I'm just a persistent wax carver, not a casting expert.
I do have a really nifty reference: "Handbook on Casting and Other
Defects, In Gold Jewellery Manufacture", produced by Dieter Ott and
edited by Christopher Corti. Copyright World Gold Council, 1997. I
don't see an ISBN on it but think that it's available from Rio
Grande. (maybe it's out of print! I just looked through the Rio 2005
Tools catalog and couldn't find it! But it's a really excellent
reference. Everyone who does, or uses, precious metal castings would
benefit from having one.) I was given mine at the Santa Fe Symposium.
Thank you again, Rio! (Usual disclaimers)
From the phrasing of your question, it sounds like you may have this
book, too. On page 35 it says:
"Case 10: Dendritic Surface - Due to shrinkage and reaction
with the investment."
They show good photo-micrographs of dendritic surface texture in
gold. I've seen a similar effect in some sterling castings I
produced. (Remember, I'm just a wax carver! )
Dr. Ott writes:
"Growth of crystallites at the onset of solidification
manifests itself as a dendritic shape. The residual melt
remains in the inter-dendritic space. If the melt does not wet
the investment at the mould wall and decomposition of the
investment gypsum binder causes the formation of sulphur
dioxide gas, the residual melt is pushed away from the surface,
leaving a skeleton of dendrites. Thus, the typical dendritic
surface structure appears. These conditions are preferentially
fulfilled in a neutral, protective atmosphere and with high
casting and/or flask temperatures."
That being said, would you please describe the dendritic texture a
bit more? Does it extend all the way through the cross section of
your ring, especially at the heavy top, or is it only on the surface?
Does the dendritic surface texture cover the whole ring, except
where you ground and polished the sprue? If the dendritic texture
goes all the way through the ring and occurs more on the thicker
sections of the ring than on the thinner sections, it could be due to
poor progressive solidification of the casting. That is, maybe the
thinner sections (the sprue and shank?) solidified first and the
thicker section couldn't get enough molten metal to fill the areas
between the dendrites. If that is the case, I expect to see the
porosity go all the way through the thick section and even show up as
larger cavities near the center of the thick section.
I cannot imagine getting a flask hot enough to keep my metal molten
and to get it into to the quench tank in time to effect the surface
texture. I know that I can get it into the quench in time to cause
shanks to crack. But not in time to effect the surface.
I think you probably hit the nail on the head with your first
thought - Gas reaction pushed the residual melt away from the
Chuck in springtime Asheville!