Hello!
I am very new to the process of making my own ingots from my scrap
material. Though I"ve successfully made a few pieces out of melted
scrap, my last ingot yielded some unexpected results. I followed the
same process I was taught and had used before, melting scrap sterling
silver into a groove carved in a charcoal block, using a
propane/oxygen torch. The scrap I used was sterling silver, I was
careful as I could be to ensure nothing else was included, and the
material was originally bought from reputable dealers. Nothing
seemed unusual in the melting/cooling/pickling/working stages, but as
I was finishing the pieces, I noticed that they had a distinct red
tinge to them. This doesn't seem to be a surface colouration,
washing, buffing and sanding don't seem to have any effect. My
sterling silver looks more like rose gold.
Though I was careful in sorting out the scrap for melting, it may be
possible for a small piece of copper to get mixed in, but it would
have had to be a very small piece, and I wouldn't think that it
would have such a drastic effect on the colour of the silver.
I rather like the effect actually, but if someone asks why this
silver pendant is red, it would be nice if I could explain it. If I
knew where it all went wrong, I could either avoid it in the future
or replicate it, if I decide that it's worthwhile. Does anyone have
any thoughts?
Thank you.
Lora
Thoughts?
yes,
I suspect its copper oxide.
If you can beg borrow some stainless steel brazing flux from a
welding shop Johnson Matthey make, apply liberally to a small piece of
the pink silver, and heat till molten supported on some small piece
of cramic fire brick.. Keep applying it in liberal amounts so the
flux runs away from the silver.
This flux will dissolve out copper oxide and it turns the flux red
leaving the silver white.
Its flouride based so ventilate your melting area well. As to keeping
sterling scrap free of oxides, you have to melt in an inert
atmosphere.
to do that your melt has to be covered with flux, borax is ok but
againg a reactive flux will clean your melt. as you raise the temp
from cold to molten.
then you also need a larger torch, say 1in dia propane et to a
neutral flame to exclude oxygen. Then you need a big propane tank say
30lbs content to back up your torch.
I use a crucible in a proper melting furnace some 9in wide by 18in
high internal dia in which I have a 6in high by 3in dia graphite
crucible. Then when the silver is melted I pour through a reducing
propane flame, again to ensure oxygen is always excluded.
The cast iron mould is preheated then flooded with olive oil. This
also excludes air during pouring and results in clean shiny ingots
that roll well into sheet and wire.
In a bullion making plant after casting most ingots are milled both
sides to remove any surface contaminants and mould lines. then its
rolled to the required thickness, with appropriate annealing in
again inert atmosphere.
Keep oxygen out of your melts. its highly reactive at molten metal
temps.
goodluck.
ted.
