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Rolled silver sheet blisters


#1

Hi

could anyone tell me what happened here… the othet day i melted
some scrap did the normal thing put a pinch of borax in and melted
the silver once it was melted i gave it a stir with my carbon or
graphite stick (not sure which so i will say both) poured the ingot
then rolled it out and it looked great, after a made a test piece
out of it, i noticed that there was like small bubbles or blisters in
the sheet just under the surface, lucky for me the item i just made
wasnt for sale, this is the first time it has happened, can anyone
shed some light on to this for me please

many many thanks
jason


#2

Hello jason,

as you know or maybe don’t know, silver attracks big amounts of
oxygen as you head it up! Add some fine charcoal to your melt which
will adsorb most of this oxygen.Make sure that this charcoal is not
poored into your ingot creating airbubbles.

Use a reducing flame to make your melt (yellow tip on the front of
your flame). Free oxygen will enter your melt contaminating the
copper (copperoxydul) and presenting itself to the molten silver.
Make a fast melt, don’t use to much time providing the silver with
time to adsorb oxygen.

Carbonize your ingot if you use Acetylene gas. The extra carbon
inside will adsorbe oxygen in the first place and acts like cold
barier prefending premature solidification of your melt whenn your
ingot is to cold to start the poor.

The poor should be made in one single but steady movement using a
dry ingot. Prehead the ingot first even when your sure that it is
dry. As you head your ingot, you will see that moisture will be
formed on the inside of your ingot due to condence. Moisture will
turn into steam which provide oxygen to the liquid silver AND causing
dangereous situations like splashing molten metal from the escaping
steam.

Make sure that you don’t overhead your metal. It will vaporate!
Oxygen will be trapped into your melt causing blisters which you will
see during the proces of your billet. Remember that your melt is
always surrounded by free oxygen if you use a torch and a crucible. A
melting device is a bit different by using a reducing atmosphere.

I’m sure that other members will give more advice and even better
then I do. However, keep in mind that silver is always looking for
oxygen in a molten state and will release this oxygen when it cools
down in the solidification proces.

Enjoy and have fun
Pedro


#3

Jason,

...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.
Peter


#4
The trick is to make your own wire. It's easier to get good silver
wire, than good silver sheet metal. 

Should help a lot, Peter. I’m a staunch advocate for using the
rolling mill - always wanted to be staunch about something ;<}

I’ll roll out silver sheet for some little job like a wide wedding
band or a model. Almost always I’m going to file the surface down
anyway. But to try to get “real” silver sheet metal for something
like a box? For all the reasons Peter lays out, plus that it’s cheap
(at least from a gold point of view) - Just buy it. It’s hard to get
really good silver sheet in the shop, and the thinner you go, the
tougher it is.

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