Is there some way, without a metallurgical lab, of determining if
porosity is caused by 'shrinkage porosity' or 'gas porosity'.
Examine the shape and concentration of the pits. Gas porosity will
tend to leave larger, spherical pits. Dirt (investment, carbon from
tools, foreign body in the wax contaminants) will present as jagged,
irregular or included pits. Over fluxing will leave rounded
depressions with a halo of bright metal -- a result of the flux
scavenging off the oxides-- and, often, glassy flux inclusions.
These pits are the result of melting, burnout or casting procedures.
"Shrink spot" or "hot tearing" are the result of sprue placement and
solidification (cooling) and occur just after casting. Hot tears
are actually torn/fractured areas usually occuring in zones where
thick meets thin. Shrink spot is most often appear as spongy areas
that just won't take a high polish that, when louped, can be seen to
actually be areas filled with fine sprays of porosity, sometimes
irregular in cross section. These areas will often be concentrated
around sprue attachments or in heavy cross sections.
When larger, heavier patterns are cast-- a gent's signet ring--
at too high a flask/ mold temprature the large mass of metal takes a
relatively long time to cool. The crystals grow larger resulting in
a coarse grain structure complete with voids and small tears-- shrink
The object of a good casting is to produce a dense, finely refined
crystal structure. Shrink spot is not this.
I've found that by considering all the factors-- the shape of the
evidence, where it's located and the mechanics of the process that's
giving me trouble-- I can often ferret out the culprit.
Summing up: in my experience, gas porosity is larger, round and
regular shaped pits; shrink spot pososity is fine and spongy in
appearance showing up, most often at final polish.
Hope this helps. Take care, Andy Cooperman