Author Topic: Heavily stained or worse silver  (Read 840 times)

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Offline redwine

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Heavily stained or worse silver
« on: January 15, 2017, 12:06:48 PM »
I hope you can come up with a solution to this.
I'm puzzled, the obverse is fine.  The reverse is bad!
How can I clean it?
Many thanks  ;D
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Offline bagerap

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Re: Heavily stained or worse silver
« Reply #1 on: January 15, 2017, 01:04:09 PM »
I've never tried this, so practise on something scrap. Hydrogen peroxide in weak solution has been recommended to me in the past. Quick wash and rinse.

Offline redwine

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Re: Heavily stained or worse silver
« Reply #2 on: January 15, 2017, 06:19:50 PM »
Many thanks.   ;D I shall pop down the pharmacist......
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Offline Figleaf

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Re: Heavily stained or worse silver
« Reply #3 on: January 15, 2017, 06:55:12 PM »
I guess it has been lying in the wrong bed in a collector's cabinet. ;)

Not clear just how worn the coin is. If its wear is relatively light, I'd recommend a silver dip. Your undoubtedly most elegant wife may well have some in stock, but if it has been in stock for many years, I would recommend a new purchase, as I think recent formulae work significantly safer. Don't follow the instructions on timing on the label. Do a 10 seconds dip on an unc coin, 30 seconds for ef and 2 minutes for vf, checking every 30 seconds. Rinse and dry well to neutralise. If it's below vf or if you insist not to touch the other side, a soft eraser will do the trick, but you'll get colour differences on the two sides. Pollution and salty air will reduce the colour difference in the long run, so leave it exposed to the elements.

Peter
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Offline redwine

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Re: Heavily stained or worse silver
« Reply #4 on: January 15, 2017, 06:59:36 PM »
Sadly this is after the silver dip (brand new)  :-\
I shall certainly try the soft eraser before I go chemical!  Many thanks ;D
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Offline Figleaf

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Re: Heavily stained or worse silver
« Reply #5 on: January 15, 2017, 07:02:44 PM »
Bad news. If the dip didn't work, it is not an oxide, as I assumed. If it is glue or another chemical, acetone should dissolve it.

Peter
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Offline redwine

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Re: Heavily stained or worse silver
« Reply #6 on: March 11, 2017, 07:39:43 AM »
After a forced absence from attempting to clean this coin.  I can now report, it's a mess! :'( :-[
Tried so far:
silver dip, more than once.
heavy duty solvent
cider vinegar - removed some rust from another German silver coin I was experimenting with
dilute hydrogen peroxide.

Any more thoughts?
« Last Edit: March 11, 2017, 08:11:05 AM by redwine »
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Offline malj1

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Re: Heavily stained or worse silver
« Reply #7 on: March 11, 2017, 11:38:31 AM »
I have seen this result from a coin left on a shelf or ledge in a shed or similar for many years; the underside stays good while the dusty side goes like your one.  ::)
Malcolm
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Offline Andrey5

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Re: Heavily stained or worse silver
« Reply #8 on: March 20, 2017, 01:48:07 PM »
Since your previous efforts proved not effective, I would suggest the following solution.

1/ Thiourea [white crystals with formula CS(NH₂)₂] - 8 g
2/ Water 100 ml
3/ Phosphoric (also called Orthophosphoric) acid 2 g
4/ Alcohol 6 ml

Components 1 and 3 could be purchased in specialized shops selling chemistry materials, maybe somewhere else also but where - I don't know. No. 4 could be replaced with simple vodka
The solution should be prepared in this order. If some crystals are not dissolved, that's not a problem, leave them at the bottom.

The solution is not very dangerous to handle, but anyway I would recommend wearing surgical gloves.

I am 99% sure that a dip from a few hours to a day in this solution and a cotton bud will clean your coin.

Don't forget to finish cleaning with good rinsing and drying the coin.

By the way, the same solution is good for cleaning rust from iron coins and conserving them against future corrosion. Naturally, you don't clean silver and iron coins in the same glass.
 
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Offline Figleaf

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Re: Heavily stained or worse silver
« Reply #9 on: March 20, 2017, 02:03:09 PM »
Very interesting, Andrey. Ingredients may be difficult to buy. If unobtainable, could you leave out the Thiourea? What do you think the pollutant is?  How would it react with your chemical soup?

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline Andrey5

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Re: Heavily stained or worse silver
« Reply #10 on: March 20, 2017, 04:21:09 PM »
First of all I am not a chemist. My knowledge in this field is limited to high school and is rather well forgotten by now. The receipt is from a professional chemist and coin collector. According to him, Thiourea is the main component of most silver cleaning dips, therefore you can't leave it out. You may even increase its percentage in this solution if the black toning (Silver sulfide - Ag
2S) on the silver coin is very deep, though based on my experience I doubt that it would be necessary in this case. It would not react with silver itself and should not harm modern silver coins.
I read that Thiourea is often used in the process of soldering silver, so it might be sold in shops selling staff for radio assembly.

Ancient silver coins are better cleaned mechanically, not chemically because silver then was different from alloys used for modern coins, but I experimented once with this solution on a Denarius. Here are before and after scans. The result is maybe not the best, but at least is shows the efficiency of the solution.
 
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Offline redwine

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Re: Heavily stained or worse silver
« Reply #11 on: March 20, 2017, 04:27:35 PM »
Very interesting, many thanks!  8)
I shall try and track some down.............
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Offline Figleaf

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Re: Heavily stained or worse silver
« Reply #12 on: March 21, 2017, 01:06:06 AM »
Since redwine already tried silver dip, I wondered if the thiourea would make a difference. Phosphoric acid (takes of iron oxide) might be effective, though. It all depends on what the black stuff is. Silver oxide is a solid, shiny black coat, while iron oxide is powdery, dull and dark grey.

I am not a chemist either.

Peter
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Offline Andrey5

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Re: Heavily stained or worse silver
« Reply #13 on: March 21, 2017, 04:13:45 AM »
I don't know the formula of patented silver dips, and the proportion might be also important. Leuchtturm dip works in seconds and minutes, while in the solution I suggested the coin should stay 6-12-15 hours with periodic control and help of a cotton bud.
Here is the result of another cleaning experiment (not mine) with this solution, I took images from another site.
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Offline Figleaf

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Re: Heavily stained or worse silver
« Reply #14 on: March 21, 2017, 10:27:16 AM »
I have no doubt your formula works like a charm. I have doubts about the sediment on redwine's coin. It sits on one side only and doesn't react to silver dip. That doesn't sound like silver oxide.

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.