CUPEL is a 5 letter word that starts with C.

1. \Cu"pel\ (k[=u]"p[e^]l), n. [LL. cupella cup (cf. L.
   cupella, small cask, dim. of cupa) : cf. F. coupelle. See
   {Cup}, and cf. {Coblet}.]
   A shallow porous cup, used in refining precious metals,
   commonly made of bone ashes (phosphate of lime). [Written
   also {coppel}.]
   
   {Cupel dust}, powder used in purifying metals.
   
   

2. \Cu*pel"\ (k[-u]*p[e^]l"), v. t. [imp. & p. p. {Cupelled}
   (-p?ld"); p. pr. & vb. n. {Cupelling}.]
   To refine by means of a cupel.