Subsurface pits can be caused by the metal/flask temperature
combination being too hot. A too hot combination will normally
produce round pits inside the metal.
Some time ago I read in a jewelry book that the sprue button
surface can tell you a lot about the temperature combination. If
the button top surface is concave the combination was too cold. If
the surface is convex the temperature combination was too hot.
When I first started casting I was told that my flask temperature of
850 degrees was too cold and that I should use a flask temperature
of at least 1000 degrees. I tried that and ended up with a sprue
button that blew up like a sponge. The casting has subsurface
Broken off investment can become trapped under the surface of the
metal. If this happens the pits/voids will have a shape to them
and will usually be larger than porosity.
Incomplete burn out normally leaves carbon on the surface of the
mold which cause pits in the surface of the metal.
Based on your comment "my casts are mostly ok but I get fair sized
subsurface pits that appear when polishing." I would suggest you
lower the metal or flask temperature. I vacuum cast with a mold
temperature of 860 degrees with a sterling melt temp of around 1830
degrees. Those temperatures work for me.
To anyone interested I have a very simple process that prevents
fire scale from forming when vacuum casting. I have produced a
paper with sketches and photographs on the subject which I would be
happy to share with anyone interested in eliminating fire scale on
The process costs next to nothing and is very simple. A little bit
of wax is placed on a solder pad under the hot flask and on the
sprue button. Place a larger empty flask around the casting flask
and add a solder pad on top of the cover flask. The wax attempting
to burn around the casting flask inside the cover flask produced a
reducing atmosphere. No oxygen is left to combine with the copper in
the silver to form firescale.
This simple process eliminates fire scale on vacuum cast parts.
Sterling castings will come out of the quench water clean and
silver colored, not black.