or suffering to endure with constancy; to strengthen; to stiffen; to inure; also, to confirm in wickedness or shame; to make unimpressionable.
2. To become confirmed or strengthened, in either a good or a bad sense.
3. To become hard or harder; to acquire solidity, or more compactness; as, mortar hardens by drying.
4. To make hard or harder; to make firm or compact; to indurate; as, to harden clay or iron.
ossify, clear, starch, bubble, unloose, falter, caseharden, liquefy, compress, normalize, cement, close, grip, indurate, season, boil over, debilitate, lead to, benumb, boil up, callous, break down, hold fast to something, have the courage of your (own) convictions, pack a punch, petrify, dull, granulate, soften, give, teach, flatten, acclimatize, contract, dry, boil away, catch, anneal, train, embitter, skew, sharpen, compact, accustom, vulcanize, gleam, stun, amalgamate, envenom, sun, bake, sear, candy, chasten, crumple, settle, color, creep, concentrate, flash, intensify, furrow, acclimate, stand firm, decide, remain, numb, cake, hold to, moderate, fix, boil, deaden, habituate, press, not give/budge an inch, express, steel, discipline, cleave to, thaw, crease, inure, stupefy, curdle, deposit, concrete, brutalize, clot, paralyze, fire, precipitate, solid, weaken, stand/hold your ground, cross, resist, flavor, coarsen, desiccate, influence, freeze, stir, continue, mollify, develop, cure, flavour, strong, atomize, inspissate, consolidate, hold out for, crystallize, blunt, cloud, bolster, beef, confirm
congeal (part of speech: verb)
strengthen (part of speech: verb)
harden (part of speech: verb)