intumesce

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 ]