...i noticed that there was like small bubbles or blisters in the
sheet just under the surface,.... can anyone shed some light on to
this for me please
As others have also mentioned, Silver absorbs oxygen when molten,
and in the case of fine silver, spits it out again on cooling. In
Sterling, it results in oxides, which can be incorporated into the
ingot. All in all, this can be problematic when reusing scrap silver,
since unless you’ve got very good control over your melting and
pouring process, there will be a strong tendancy to get at least a
few such blisters in sheet metal. Not always, but it’s sometimes
difficult and unpredictable to avoid.
There are two approaches I use with reasonable success. Neither is a
really reliable preventative, but they’re usable work-arounds.
The first is that after rolling the sheet metal to it’s final
thickness, I firecoat it properly to avoid firescale, then anneal the
sheet. Any areas with blisters that would show up later in
fabrication, will show up now, allowing you to simply avoid those
parts of the sheet metal in your work.
The second is simply to buy your sheet metal from a decent supplier.
The big guys generally roll sheet metal from considerably larger
ingots, which in some cases they actually machine, first, milling off
a surface skin where the majority of the inclusions/imperfections
seem to concentrate. That’s one reason commercially rolled sheet
metal is pretty good. That plus the better atmospheric and
temperature controls the refiners will have available.
In the latter case, of course, then there is the question of how to
reuse your scrap metal. The trick is to make your own wire. It’s
easier to get good silver wire, than good silver sheet metal. In the
first place, pouring wire ingots tends to be easier to do without
problems since the narrower ingot is less likely to trap impurities
and bubbles. With a closed mold poured from one end, more of the
defects end up at the end than in the middle, especially if, when
preheating the mold, you try to get the top end of the mold hotter
than the bottom, so the metal solidify progressively from bottom to
top. There’s often a bit of a shrinkage hole in that end anyway,
which you’ll want to cut off before rolling. If not, well, watch the
end while working to see if problems develop.
With an open mold (which can be harder to get a good ingot with, in
silver), impurities tend to end up on the surface, where you can see,
and file them off before rolling. Then, when you roll square wire and
then draw it down to usable wire sizes, if there are imperfections
remaining in the wire, they tend to be centered down the middle of
the wire, becoming threadlike structures in the middle of the wire,
rather than a blister. If they’re severe, the wire simply won’t
survive drawing, or will obviously have cracks or defects you can
avoid. If they’re not severe, the minor defect centered in the middle
of the wire won’t cause you trouble.
Hope that helps.