Sunday, January 18, 2009

Refining Gold

This is a method for taking old gold and refining it to about 23kt. When I do this I regard the gold as 22kt, and I alloy it accordingly down to 18kt or 14kt. This gives me a safety margin so the gold is a slightly higher purity than I stamp it. Safety disclaimers notwithstanding, this process uses Nitric acid at 70% concentration, and caution must be used. Generally spoken, when one makes a mistake with strong nitric acid, one makes that mistake only once..... After that, the lesson is well learned. Only yellow gold can be refined, since nitric acid does not dissolve palladium. Nor platinum, for that matter. I have not tried it with 14kt white, since I do not work in metals that are derived from the devils armpit.

The old gold is weighed out and five times the weight of copper is added. So 40 grams of gold will need 200 grams of pure copper.

Since I am refining a large amount of 14kt and since that is more than I can melt at one time with five times the weight of the copper, I divide the gold and copper up into four, and for this Hidi (How I Do It) I shall only be showing a portion of the melt.

I use a large refuse drum, which I fill up with water to make shot. A steel one would be better, but hey, I use what I got.....:)

At the bottom I use a stainless steel bowl and a plate below that to catch the shot, just in case it missed the bowl. It does sometimes.
Then I melt the copper.

And I add a portion of the old jewellery to the molten copper.

And then, when the gold is mixed with the copper, I pour it into the water. This makes the shot, and the finer the beads are, the quicker the nitric dissolves the mix. There are better methods, using steel baffles, but this is a quick method, without to much added hassle.

This is the resultant mix of copper and gold.

Then the fun starts. The bowl of alloy, a bottle of 70% fuming nitric acid, a bucket of water. The water is there to wash and spills off quickly, just in case a spill occurs. Much better than running to a tap screaming, If you dumb, you will not use safety glasses and gloves, so it's lucky you are clever, not?

There is a decent reaction when nitric acid hits the copper/gold solution. A cloud of dangerous gas is released, so it is better to do this outside and stand up wind at the same time. And maybe make sure your downwind neighbors are not at home.....Fun stuff, this.

After the reaction is finished, and all is calm, more acid is added, until there is no more reaction. This means that all the copper, silver, and any other contaminants are dissolved and only the gold remains.

And this is the before and after picture. The fine gold looks just like coffee grounds, and if you take it between your fingers, is crumbles just like coffee. I doubt very much that it tastes like coffee, though.

Then I mix it with alcohol and borax and saltpeter ( potassium nitrate) all available at your friendly chemist. It is not really necessary, because one can just melt is as is. The alcohol dries out the water and the borax and saltpeter makes melting easier.

This picture shows the water/alcohol burnt off and the top of the mix melting. Suddenly, gold appears. This is only one reason why it cannot be used as a coffee substitute. There are others.

After the gold it melted in the normal fashion and cast into an ingot, it looks like this, ready to be alloyed down to whatever karat is required.
This will be the first part of a piece of jewellery that I am going to make in the coming week.
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