Stones damaged by AR?

[MGH]

Hoping someone has had this same experience. I’ve just started getting into stone removal using alternating digestions in AR and ammonia. This is something I’m doing as a service rather than working on my own material. So far everything has gone as expected, but I’m thinking with this batch there were some fake stones that are now an ugly grey mess.

There was one piece in this lot that was 10K yellow gold with quite a few white stones in it (sorry, I don’t have a picture of the original piece). They formed kind of a wide, shallow “V” shape, and most of the stones were baguettes. I should have looked at them more carefully with so many of them there, but I simply did not. I assumed the customer would have only included the piece in the lot if he already knew they were real.

I’ve done four cycles of AR to ammonia, and now I have the metal all digested. I’m left with what you see in the picture. It looks like all those baguette stones from the piece mentioned above have now become the grey mess mixed in with the other stones. Most of it has crumbled, but you can still see the rectangular shape in places. Also, just from an eyeball estimate, it looks like the total number of stones is missing the contribution that would have come from the one piece.

If I’m not misunderstanding what I’ve read on the forum so far, cubic zirconia would survive AR, but would then be frosted by HF. I’m not even touching HF in my processes. So if these aren’t cubic zirconia, what could they be?

The customer knows his stuff. Perhaps he just wanted to include this in the lot to get the metal digested from around the stones and returned to him with the rest of the metal. Or perhaps he was careless and didn’t check the stones (not likely). I’m just hoping I can arm myself with some more information before I return the lot to him.

Thanks for any insight you guys can provide.

 

[Lou]

Aqua regia damages oily stones (emeralds, rubies) but doesn't hurt sapphire, zirconia, zircon minerals. Diamonds are unfazed. Opals are destroyed.


My advice on the stone removal is to avoid the darn ammonia if at all possible--you run the risk of forming explosive gold compounds that and it takes up finely divided gold into suspension. Much better to do the 10K material in a 304L stainless sonicator using 30-40% nitric acid--even 14K will go; 18K needs aqua regia. That still risks the diamonds scratching one another and the other stones.

Some refiners I've consulted for once before were doing the ol' AR, decant, rinse, ammonia, repeat to get it done and wondering why they were low on expected gold.
In one instance, we found over 100 fine troy in all the ammonia waste!

 

[MGH]

Thanks Lou. I'll have to give straight nitric a try next time.

 

[nickvc]

The whole point in stone removal is to do what it says, remove the stones undamaged.
First attempt should be in hot nitric, this will remove some base metals and silver and may allow the stones to drop out, if so job done. If after that nitric soak they are still in situ go into hot AR, and I do mean into hot AR, not cold AR that needs heating, this has to be done in a fume hood and with care and with small amounts of hot solution to avoid boil overs with small additions of acids to keep it going.
This should release the stones due to the rigorous action of the AR on any visible gold especially after the hot nitric soak, Lou,s method using ultrasonic is good but probably too expensive for most members but the method outlined above should work for 90% + of all stone set jewellery.

 

[MGH]

Thank you also, Nick. Glad to hear it should work (most of the time) without the sonicator.

True, the customer is most concerned with getting the stones back, though he likes having even the once refined [recovered] gold returned as a physical metal as well. I return recovered gold, undigested gold alloy, and the stones, and he is happy.

Update, the customer already suspected these stones in question were either not real or pretty poor quality to begin with. He was still happy with the job, and I have everyone at the GRF to thank for that. So... thanks!

 

[bswartzwelder]

Many thanks, Lou. Although I doubt I'll ever be working with anything containing stones, I always thought that ALL stones were free from being attacked by the chemicals we use. This was a great eye opener.

 

[4metals]

Once, many years ago I had a customer come in with a large lot of pearl earrings for stone removal. They were high end pearls and set in a prong type setting made to grip the round pearl tightly, so they weren't drilled and set on a post like less costly pearl earrings. There were a few hundred pair to process but I know pearls were destroyed by the acid so I had a few guys sit around a table with jeweller's snips and cut the prongs so the pearls fell free. Then we melted the gold and paid the customer for his gold, returned the pearls and charged him the stone removal rate.

He wasn't happy at the charge so he took his next lot to a competitor and asked for the same thing, stone removal. Well he got what he asked for and lost every pearl.

When he had another lot he came back to me and asked why they came out so nice when I did them and the other guy (whom he wouldn't name) ruined the pearls. I played dumb (often easy for me!) and said I don't know we just did a stone removal. I wasn't lying, we did a stone removal, just manually. Anyway he was a customer for years after that!

 

[g_axelsson]

Good story 4metals! I bet you could charge him the higher rate even when he came with diamonds or other stones that could withstand the acid. :lol:

Göran