intumescing

It is very fragile and brittle, however, and is never perfectly transparent, but quite opaque; its specific gravity is 2.9, and it is readily fused before the blowpipe after intumescing.

Scientific American Supplement, No. 344, August 5, 1882 Various

Natrolite resembles stilbite, but may be distinguished by gelatinizing readily with hydrochloric acid and by not intumescing when heated before the blowpipe; from the other minerals by the form of the crystals and their setting, also the locality in the tunnel in which it was found.

Scientific American Supplement, No. 344, August 5, 1882 Various

It gelatinizes by boiling with acid, and after intumescing before the blowpipe, fuses to a frothy mass.

Scientific American Supplement, No. 344, August 5, 1882 Various