The other day I was annealing newly purchased pure silver ingots to
roll them out and saw that when heated, the silver “bubbled”. And
when the metal was rolled out and continued to be worked, the bubbles
didn’t go away. In fact, more appeared as I repeatedly annealed.
Has anyone experienced this with newly purchased fine silver ounce
ingots? Any ways around this? I couldn’t use the silver for rolling
out. I guess it goes into the crucible for casting. Will the
"bubbles" carry over there as porosity?
Looking forward to
Jim
Hi Jim,
The bubbles that appear each time you anneal are the remains of
actual bubbles in the metal when it was cast. They may have been
rolled flat afterwards, but they are still there and those at the
surface will rise up when annealing.
Bubbles or porosity occurs because silver will absorb oxygen when in
the molten state. I would re-melt the silver using an oxy/acetylene
(or other) torch with a reducing flame (a bit more fuel, or less
oxygen in the gas mix), and just before pouring throw a few match
heads onto the molten silver. The phosphorous in the burning match
heads will “suck” any oxygen out of the melt. There are proprietary
fluxes that will de-oxidise silver very well. Also do not overheat
the molten silver, just get it liquid enough for a quick pour into
the mould or trough.
Alastair
Hi Jim,
Sorry for this belated reply. I’m days behind in emails (first
grandchild was born Feb 13th and he and his mom live with us).
In Metalsmith (Tim McCreight) I was just reading that sometimes
bubbles appear (though he mentioned it with regards to granulation in
a side-bar). He says they will stay unless you go over then with
either a feather or a cloth-stringed mop. I imagine the mop is easier
to use. It said nothing about temp to do it at, etc.
he does not, however, mention much else about them. I’m sure someone
has the book handier than I do. Or, more likely, someone else
answered the question more fully. I’m pretty new to this world, and
some of my hasn’t been ‘tested’ yet. The bubbles I think
I could do without. 
Good luck,
Kim