On Tue, 02 Mar 2004 19:56:02 -0800, in rec.crafts.jewelry
Post by TLIs it true that in the ancient method for refining silver over an open
flame, that the stopping point was approximately when the refiner
could see their reflection in the silver?
Out of curiosity, what method are you referring to? Just melting silver
doesn't refine it very well. it's most common natural alloys are with things
like gold and copper, which are not generally driven off by heating to silver's
melting point. Certain fluxes can be used to help remove impurities like iron,
or some of the lower melting metals like zinc, lead, etc, but at least in
naturally occuring silver deposits, these impurities aren't that common, or
that much of a problem, so far as I know...
also, though I'm unsure of the method you refer to, in general, melting silver
alloys is done in reducing atmosphere, and in those conditions, the molten
metal is bright and shiny without needing to be pure at all. Even in oxidzing
atmosphere, it will tend to be reasonably bright before it's all that pure,
especially if the impurity is gold, a common admixture with silver.
Now, perhaps you're referring to a traditional (and still used) method for
assaying gold in particular. In this process, called cupellation, the impure
gold is wrapped in a bit of lead foil, and melted in a porous bone ash crucible
called a cupel. The lead combines with oxygen and the baser metals in the mix,
and the combined slag of these impurities are absorbed by the crucible, leaving
just the gold and silver. In that process, it's complete when the molten
button of metal in the cupel is bright and shiney, indicating that the lead
oxide is fully absorbed by the cupel. I don't recall ever reading, though,
that this was ever used as an actual means of refining precious metals, since
the cost for the lead itself would end up being rather high.
Post by TLAlso is it critical to not get the silver too hot or it can be
damaged? Thus necessitating that the refiner pay clos eattention to
the process.
well, if you heat it hot enough, you'll start to vaporize some of it. And in
any melting environment, molten silver is capable of dissolveing considerable
amounts of oxygen, and some other gasses, which then come back out of solution
when the metal solidifies. This doesn't ruin it in chemical terms, but it sure
can make a mess of an ingot if you wanted solid metal without pinholes,
porosity, or the like. If you melt pure silver without protective flux or a
reducing atmosphere, there will be enough dissolved oxygen that when it
solidifies, that gas coming out of solutiona again can cause spitting and a
very very rough surface on the silver. Either pretty, or a mess, depending on
what you wanted to achieve. Again, fluxes can be used to protect the metal
during melting to minimize this, and it's less a problem melting alloys of
silver if the alloys are baser metals (like copper), since those then combine
with the oxygen forming oxides, which again, can have negative effects, though
as the oxides tend to slag off to an extent (baser metals especially), it does
have some refining effect. But gold isn't removed this way, for example
Post by TLThis is strictly for personal reference and I am not actually trying
to refine silver.
toposting it here.
Peter Rowe