The Collaborative International Dictionary of English v.0.48:

Crucible \Cru"ci*ble\ (kr[udd]"s[i^]*b'l), n. [LL. crucibulum a
   hanging lamp, an earthen pot for melting metals (cf. OF.
   croisel, creuseul, sort of lamp, crucible, F. creuset
   crucible), prob. of German origin; cf. OHG. kr[=u]sul, LG.
   kr["u]sel, hanging lamp, kroos, kruus, mug, jug, jar, D.
   kroes cup, crucible, Dan. kruus, Sw. krus, E. cruse. It was
   confused with derivatives of L. crux cross (cf. Crosslet),
   and crucibles were said to have been marked with a cross, to
   prevent the devil from marring the chemical operation. See
   Cruse, and cf. Cresset.]
   1. A vessel or melting pot, composed of some very refractory
      substance, as clay, graphite, platinum, and used for
      melting and calcining substances which require a strong
      degree of heat, as metals, ores, etc.
      [1913 Webster]

   2. A hollow place at the bottom of a furnace, to receive the
      melted metal.
      [1913 Webster]

   3. A test of the most decisive kind; a severe trial; as, the
      crucible of affliction.
      [1913 Webster]

   Hessian crucible (Chem.), a cheap, brittle, and fragile,
      but very refractory crucible, composed of the finest fire
      clay and sand, and commonly used for a single heating; --
      named from the place of manufacture.
      [1913 Webster]

WordNet (r) 3.0 (2006):

crucible
    n 1: a vessel made of material that does not melt easily; used
         for high temperature chemical reactions [syn: crucible,
         melting pot]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "crucible":
   Bunsen burner, Erlenmeyer flask, Kipp generator, acid test,
   affliction, agitator, alembic, anvil, aspirator, assay, beaker,
   beater, blank determination, blender, blowpipe, brouillon, burette,
   caldron, calvary, capillary tube, cement mixer, churn, condenser,
   criterion, cross, crucial test, deflagrating spoon, desiccator,
   determination, distiller, docimasy, eggbeater, emulsifier, engine,
   essay, etna, feeling out, first draft, kiteflying, lathe, machine,
   matrass, melting pot, mixer, mortar, motor, ordeal,
   pestle and mortar, pipette, precision balance, probation, proof,
   reagent bottle, receiver, retort, rough draft, rough sketch,
   sounding out, standard, still, test, test case, test tube,
   touchstone, transducer, transformer, trial, tribulation, try,
   verification, visitation