Non-Chemical Removing Discoloration During Melting

[chemist]

I melted some gold powder into a beautiful button. After cooling, it had a slight silvery discoloration on the top surface.
The bottom side (in the borax flux during the melt) is shiny and gold. The pipe looks great and there is good crystallization, so it is probably over 99%. During the torch melting process there was some small amount of white material on the surface of the button that got pushed around by the flame.
What should I use as a flux to remove this contamination during melting? Is there a better technique?

 

[4metals]

There is an age old process referred to as toughening where base metals which are in low concentration are removed as an oxide by slagging them out of the melt. The oxygen is provided by sprinkling a few granules of potassium nitrate on the molten metal, the niter will react with any base metals and oxidize them and they will collect on the surface of the melt. If you add just a pinch of borax, it forms a sticky dot which the oxidized metal will stick to when it makes contact with the borax as it is dancing around on the surface of the molten gold. The niter has to come into contact with the molten metal so this is not as effective if you have borax covering the molten metal, just a pinch is needed. I have not melted as much gold as others in small melting dishes so others should chime in here but I would assume the thin film of borax on the dish used to prepare the dish for melting may be enough to trap the dancing oxides on the scorifier wall. In a large crucible with a pool of gold a dot of borax works well.

This is only a measure to clean up gold that is not quite as pure as you would like it to be. Better rinsing and filtration can usually prevent this before hand. Refining small lots of gold is actually harder to do than large lots because of the rinsing required so it is usually refined twice to get the purity. (inquarting with silver and parting the metal to leave the undissolved gold behind counts as the first refining because the end result is usually 99% pure after parting) The second run through in aqua regia usually cleans it up. If not toughening is a trick to try.

 

[philddreamer]

Veeeeeeery interesting!

Thank you gentlemen!

Phil

 

[chemist]

Thanks 4metals.
Toughening the gold. I learn new stuff every day.
I just finished reading GSP's book today and he had discussed a similar technique.

 

[Lou]

Just make sure you toughen it under a good draft, the sodium or potassium nitrate is quite volatile at that temperature and perhaps a small portion of it may go up in the smoke as a basic oxide.

Also, let me remind people to be careful when melting their gold if it is impure--any silver chloride present during the melting process will be volatilized as a white smoke, one that is very hard on the lungs and eyes.

Lou