Hola again: One more interesting consideration related to porosity. If
it is in the form of small round bubbles it has to do with gas, most
probably coming from carbon residues; if has other geometrical form
such as squares or triangles then it comes from investment particles
that broke probably when the molten metal was being poured; if it
shows always in the same part of the piece then it is related to a
wrong sprueing technic.
David C. Duhne