Bronze Disease

[bruce61813]

Update to some of my add information. Copper II is the real problem, and in truth, Bronze disease should be called Copper disease. Copper II comes in 2 basic forms. the Anhydrous form is dark brown and inactive. as it slowly absorbs atmospheric moisture, it turns the characteristic cyan color we have called Bronze Disease, http://www.amazingrust.com/Experiments/background_knowledge/CuCl2.html shows its transformation from anhydrous to saturated. In the green form, it starts doing its damage.

Bruce

[Figleaf]

When it comes to bronze disease, I am like a car driver who knows nothing about explosion engines: I know what to do, but don't understand the mechanics. Your paper suggested to me that the problem is caused by HCl, so I wondered how the coin got in contact with that substance. However, further on, I got the impression that the paper was written for coins found in the sea. I can understand that at least. My grandfather, who was into repairing fishermen's ships, would insist on having the propellors sprayed clean of salts when a ship went into dry dock.

Yet, coins not found at sea also suffer from bronze disease. Detectorists tell me the problem is artificial fertiliser. AFAIK, chlorine is normally part of artificial fertiliser. That would extend my understanding to coins found on land. Now, how about coins that have been in a collection of a very long time? Would they get bronze disease from polluted air?

Also, if there is copper II, does that mean there is copper I? What's the difference?

What I am after is some expertise on which coins are at risk and how to prevent bronze disease...

Peter

[bruce61813]

It is difficult to say with precision what coins are at risk. The general answer is that the coins that are found buried or have been in contact with the soil for a period of time. It is not HCl that is the direct cause, as copper does not easily react with the acid. Here is a bit from my latest paper, as yet in finished.

What Happens and To An Extent Why.
    Copper in moist air slowly acquires a dull green coating, called Verdigris, because its top layer has oxidizes with the air. Some architects use this material on rooftops for this interesting color. The simple chemistry is as follows:

The green material is a 1:1 mole mixture of Cu(OH)2 and CuCO3.
2 Cu(solid) + H2O + CO2 + O2 → Cu(OH)2 + CuCO3(s)
Malachite is (Cu2(OH)2CO3). Copper II can  be generated by treatment of the hydroxide, oxide, or copper(II) carbonate with hydrochloric acid. In many cases it is the simple formation of Hydrochloric acid from Sodium chloride [NaCl] and water [H2O].
NaCl + H2O → NaOH + HCl

2NaOH + 2HCl + CuCO3(s) = NaCO3  + CuCl2  + 2H2O

Why is it so hard to detect?
   Anhydrous Copper II Chloride may appear in color from a dark brown to the cyan green, depend on the amount of atmospheric moisture that is absorbed. If it was in a desert region with very low relative humidity and dry air, it may never be seen. In moist or very humid areas, it changes color rapidly and starts to work on the solid copper. See the picture I posted earlier.  The color change of the copper II does explain why Bronze Disease is so hard to detect.

Back to your original question. All very old coins, especially finds by metal detectors, even small silver coins, should be treated in a soda bath.  this should be done as soon as possible after their exposure to air. I don't mean radical attacks on the coins, but get the mud, clay and dirt off, and allow the coin to soak in the soda bath or a high pH bath for a day or two. This may not eliminate the problem, but will help minimize the damage.

Exposure of any copper based alloy to salt spray or environs that have common salt, I am referring to NaCl or basic table salt, in the air, may be at risk, but it is limited. Think about modern coins, they are just as exposed, but because of being rubbed and handled , they for less of the needed oxides. The oxides are part of the equation.

I hope this helps.

Bruce

[Figleaf]

OK let me see if I grasped that. How about this re-formulation:

Bronze disease is a common oxidation process, like rusting iron. However, where iron will rust from exposure to air alone, in the case of bronze, a catalyser is needed. The most common one is salt (think of sea water and coastal air), but artificial fertiliser will do the trick also. The catalyser will accommodate the process, but oxygen (and water?) is still necessary.

Another difference with iron rusting is that the process feeds on itself. It will continue until the whole coin is gone. As the process eats into the copper, leaving traces, it is important to act as quickly as possible. When in doubt, act.

The coins most at risk are those found in the sea or buried on land. Give them a hearty soda bath immediately. If you just bought a copper coin, keep an eye on it. Look for changes in appearance, in particular brown, green or blue-green spots that grow bigger. When that happens, treat it as described above in reply #4. Be thorough. If you miss something, you'll have to start again later.

Peter

[bruce61813]

Bronze disease is not common oxidation, it is one step beyond. Also the formation of Copper II causes a lot of damage. It really begins etching the metal surface. I noted that a brief 5 minute contact between my lab grade Copper II and a stainless steel funnel, made a notable etch pattern into the metal of the funnel.

Common copper oxidation is known as verdigris, and is mild, giving copper a dark green colored cover and is somewhat protective. whereas Bronze Disease is cyan in color and is self renewing. In the presence of NaCl it forms CuCl and will really do damage.

Bronze coins are not too long lasting in salt water, that is true. But it is due more to the reaction of the copper with other metals in an electrolyte.

Bruce [aka bruce61813 ]

[Figleaf]

Thank you, Bruce!

Peter

[Levantiner]

Theres another technique you might want try  But on a space filler coin first. I use it on Silver coins that have developed Verdigris ( and only when I want to stop the problem getting worse)  due to teh copper in the alloy corroding. The ingredients  Hot water, Household soda, Aluminium foil and a glass bowl.   Water and about two heaped spoonfuls of the soda go into the glass bowl.   put a small sheet of aluminium foil in the water.  Take the coin you wish to get rid of the Verdigris   imerse it in the water soda solution and ensure that it is touching the aluminium foil.        A minute is about as long as it should take ( 10 seconds is usually ample)    Take the coin out and wasjh it with cold water.    What happens is the Aluminium ends up corroding  while the oxides and sulphates on the coin are reduced. When done with a lightly corroded silver coin...you end up with a surface that looks like it has copper plating where the corrosion was......so it is only good for  Space fillers....the good thing is it stops the rot dead in its tracks!! Nver use it on a toned silver coin.....in a blink the coin will be "untoned"

[bruce61813]

That technique is a chemical form of electrolysis, and has been used for years. If it is just verdigris, a solution of distilled vinegar and water with a soft cotton cloth should dissolve it, without harming the silver.

bruce

[Figleaf]

Wouldn't the vinegar dissolve the patina also?

Peter

[bruce61813]

On Silver, it is possible, but if the object is to get rid of verdigris,then it is worth it.  Silver will darken on its own rapidly, sunlight and open air will do it.


bruce
 

[Pellinore]

@Bruce, how would you comment on the Wikipedia article about Bronze Disease?
-- Paul

[bruce61813]

It is the the same as what I have written. Neither of us is the first by a long shot. The process was developed by one of the major museums back in the 1920 - 30's and has proven effective, but slow. The benzo they talk about is fine, but highly toxic and a known carcinogen, that is why I stick to the soda/soda mix. It is cheap and effective.

Bruce

[bruce61813]

JAIC online       
Journal of the American Institute for Conservation 
JAIC 1994, Volume 33, Number 2, Article 6 (pp. 141 to 152)   
 
A PERSPECTIVE ON THE HISTORY OF THE CONSERVATION OF ARCHAEOLOGICAL COPPER ALLOYS IN THE UNITED STATES
TERRY DRAYMAN-WEISSER
 
This is the paper that discusses the use of sodium sesquicarbonate [ home made by using 5 parts sodium bi-carbonate to 8 parts sodium carbonate].

Bruce

[Pellinore]

Excellent, that's useful to know! Thanks.
-- Paul

[Levantiner]

You might want to check out a product called Verdicare... its, appernetly designed for coins, but I have had no experience with it my self  It may be a commercial form of sodium sesquicarbonate