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FORVM`s Classical Numismatics Discussion Board  |  Numismatic and History Discussions  |  Uncleaned Ancient Coin Discussion (Moderator: bruce61813)  |  Topic: Silver nitrate bronze disease treatment HELP!?! 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: Silver nitrate bronze disease treatment HELP!?!  (Read 3102 times)
Xerxes King of Kings
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« on: February 09, 2011, 10:34:31 am »

After reading around the forvm I decided to try out the silver nitrate treatment on one of my rougher coins (Hermaios ae tetradrachm), how's it progressing?

Added a 'pinch' of the nitrate to a small bowl of water the coin was immersed in, I took photos of the coin before and two others at 5min intervals. The water became cloudy and now I can see bits/crud floating around, is this normal?!

Pics:

1. before
2. just after adding the AgNO3
3. 5mins later
4. 10mins 
   


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Xerxes King of Kings
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« Reply #1 on: February 09, 2011, 11:28:29 am »

About 80mins later, water has become clear, more silver salt or maybe a rinse and rub? (patience perhaps?  Grin )
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Xerxes King of Kings
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« Reply #2 on: February 09, 2011, 12:36:26 pm »

After about two hours I removed the coin from the water, gave it a rinse and a sponge and this is the result.
The patient seems unresponsive to treatment, however some surfaces appear to have been silvered slightly. I gave it a quick dry with paper and a hair-dryer (what's the safest way to dry a coin?)

Maybe I should add more AgNO3 to the solution?
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slokind
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« Reply #3 on: February 09, 2011, 04:33:32 pm »

I think you did no harm, perhaps improved detail a bit, BUT I think you also revealed (not caused) deep bronze disease.  Note the characteristic light somewhat "chalky" green hue in all the visible pits and crevices.
Pat L.
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renegade3220
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« Reply #4 on: February 09, 2011, 05:26:22 pm »

I think you did no harm, perhaps improved detail a bit, BUT I think you also revealed (not caused) deep bronze disease.  Note the characteristic light somewhat "chalky" green hue in all the visible pits and crevices.
Pat L.

I see it in the original picture.
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« Reply #5 on: February 10, 2011, 12:44:56 am »

I have never used the silver nitrate method.  But for other anti-BD soaks (Gringott's, etc) I have been told to, and always try to, remove as much of the soft green stuff prior to the soak. 

I do this with brush (tooth or soft brass depending on coin condition) and mechanical (pick, pin and/or scalpel, often under magnification).

I wonder if this is something one should do before the silver nitrate soak as well.

Shawn

 
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SC
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« Reply #6 on: February 10, 2011, 09:13:27 pm »

This brought back memories of silver nitrate from my long ago college chemistry days.
You definitely don't want to get it on your hands. The blackening eventually wears off,
but I remember some story about sorority sisters using silver nitrate to write on one
of their pledges, perhaps on her forehead. According to the story it scarred her and she
filed a lawsuit. Jim A 
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Mayadigger
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« Reply #7 on: March 09, 2011, 06:06:57 pm »

Ave!

Always remember that any BD pockets need to be removed/scraped away before any chemical soaks. We always use a common dental pick for this method.

Not sure about using silver nitrate...but from our end, GG's BD Killer works every time.

Best,

Kevin
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« Reply #8 on: March 10, 2011, 10:51:50 am »

The grey or whitish stuff that forms around the coin is silver chloride meaning that there is a lot of salt (sodium chloride) coming out of the coin and reacting with the silver ions in solution. Just wash it off -it's harmless but it indicates that the coin you have is very salty and you might want to pre-soak it a bit in plain water before applying the nitrate treatment. Getting  salt out is important

The 'silvering' that you get on exposed bronze surface is very thin and can be rubbed off with a Q-tip in my experience. I would ignore it.

Silver nitrate will blacken your fingernails for a while unless you wash it off promptly. Harmless but gloves or tweezers for handling coins is recommended.

Really really IMPORTANT point:

Once you have treated the coin make sure it is dried out thoroughly. Not with a dish rag but BAKED OUT at oven temperature. I place the coin on an inverted stainless steel (not aluminum) collander over a range element set to high heat for about 20 minutes. Regardless of what treatment you use this step is very important.

Glad to see someone using this method.

Regards, Dave

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daverino
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« Reply #9 on: March 10, 2011, 11:16:07 am »

Just to be clear - I wouldn't recommend the oven treatment unless it were a coin that was actually showing serious BD. Getting moisture out is important  but there is no sense in strenuous treatments of any kind unless necessary!

Little green/blue patches on a bronze coin are common and not BD.
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mwilson603
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« Reply #10 on: March 10, 2011, 06:14:13 pm »

BD seems to be a problem that comes with coins that were buried in acidic and salt-laden soils.

Also a lot of "desert patina" coins have BD under the sand if cleaned off.
regards
Mark
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« Reply #11 on: March 10, 2011, 10:18:18 pm »

Two interesting questios above

1) What is in the GG mix? I supposed it to be a mixture of sodium carbonate and bicarbonate which can be quite alkaline, pH about 12. Right or wrong?

2) What exactly is BD? When copper reacts with acids in the presence of chlorides and without air it forms a white or pale green layer of copper(I) chloride on the metal surface. It is something that you can even see in some cases. When exposed to  air and moisture this reacts to form the fluffy green stuff (copper(II) chloride)that "bubble up" to the surface. In time, if a lot of  this underlayer has built up, the whole patina can dissolve. If the coin is made free of moisture, the reaction of copper(I) chloride with air will be slow so drying is very important.

 Mark mentions that desert coins also have BD.  If coins from the desert stayed in the desert there might not be a problem but they are exposed to moist environments after they are collected.

BD is different from ordinary green copper corrosion which is pretty common on ancient coins from many environments. This is mostly stable and mineralized.
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mwilson603
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« Reply #12 on: March 11, 2011, 02:42:17 am »

Mark mentions that desert coins also have BD.  If coins from the desert stayed in the desert there might not be a problem but they are exposed to moist environments after they are collected.
Also a lot of "desert patina" coins have BD under the sand if cleaned off.
I should have made myself a little clearer maybe.  Frequently when you clean off the sand there are the remnants of bronze disease lying underneath.  And yes, in a moist environment the reaction can start again quite easily.

If the coin is made free of moisture, the reaction of copper(I) chloride with air will be slow so drying is very important.
I hate to post this because you have picked me up on BD before Dave, but here is something that I posted previously that would seem to disagree with that point.

Excerpt from e-conservation magazine online article below, (full article can be found at http://www.e-conservationline.com/content/view/863/281/)
"Copper (Cu) and cuprous chloride (CuCl) powders were used to establish the critical RH value that CuCl transforms into copper trihydroxychlorides, the corrosion products of the so-called “Bronze Disease”. XRD analysis of the tested samples showed that the rate of transformation is fast above the deliquescence point of CuCl (68.4% RH at 19.4º C) but very slow below it. The critical RH value for CuCl transformation was found to be at 63% RH. However, subtle variables such as air movement, composition of samples and type of substrate may result in the depression of the RH value that this transformation occurs. Nevertheless, the results of this study suggest that copper artefacts would be safe from the occurrence of “Bronze Disease” in the ambient museum environment (45-60% RH), provided that the upper limit is not exceeded."

regards

Mark
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daverino
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« Reply #13 on: March 11, 2011, 08:49:21 am »

Mark

Thanks for the link. After reading it I believe that they are supporting what I have said. The RH (relative humidity) should be kept low but it may be less critical than some had thought, they claim. BD really takes off above the "deliquescent" point. This is the RH where the copper(I) chloride actively gains water.

Since coin cleaners frequently soak their coins for days on end this is (I think) why BD is such a big issue with them. To reverse the absorption of water some aggressive heat drying may be necessary.

Art

I haven't used GG so I shouldn't knock it. Alkaline solutions are beneficial for the main problem of coin-cleaners: getting the crud off. If they also neutralize some of the acid build-up in the coin that is all to the good.

Regards, Dave
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mwilson603
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« Reply #14 on: March 11, 2011, 01:30:45 pm »

Thanks for the link. After reading it I believe that they are supporting what I have said. The RH (relative humidity) should be kept low but it may be less critical than some had thought, they claim. BD really takes off above the "deliquescent" point. This is the RH where the copper(I) chloride actively gains water.

Hi Dave, I knew that you would prove me wrong again.   Smiley 

However, the "deliquescent" point mentioned is 64% RH at a temperature above 19.4 degrees Centigrade.  According to www.climatetemp.info, the average RH levels in London over the last year have been somwhere in the high 70's.  (Germany appears to be slightly higher, Spain possibly averaging just below the "deliquescent" level, France higher than the UK etc etc)  Maybe large parts of the US have a lower RH?

Couple that with the average household temperature in the UK estimated at being between 20 and 21 degrees Centigrade, and I reckon we fall into the category where BD could be an issue.  I am only guessing, but I would be surprised if most northern european countries had a yearly average house temperature of less than the crucial 19.4 degrees C so they would also fall into the same category.

And that's why regardless of how long I dry any coins I have soaked, I always keep them with a few sachets of silica gel in the vicinity  Wink

regards

Mark
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daverino
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« Reply #15 on: March 11, 2011, 04:14:11 pm »

Hi Mark

Thanks again for the article. It raises some interesting points. The typical RH around here is probably  ~50%. Here in New England it occasionally rains. In Olde England it is more accurate to say that it occasionally doesn't rain!

 RH is a function of both water content and temperature. As temperature falls the RH of a given percent water vapor climbs until it reaches 100% (the dew point). This calls to mind the problem of keeping coins in plastic 'flips'. I have heard it said that the plastic reacts with the coin, which makes no chemical sense. More likely they are moisture traps. They are open enough that over time they fill with ambient humid air but when the temperature falls the RH climbs and water may condense out of the trapped air. Obviously bad for the coin and a breeding ground for BD. Better to keep coins in a dry but well-vented place as you suggest.

I have always been puzzled as to how BD generates acidity. These authors repeat that assertion and (reading between the lines) seem puzzled by it themselves.

Regards, Dave
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« Reply #16 on: March 22, 2011, 04:13:43 pm »


Since coin cleaners frequently soak their coins for days on end this is (I think) why BD is such a big issue with them.


Silver nitrate treatment works, but if you forget about the coins overnight, it can be pretty hard to get the silver plating/traces off of the coin!  I have not had a recurrence of BD on a coin I have treated with silver nitrate.  Thanks to daverino for making for making me aware of this method!

FWIW, I live in a humid area, and am thinking of abandoning cleaning coins in water entirely.  A couple of years ago, I purchased several hundred uncleaned coins from a collector who had tired of coin cleaning.  About half the coins had been pre-soaked in olive oil and the other half were dry.  I finished the olive coins with olive oil, and cleaned the rest with distilled water. 

Since that time, about 10% of the DW coins have broken out with bronze disease, even though I dried them at high heat and waxed them, and only a few of the olive oil coins have had problems.  While DW might be easier and less messy, if using it means that I spend more time treating and retreating coins my coins for BD, and less time enjoying my collection, then goodbye DW.

Before I switch to olive oil, however, I am trying another oil-based product which has been discussed here: Bag Balm.  See thread: http://www.forumancientcoins.com/board/index.php?topic=67407.msg422373#msg422373 

While I have only cleaned a few coins using Bag Balm, I am impressed with it.  I smear it on the coins, then come back a day or two later with a bamboo skewer and pick at the grime.  Plus, it makes my udder hands nice and soft.  Has anyone else had any luck with Bag Balm?

Cliff
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« Reply #17 on: March 25, 2011, 02:05:29 pm »


Since coin cleaners frequently soak their coins for days on end this is (I think) why BD is such a big issue with them.


Silver nitrate treatment works, but if you forget about the coins overnight, it can be pretty hard to get the silver plating/traces off of the coin!  I have not had a recurrence of BD on a coin I have treated with silver nitrate.  Thanks to daverino for making for making me aware of this method!


Hi Cliff

Ooh! Did I forget to mention that if you soaked your coins overnight in AgNO3 solution the silvering could get a little sticky?

Anyway, thanks for doing a really systematic study of the silver nitrate method and I am not surprised at the favorable results. Maybe others will give it a whirl too. From now on you are the expert.

Regards Dave
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« Reply #18 on: March 26, 2011, 12:05:09 pm »

Uh oh. We're in big trouble when I am considered an expert in anything involving chemistry or hazardous materials. I'm just a friendly neighborhood dilettante sharing my scant knowledge!
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Sosius

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« Reply #19 on: March 27, 2011, 05:18:50 pm »

Cliff, You are too modest and if being a dilettante means ready to try something different - well, there is a lot to be said for that. Perhaps you can reassure people that their fingers won't turn black and fall off? As far as other hazards go, I have always assumed that ancient coin collectors have above average IQ's and don't swallow stange chemicals just to find out what will happen!!

My current guess as to why the method works is based on the similar chemistry of silver and copper in their (+1) oxidation state. It is generally thought that the basis of BD is Copper(+1) chloride  (CuCl) formed in the patina and at the metal surface. When it reacts with oxygen the copper(+2) state is formed and this kicks off the problem. Silver has a higher affinity for chloride ions than does copper so silver nitrate substitutes silver(+1) ions for copper(+1)  and the resulting layer of silver chloride won't react further with air/oxygen. For instance:

3Ag(+)  +  -Cl-Cu-Cl-Cu-Cl-Cu-   --> -Cl-Ag-Cl-Ag-Cl-Ag- + 3Cu(+)

The longer that one soaks the coin the better to protect it from further outbreaks of BD but you don't want to do it so long as to get a thick plate of metallic silver on exposed copper as Cliff points out. A few hours is good and it shouldn't otherwise have any effect on the patina of the coin.

Regards, Dave
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