The Collaborative International Dictionary of English v.0.48:

Intumesce \In`tu*mesce"\, v. i. [imp. & p. p. Intumesced; p.
   pr. & vb. n. Intumescing.] [L. intumescere; pref. in- in +
   tumescere to swell up, incho. fr. tumere to swell. See
   Tumid.]
   To enlarge or expand with heat; to swell; specifically, to
   swell up or bubble up under the action of heat, as before the
   blowpipe.
   [1913 Webster]

         In a higher heat, it intumesces, and melts into a
         yellowish black mass.                    --Kirwan.
   [1913 Webster]