Diamonds turned black in aqua regia

[goldsilverpro]

https://www.finishing.com/608/14.shtml#new

This guy has an interesting problem and I have no idea on how to help him. You don't have to register to post on finishing.com. I've been posting on there, occasionally, for about 15 years.

 

[4metals]

I have seen many bags of diamonds with that dark color and always assumed it was the diamonds color. He says they are VS 1 quality which shouldn’t be like that. I always like to boil the stones in a small beaker of fresh aqua regia after removing the chlorides. Never drying the stones between the thiosulphate treatment and the final aqua regia boiling.

I wonder how he separated the chlorides from the diamonds. I have seen stones darken when dried after thiosulphate treatment if not well washed. Maybe he should try a soak in HF.

 

[nickvc]

Chris the stones appear to be smoked which could have happened during the casting process and there is no chemical process to bring them back, if they are smoked they will need repolishing to brighten them.

 

[goldsilverpro]

I have seen many bags of diamonds with that dark color and always assumed it was the diamonds color. He says they are VS 1 quality which shouldn’t be like that. I always like to boil the stones in a small beaker of fresh aqua regia after removing the chlorides. Never drying the stones between the thiosulphate treatment and the final aqua regia boiling.

I wonder how he separated the chlorides from the diamonds. I have seen stones darken when dried after thiosulphate treatment if not well washed. Maybe he should try a soak in HF.

I was also thinking about HF. The guy sounds pretty astute and could probably handle it OK.

 

[4metals]

Honestly I have always found those with experience as electroplaters to have much better experience in chemical handling than refiners. Whenever a client needs to hire a refiner and there are none available, I always recommend looking for someone with plating experience, they have the chemical handling / safety thing down pat, usually, and to teach the refining isn't all that difficult.

Chris, why don't you post on the finishing forum for him to try HF? Also ask how he separated the AgCl from the diamonds.

 

[nickvc]

I have seen many bags of diamonds with that dark color and always assumed it was the diamonds color. He says they are VS 1 quality which shouldn’t be like that. I always like to boil the stones in a small beaker of fresh aqua regia after removing the chlorides. Never drying the stones between the thiosulphate treatment and the final aqua regia boiling.

I wonder how he separated the chlorides from the diamonds. I have seen stones darken when dried after thiosulphate treatment if not well washed. Maybe he should try a soak in HF.

I was also thinking about HF. The guy sounds pretty astute and could probably handle it OK.

If those diamonds are smoked which I suspect it won’t do any good at all, smoking is a layer of carbon which forms from overheating the stones , I believe it’s sub surface but what do I know, I was only a jeweler and refiner, he’s lucky they are small stones I’ve seen this in very big stones worth a lot of money and the only course of action is repolishing, not cheap but better than losing all those stones.

 

[4metals]

He did say the stones were VS-1 to start so the black is not sub surface, that is a far cry from VS - 1. I have seen stones darken from heating before they are clean. Usually from when they are dried before they are separated, probably from getting too hot. That's why I like to keep them wet until the final aqua regia boil.

 

[nickvc]

Most of the stones I have seen that color appear from the fluxes after melting, usually small or just missed, I used to find a few in my fluxes when I re ran them sometimes and my friend who is a dealer and stone cutter told me the only cure was to repolish them, I tried most acids including HF and he was right it made no difference, a trick a friend of mine used when working with heat near diamonds was to cover them in liquid flux first to avoid the problem, not sure how it worked but he never had a problem.

 

[autumnwillow]

As a jeweler myself, my guess is that they are fracture filled diamonds.

It would be better if he could provide us with a magnified image.

 

[grainsofgold]

Been busy but have a minute-

1. Were the stones in ? cast in place- if do might they have gotten burned by not adding the right amount of borax to the investment to protect the stones during the burn out-

2. Maybe the flasks were quenced at to high of a temp

3. Were the stones known as natural with no treatments before they set in the metal ? coatings , fillers, etc?

4.Basic diamond cleaning is done with 1 part Nitric / 2 parts H2SO4 heated under a hood - this will clean the surface of diamonds

5. If you want to frost the cz then follow up with HFL

6. Then final step would be to deep boil them- put them in a pressure vessel with cleaning acids and allow to clean under heat and pressure- there are indian diamond companies that provide this service-

GOG

 

[prakash2016]

Dear Chris,
Thank you for posting my query in this forum. Thank you again for taking out precious time for solving my problem. My replies to all the gentle men who are out there is...
This is the first time such a problem has been face... I never had such issues & have been constantly boiling diamonds in the Aqua Regia for the past 25 years. The picture I have posted is of original diamonds without any treatments or Laser Drills done to remove the inclusions.
My reply to 4metals: I have tried soaking them in HF for over an hour without any results. Also we have a professional refining plant for refining gold.
Second reply to Nick: sir, For now i have re polished these diamonds shown in picture & have used them.But my question is how to prevent this from happening?? What was the cause of blackening?? If you can please explain a bit in detail...
Normally at times we do hold the flasks in furnace at 600, as at times it happens we are issued metal for casting at quite late. (for example: I have a 12 hour overnight burnout cycle & my time for casting, normally is 8.30 am next morning,but due to delays in getting metal the castings are help up till 4.30-5.30 pm. We do get metal at 4.30 -5.30pm at times.. so the flasks are held up till then).
Can this be the cause of blackening ??? (Even though we add 4% additional Boric acid to investment mixture). Can you suggest any other chemicals to prevent the same???

my reply to autumnwillow: NO these are not fracture filled diamonds..sir..

My reply to Grainsofgold: sir my replies are as follows;
1. YES sir these are wax set (stone in cast place)
2."NO" sir we quench ROSE color after 15 minutes as per the manufactures Tech sheet specifications.
3."YES" The stones were absolutely natural stone without any treatments done initially.
4.We do perform the process with same acid of Nitric +Sulphuric accordingly.
5. I personally know the Pressure + Heat process, which I used to perform on Laser Drilled diamonds earlier. I learnt the full process from Mr. Max Greeblatt at Laser Processing corporation..in NY.

Thank you Chris for this wonderful opportunity you have given me ... I had registered at this forum earlier & had totally forgotten about the same.. Thanks for reminding...
Awaiting for your reply,
Love & Warm Regards to you all,
Prakash V Pai
INDIA.

 

[nickvc]

Prakash
I’m not sure I’m the one that can answer why but I have seen stones from scrap melts looking exactly like yours and my friend who buys and sells stones as well as cutting them told me the only way to clear them is to repolish them so I’m assuming that it’s a mixture of heat and gases that causes the problem. A friend of mine who was and is a craftsman jeweler used to cover any stones with liquid flux to avoid the problem and he claimed it worked, I presume it stopped the air getting to the stone regardless of the heat.
I hope that can give you some clues or ideas to help.

 

[prakash2016]

Dear Nick,
Thank you very much.
I am still not clear about the problem & what the causes are. But will definitely share the solutions with you all once I get results...
I must thank you again whatever you shared was of utmost importance...to me & could learn a lot...
Thanks again,
Warm Regards,
Prakash V Pai
INDIA

 

[4metals]

I have seen the stones that appeared identical to the bag of blackened stones that you posted on the metal finishing forum. Most of the time the stones emerge from the refining process clean like the stones in the photo you posted to show the contrast. When I actively refined my process was consistent yet occasionally some stones were blackened. Your situation makes it very obvious because you had an entire batch of stones that went through the same process from manufacturing through refining so they all turned dark from something they were exposed to in their processing. Most refiners see diamond recovery lots made up of pieces from many different manufacturing processes so if a few are dark it is not as noticeable as it is in the case of your stones which were treated the same throughout.

The only thing that I ever experienced that blackened stones in an entire lot was poor rinsing of the thiosulfate leach used to separate the stones from the silver chlorides. If the lot was poorly rinsed and dried, the stones darkened. Were the stones separated from the silver chlorides with thiosulfate or was the ammonia process used?

Based on my experience, I would be looking at the manufacturing process rather than the refining process because, as I said, I have seen lots with some stones darkened and others perfectly clear. This can only happen from something encountered in the manufacture of the jewelry or the setting of the stone because the refining process was the same and some stones emerge perfect and some dark.

I can't help but remembering a very popular thread here on the forum from a refiner in India who precipitated gold with stannous chloride, this is very odd but where they were taught to do it that way, and it seemed to be popular. I would think that the colloidal gold dropped by the stannous chloride would in time be in all the vessels and could possibly contaminate the system resulting in a dark coating. Just a thought, may not be the issue, but how is the gold precipitated in your refinery? The refinery in the thread I reference was also a professional refining plant, you can reference that thread from this link.http://goldrefiningforum.com/phpBB3/viewtopic.php?f=85&t=22243

 

[autumnwillow]

Is it just the surface or is the entire diamond affected?

If its just the surface then one of the solutions provided here should work.

If its the entire diamond you have altered its crystalline structure somewhere in your process(too much heat or pressure will). Casting w/ stones should work fine. Even AR will not damage a diamond of Vs-1 clarity.

Could it be fine silver chloride attached to its surface? Have you tried washing with 3% ammonia at room temperature?

 

[grainsofgold]

Prakash V Pai

I have to agree with 4 metals last post. Diamonds do not turn black without contact with something or being subjected to thermal shock or heat.

Try to use Nitric /sulfuric to clean a few stones. If this doesn't work , then send a few to be repolished and have the polisher report back to you on the removal of the black.

If the black is stuck on the surface but not embedded in the surface you may need to use something that will rid the film off the surface with abrasives that will not damage the stones-

More than likely I suspect you will need to re polish the goods- I'm familiar with cutting costs in India and it shouldn't be much for the lot-

If you cast in place often - I doubt there would be a contaminant in the investment that is causing a problem - unless it is reacting to the increased amount of copper in the rose gold alloy-


You might want to run another flask but one ring with a few new stones and try to duplicate the error, but track all your steps one by one along the way -

My .02cts is all

Good luck

GOG

 

[prakash2016]

Dear 4metals,
Sir We do use Ammonia for removing chlorides in refining for separating silver chlorides. We wash the lot with Ammonia & reprocess again.
We process gold in Aqua regia process,On filtration we use Urea to remove access Nitirc Acid before precipitation & then wash with Distilled water several times & then boil in Diluted or Concentrated HCL to purify the powder to purify.
We do have a plant which is designed to handle large lots as we regularly require metal for alloying and creating new karats for casting. We cast around 60-100 flasks in a day each at our five units manufacturing jewelry for exports...
Regards,
Prakash V Pai

 

[prakash2016]

Dear Autumn willow ,
sir,
Yes it is on the surface of the diamond. The diamond is excellent & is shining but the shade has gone from white to blackish... You can see it in the post which i have attached the picture of diamond.
Yes we do wash with ammonia to remove silver chloride ....
IF you wish to see the picture of diamonds kindly refer to this post at this thread:.http://goldrefiningforum.com/phpBB3/viewtopic.php?f=85&t=22243
Regards,
Prakash V Pai