Re: granulation

On Thu, 26 Apr 2007 00:31:33 -0700, in rec.crafts.jewelry mbstevens
<NOXwebmasterx@xxxxxxxxxxxxxxx> wrote:

On Thu, 26 Apr 2007 06:08:24 +0000, Peter W.. Rowe, wrote:

However, I'd guess that when the silver melts, the gold surface in contact with
it at least slightly melts too, forming a thin layer of eutectic alloy.

This is at least possible, since the Krugerrand is alloyed with copper.
However, the granules had no treatment save a light coat of Handy flux,
and I rather doubt it.

The copper doesn't play a major part in this, I don't think. In the usual
methods of granulation, the copper forms a lower melting alloy when it mixes
with the silver or gold with which it is in contact. But don't make the mistake
of thinking copper is the only metal this can happen with. Because silver,
mixed with gold (the two are completely soluable in each other), melts at a
lower temperature than do either of the two alone, a contact between silver and
gold will, if there IS a lower melting combination of the two metals in contact,
also mix with each other at the contact point, and can do it below the melting
points of either metal. Silver mixed with gold does not lower the melting point
as drastically and quickly as does copper with either one, but it DOES lower the
melting point of high karat gold. If the gold had lots of copper, then adding
silver might, with some gold/copper alloys, raise the melting point of any
resulting mix, so then this would not happen. But because there IS a mix of
your high karat gold with silver, that melts below that of the gold alone, then
a eutectic alloy will form when a small amount of the gold dissolves into the
silver, alowing both to melt together. What I'm usure of is whether this
potential alloy also melts significantly lower than the silver itself. I
suspect not, which is why you end up with the silver almost melting before you
get bonding, but then when it happens, I'll bet you find the gold grains seem to
sink rather easily into the silver. The above mechanism would explain that.

This is similar to what happens if you place a piece of brass, with flux, on a
piece of silver and heat them. Because brass alloyed with silver forms a lower
melting alloy (silver solder, to be exact, any of several grades depending on
proportions), at the melting temperature of the lowest melting of the
combinations of silver with brass (the eutectic temp of that system), the two
still solid and unmelted surfaces, of brass and silver, will combine without
first melting. They do that because the contact area itself, rather than one
surface or the other, allows that eutectic alloy to form, melting the interface.
This proceeds, of course, dissolving more and more of both surfaces into the
mix, forming more of the eutectic mix. With silver and brass, the potential
drop in melting point, ie the difference in temp between the melting point of
the eutectic mix and that of the two parent metals is rather large, leading to a
rather rapid combination when an unsuspecting jeweler heats the two together,
perhaps trying to solder them together. The brass can quickly sink right into
the silver as it does this, combining the two sometimes disastrously. I
suspect, but don't have data in front of me, that the reduction in melting point
that occurs at the contact between gold and silver is slight, making it a
controllable situation.

Again, in this case, you don't need to treat the granules, because the existing
dissimilarity between the metal of the grains, and that of the silver, already
provides the needed conditions for the bonding to take place.

That is, of course, all dependent on whether the addition of silver to your
kruggerand alloy raises or lowers the melting temp. Because there is already
copper in the gold, it's melting point may already be lower than what is
achieved by adding silver. If that is the case, AND if adding gold to the
silver does not then lower the melting point of the silver, then what would be
happening, as i said before, is simply that the silver is melting first and
fusing to the gold, without any formation of a eutectic. If this is the case,
then even so there will be some diffusion of the gold into the silver and silver
into the gold, which is why the bond then has strength beyond just "wetting".

Peter
.

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