This is very interesting, and I would like to know how it fits in
with the film "Puzzled by Porosity" from I. Shor International, ca.
1985, that says if the metal is overheated one can get porosity, and
a clear sign of overheated metal is that the sprue button has the
texture of a shriveled prune. I believe the shriveled prune
phenomena goes with a concave (i.e., indented) surface, not a convex
surface. Can someone straighten me out on this? Thanks.