October 16, 2019, 08:51:34 PM
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Topic: Substitute for 70% Nitric acid as CuO solvent in silver work  (Read 601 times)

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Offline srt9969

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Hi all,
I'm posting this here, but I'm not really a student, so if it's inappropriate please feel free to move it elsewhere.

Quick bit of background:
I'm not a chemist - but I do have a physics degree. Feel free to science at me.
I'm enamelling on to sterling silver (92.5%Ag, balance Cu). In order to avoid adverse colour reactions, I need to 'depletion silver' the surface of my sterling. This is generally done by heating the sterling to 760C, at which point surface copper oxidises, then cooling and washing in a dilute sulphuric acid soln (which jewellers call 'pickling'). This dissolves the CuO and leaves purer silver on the surface. This process is done several times until no further CuO can be seen.

Now, the protocol I'm following comes from a superb enameller, but I don't think he's a chemist, hence this post. Here's an extract from the book:
"pickle (dilute sulphuric acid)...will not remove the fire stain (CuO), it is more a bleaching process, and the copper oxide is still there as a fine layer over the silver surface. The aim is to remove the oxide, and this is done using commercial standard nitric acid ,which is graded at around 70%..."

Now, I can't get nitric acid without a licence from the home office.
So, the questions:
  • Does anyone recognise this bleaching effect? Is it true that sulphuric won't fully dissolve CuO?
  • Is the another reagent, or combination of reagents, that will fully dissolve surface CuO without attacking the silver?

Thank you for your kind attention, I'll try to answer questions... :)

Sean

Offline srt9969

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Re: Substitute for 70% Nitric acid as CuO solvent in silver work
« Reply #1 on: June 19, 2019, 04:16:38 AM »
So, no responses. Was it a stupid question?

Offline chenbeier

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Re: Substitute for 70% Nitric acid as CuO solvent in silver work
« Reply #2 on: June 19, 2019, 05:39:04 AM »
The sulfuric should dissolve the copper oxide. What temperature do you use. Heating might help and add a small amount 3% hydrogenperoxide. Nitric acid is to strong and will also disslove silver.

Offline Enthalpy

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Re: Substitute for 70% Nitric acid as CuO solvent in silver work
« Reply #3 on: June 19, 2019, 05:50:34 AM »
Was it a stupid question?

Definitely not, and welcome here. I hoped other people would react before I put my answer, which isn't exactly the one you expected.

I just wonder why you should repeatedly heat the sterling silver and pickle it by acid just to remove copper form the subsurface. Would it be more difficult to deposit a fresh layer of pure silver? Usual solutions exist for that, used among others for silver on copper for radio-frequency electronics. As simple as: clean the copper, dip the part in the solution at room temperature for few minutes, wash, done. It's a kind of battery where copper dissolves and silver deposits, only at the same place: maybe it works with sterling silver too.

To make woodwind instruments, silver is deposited on nickel silver keys, and they use electric current. Simple enough for luthiers whose vast craftsmanship and expertise doesn't include academic chemistry.

Neither did I grasp whether you need sulphuric or nitric acid. Is the process with nitric acid a better variant? Or does nitric acid act after sulphuric acid?

Offline srt9969

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Re: Substitute for 70% Nitric acid as CuO solvent in silver work
« Reply #4 on: June 19, 2019, 12:52:55 PM »
The sulfuric should dissolve the copper oxide. What temperature do you use. Heating might help and add a small amount 3% hydrogenperoxide. Nitric acid is to strong and will also disslove silver.

Jewellers usually heat the pickle (dilute sulphuric). For enamelling, if used, it is used cold (room temp), as warm pickle can attack the enamel (soda glass, essentially).
It had also been my understanding that the sulphric should dissolve the CuO just fine - ubt this guy says nitric is needed, and he has way more enamelling experience than me! Hence my post, pretty much...
I did wonder if peroxide might help, but was going to try a higher conc. so thanks for that.

I just wonder why you should repeatedly heat the sterling silver and pickle it by acid just to remove copper form the subsurface. Would it be more difficult to deposit a fresh layer of pure silver?
Pickling is the traditional method of preparing the surface for enamelling. Plating had never crossed my mind. Although I would have concerns about the integrity (stickiness) of the plated layer...

Quote
Neither did I grasp whether you need sulphuric or nitric acid. Is the process with nitric acid a better variant? Or does nitric acid act after sulphuric acid?

The author contends that the usual method involving sulphuric doesn't properly remove the CuO, and recommends 70%  nitric to both oxidise the copper, and to dissolve the resulting CuO. He is not a chemist, so it's quite possible that he misunderstands the process; he describes heating the sterling, allowing to cool, then washing in nitric, watching for colour change first to grey/black (oxide formation), then to white metal (oxide solution), before neutralising and washing. He suggests this should be done several times, until no initial darkening of the sterling surface is seen, at which point it is assumed that the surface layer is pure silver. Does that make sense?

Thank you both for your replies.

Sean

Offline Enthalpy

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Re: Substitute for 70% Nitric acid as CuO solvent in silver work
« Reply #5 on: June 20, 2019, 05:34:35 AM »
[...] I would have concerns about the integrity (stickiness) of the plated layer[...]

What I can tell is that at woodwind instruments, the silver layer electrodeposited on German silver sticks. It covers keys where the musician strikes many times a minute for decades. Would that compare with jewels?

A video showing the electrodeposition of silver over German silver, starts at 3:16, the deposition itself at 3:45
https://www.youtube.com/watch?v=x3yasK8UgoA
others videos must exist in English. This one by a manufacturer of music instruments, maybe not very far from jewellery.

Offline srt9969

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Re: Substitute for 70% Nitric acid as CuO solvent in silver work
« Reply #6 on: June 22, 2019, 06:09:58 AM »
Thank you for that - it's definitely a direction to consider, one which no-one else is using for this specific application, to my (limited) knowledge.
From my (so far limited) googling, it seems that the piece to be plated needs cleaning in aqua regia (1:1:1 nitric, HCl, sulphuric) in order to get the plate to stick. This somewhat defeats the object of the exercise. Nevertheless, it's worth exploring. Thanks all for your contributions, if there is interest I'll post again with the results of my stumbling about in the dark...

Sean

Offline Enthalpy

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Re: Substitute for 70% Nitric acid as CuO solvent in silver work
« Reply #7 on: June 24, 2019, 05:05:38 AM »
Aqua regia would be less nice, but the linked video doesn't mention it. The operations pass directly from polishing to electrolytic steps, which includes an "electro-clean" step used in this video too
https://www.youtube.com/watch?v=9Yv4yPGsNuQ

Neither did I use aqua regia before sliver-plating copper parts for electronic circuits: standard cleaning sufficed - but I didn't try these parts for decades of skin rubbing, as opposed to woodwind keys. It resembled this
https://www.youtube.com/watch?v=bZcjJFIL4T4
I didn't even use electricity nor a silver electrode. Just immersed in the bath.

Sure, please tell us what you tried and observed!

Maybe someone else here would tell more about the choice of acids to remove CuO?

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