The Collaborative International Dictionary of English v.0.48:

Cupel \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.]
   [1913 Webster]

   Cupel dust, powder used in purifying metals.
      [1913 Webster]