Typically, at least with Satin Cast 20, you find increased surface
roughness of the casting, and some increased surface porosity, due to
a metal/mold reaction.
That's interesting Peter. I do primarily vacuum casting and when
everything is done just right, my palladium comes out smooth enough
to polish up with a few rubs of the thumb, a very light gray, not
mirrored but very smooth and a bit reflective, with no apparent
interaction with the investment. And no porosity with a very crisp
ring to the metal. I use a dead neutral flame... with quick heat in
and out...that seems important too. Years ago, if I lingered with
heating the metal due to too small a flame for the amount of metal or
even just a bit too rich or sharp a flame, I will not get good
results. Now I have certainly had what you describe happen when I get
the metal a bit too hot too. For this reason I often do a two melt,
one to homogenize and mix the alloy thoroughly with the gold which
requires a very high heat. ( I do this in a charcoal block which is
the easiest way for me to control the atmosphere) and another melt to
cast, pouring as soon as my graphite rod says is has melted, which I
monitor closely, keeping the rod in the flame so as not to have a
false cool reading of the stir. I don't know all the science involved
but this works well for me.
Maybe the mold temp being too hot also contributes to failure,
although those experiments were too many years past for me to
actually remember all the details. But the neutral flame seems the
most important. And definitely overheating the metal just a little
will fuse some investment into the metal and porosity becomes evident
as well, usually to the scrap out point as the porosity often runs
deep and the metal is not sound or dense.
I had these same good results with a standard gold investment from
Westcast, in NM or TX I think, about ten years ago... Rio possibly
distributed it if not mfg it Discontinued quite a few years ago
though. I haven't used Satin Cast since the early 80s, not for a
particular reason that I recall.