Questions: 1. What might make an old 18k alloy smoke when melted?
I guess it is some metal burning off, but what?
My guess is a higher zinc content than is normal for 18K alloys.
That, combined perhaps with a higher copper content, if the alloy is
rosier than usual, might easily do what you’ve described.
2. What might cause the brittleness in the casting?
Too slow a cooling of the metal, if it’s a rose gold, can do that.
Or, more seriously, if the metal is contaminated with iron, lead,
tin, or some solders, you can also get brittleness. I’d suggest
melting the metal with a quite reducing flame (and very good
ventilation), on a standard charcoal block (hollowed out cavity
thereon), and when molten, adding a pinch of ammonium chloride, which
won’t melt, but which will skitter around on the surface causing much
blue fumes. As it does that, stir the melt with a carbon rod.
repeat this, then sprinkle on some boric acid and let the button
cool. The Ammonium Chloride will tend to form chlorides of baser
metals, including iron, tin, lead, etc. These chlorides are
insoluable in the melt, and then slag off with the flux, resulting in
less contamination. It also removes zinc. The downside to this is
mostly the fumes, so do it with good ventilation, or outside. I’d
suggest then trying to recast it. this time try a hotter mold, and
heat the metal a bit hotter too. Sounds like it wasn’t fully liquid.
Be sure to use a casting flux of mixed borax and boric acid, or a
good commercial casting flux. The combination of the two works better
than either one alone.
Also, on the possibility that the casting was brittle from slow
cooling, after you’ver removed it from the investment, before doing
anything else, anneal it. Reheat it to a low red, then at just the
point where it looses that glow, which will be around 900 degrees,
quench it IN ALCOHOL, not water. This is a gentler, slower quench
that water, as some alloys will tend to crack with a water quench.
The alcohol quench is sufficient to keep it from age hardening as it
cools. If it’s still brittle, then you’ll just have to be careful.
In that case, cut off cast jump rings for the cross, and solder on
new ones made with good wire. If the heavier cross is brittle, it
might still survive, but a brittle jump ring or bail is likely to
break off. When you then cast this, again be sure you’ve got a gentle
flame (you’re using natural gas or propane with oxygen, aren’t you?
Not oxy/acetylene, I hope? That can easily overheat your metal, also
causing problems. In golds, acetylene can also cause formation of
carbides if the flame is too reducing, which are hard spots in the
3. Do you take in old gold to recast? If so, what kind of
guidelines do you use in deciding if it is appropriate for you to
accept? These rings did not seem to have solder on them.
The trouble with old gold is that even if it’s the right alloy,
which you never know, it’s not as likely to give a good casting.
Usually, the cost savings for using old gold don’t make up for the
reduced quality of the finished piece, so the only real reasons to
use old gold like this would be sentimental, not economic. I’d
actually charge the same as if I were supplying the metal, and in some
cases, maybe even more, since sometimes you’ll have to add a bit of
your own to have enough for a sprue, which is then scrap you won’t
want to reuse again, but will have to sell the customer, or scrap
out. Customers should be warned that the finished casting will
almost certainly have more porosity than normal, in some cases it can
be quite serious, more spongy and full of holes than they’d expect.
That’s what you tell em. Along with possible brittleness and other
casting faults. If they then still want to proceed, they’ve been
warned. And though you’ve told them to expect porosity and crappy
castings, as often as not, they turn out somewhat better than that,
and sometimes are just fine. Depends on the alloy and your luck that
day. If they’re told there are risks up front, then no matter how
bad it turns out, they’ve been warned, and any success is better than
they may have been led to expect.
4. What should I tell the clients to make them feel better about
this little disaster? I will tell them the truth, of course, but
in how much detail?
Tell em the truth. No sense smoothing over it. But you might want
to retry it one more time before telling them it’s a total failure.
5. Is there any reason I should not feel badly about this? I
thought I could do it and that it would work ok, but now I'm
feeling kind of incompetent and fearful about trying it again. What
if there's cadmium in the alloy? Why risk my health just to attempt
to please some client?
There’s not likely to be cadmium in the alloy, unless it’s
contaminated with solder manufactured since WW2. Even then, amounts
will be very small, and you probably burned most of it off the first
time, if there was any there. but there may be other metals there,
and even with new good golds there are componants you don’t want to
breath, so you should always melt metal with good ventilation. Zinc
fumes, flux fumes, etc, aren’t good for you. I doubt your metal
presents a special case in this regard. As to feeling bad, well,
that’s normal. But don’t dwell on it. You cannot predict how unknow
metal will react till you try it. Sometimes even supposedly good
metal can “go south”. I recently had a commercially purchased bit of
14K nickel white gold that was just terrible. We’d cast it once,
which was fine, but now attempting to fabricate parts with the sprue
metal, was a total failure. I couldn’t even get the stuff to pour a
good ingot, and I’m not exactly a beginner at this. Hollow
sections, big shrinkage voids into the side, sludgy pouring
characteristics. And the portions I could get to cast wouldn’t roll
into good metal either. I’ve no clue what the hell got into that
melt. Something, I’m sure. But beats me what. The full ounce of
that crap is in our refining bin now… Not worth the time to fool
around with it. I just got another sprue button and used that, with
6. Is this why some jewelers don't take in old gold to recast? -