'Xylene' - Is it a suitable product for use on Bronze AND Silver coins?

According to you the difference between verdigris and bronze disease is that the first is stable and the second "spreads." That is a good informal way to distinguish them. What you are not observing, however, and what I was pointing out, is that copper chloride tends to be soft and "spreads" while the acetates and carbonates tend to be stable. So the chlorides are in a different class of danger to a coin than the acetates and carbonates. Chloride ions are the essence of bronze disease, and that powdery green requires attention. Acetates and carbonates tend not to be so invasive, and do not require extreme treatment, even though they are hard and green. But to remove them requires different chemicals than the neutralization of chlorides. There's not much more to it than that.

BTW the questions I posed earlier were rhetorical. I have used sesquicarbonate for years and am very familiar with what it does to the patina on a bronze coin. You don't use it to remove verdigris unless you want to strip the coin. But it is indeed the treatment of choice for bronze disease. That tells you right there that these problems are not all one kind. At this point that's all I intend to say on the subject.

 

Log in or Sign up to hide this ad.

lrbguy, posted: "At this point that's all I intend to say on the subject."

I don't know what the rest of you reading this thread think but this is my take:

1. The chemical composition of the deposits on our coins has been analyzed and published. These compounds have names. Unfortunately, all sorts of names are tossed around, often for the same substance, and often not correct.

2. I think the term "bronze disease" has been locked down specifically as to the chemical reaction, its composition, its effects on coins, and how to minimize it.

3. Verdigris is another thing. Someone posted it is green, hard, and calls it Malachite. That's what causes problems - misinformation. Verdigris and Malachite are not the same thing.

4. Apparently, "verdigris" and "patina" have become merged to describe a hard coating on alloys of copper. In my opinion that is unfortunate. In the good old days a "patina" was the hard green coating on ancients. Then its usage spread to toned coins of any composition as: "This gold coin has a nice old patina." Back then, we used "verdigris" to describe the soft, green, residue or scum often seen on copper coins. It was easily removed with a rose thorn or acetone. The brass snap on my alligator key case (only used on special occasions) had to be wiped clean of this green scum before each infrequent use! Verdigris has a specific chemical composition. I don't know what the green scum was.

5. Finally, I get a very sick feeling of loss and may even begin to doubt what a member posted when someone who appears to be informed about a topic we are discussing "quits posting" and leaves the thread. IMO, it's very unprofessional and a sign that their opinion may not stand up to discussion. Hopefully, they will return.

The truth is out there one way or another.

 

How such occur is of course interesting scientifically, especially ancient coins due to the long time period exposure of the regions environment, and sometimes the sometimes questionable composition of metals. But most are concerned more about either how to remove safely if present somehow on ones coins, or how to prevent from occurring.
It is my opinion that for copper/silver composition coins, preventing moisture as much as possible, and not using products with PVC plasticizers. So strongly limit exposure to water and and progenitors of an acidic environment. I mainly collect modern ( post 1800) varieties, and often have purchased some with initial reactions. I have tested both the old product "verdigone" ~past posts here. and the current products and I have found them extremely useful for "conservation" and stabilization. I have only used them on 2 ancient copper containing coins, which were much more corroded than most moderns, and results were much less ( as expected), but better than before, and the coin hasn't resumed such corroding, due to proper storage. With ancient coins, harsher methods of cleaning seems to be acceptable as well as using coating material on the surface of the coin as a preservative that modern coin collectors do not accept, indicating different traditional methodology.

 

Wow...lotta stuff here, most of which I agree with... First a link for the Biox (I think) http://www.samson-kamnik.si/en/biox---cleaning-agent then some comments. "But acetone, water, xylene and nothing else that I've ever heard of, besides some acids, will dissolve verdigris." C'mon, you even mentioned the number of times sodium sesquicarbonate (or a mixture of sodium carbonate and sodium bicarbonate) has been mentioned as a solution that will remove verdigris or bronze disease. @BadThad usually checks in on weekends, let's see what he has to say. "I never knew chemistry could be so complicated".

 

GDJMSP you can write a lot of words, but Verdigris and bronze disease are not the same thing, no matter how many times you want to say it. Bronze disease is a very specific thing that is active and will destroy a coin very quickly if not treated, sometimes as quick as a few weeks. Verdigris is stable and can sit on a coin for hundreds of years and not cause any more damage. Bronze disease will spread from one coin to another, Verdigris will not.

As they are chemically different, their removal is completely different. Verdigris can actually be removed mechanically in most instances and it won’t return. If bronze disease is not chemically treated it wil continue to return, it’s almost impossible to remove mechanically.

Barry Murphy.

 

I stand corrected, was thinking of VerdiCare.

 

Fair enough. It was not my intent to storm away, but to make my point one last time and drop it. But let me put the facts before you and give you some references along the way. Please be patient, it is an involved topic, but it is not necessary to be a professional chemist to make sense of it. Barry Murphy has it straight, and I have met others in the hobby that do as well.

Though not a credentialed expert on all this, I'm no babe in the woods either. That is, I am not a chemist, but my father was a research chemist in a major corporation for his entire career. My training in chemistry began with him when I was 9, but not formally until high school and college. In terms of the topic under question my learning really took off when I was a graduate student at the University of Chicago, where part of the time, I worked in the Oriental Institute Museum artifact registration division, and had lots of conversations with the conservators about how to deal with bronze disease and other threats to our artifacts. By then I was actively collecting ancients and could try solutions directly. I will share what I have learned from them.

I'm going to try to put this in terms you guys can follow, but some of it is going to stretch you a bit. Please bear with it. It will help you get started to look at this Wikipedia article: https://simple.wikipedia.org/wiki/Copper

When a seller offers a bright shiny very "coppery" colored ancient, those of us who know better will avoid it. Why? Because, we say, it has been "stripped" of something down to the "bare metal." Okay, what's missing, and how can you tell? First of all, the color is wrong. A well preserved bronze or copper ancient should look reddish brown; maybe greenish, but certainly not like metallic copper. Secondly, some detail is missing; the coin features appear flatter than they should. They are. Something IS missing. We say the coin has been stripped of its "patina," but what is that?

"Patina" is what forms in the conversion of the outermost surfaces of a coin into a non-metallic salt. It results from the interaction over time of the material of the original coin surface with materials around it. Not all chemical reactions at the coin surface will result in products that are stable, so the term "patina" is reserved for the non-metallic conversion products that are stable. Since it is a conversion product, patina becomes part of the fabric of a coin, and its removal should be thought of as a removal of part of the original mass of the coin as it was first made. But because it is a non-metallic conversion component, it does not have the same physical properties as the original coin metal. The most notable differences are color and hardness. For example; whereas metals are malleable, non-metals may be brittle, so an intact fully formed patina is less susceptible to scratching than bare metallic copper would be. But let's look at color. For bronze coins "patina" may come primarily in two colors and still be a stable coating; reddish brown and azure (a bluish green). Which is preferable is a matter of personal taste. Sestertii with a uniform azure patina have long been regarded as the most elegant. I prefer to see them in a rich reddish brown. De gustibus (et coloribus) non disputandum est. Irrespective of personal preferences, how do we account for this color difference?

The "chocolate" surface is what appears when the patina consists of copper oxide. The greenish color is most often the result of copper carbonate. Other stable combinations may also exist, and vary in color tone. Copper salts derive their colors from the "valence" or oxidation state" of the copper that has chemically bonded. Because of the distribution of electrons in the outermost shells of a copper atom, copper may take on one of two "valence states" in forming bonds with other materials; +1 for a single free electron, and +2 for two. It varies dependent upon the nature of the other component to the reaction. Copper oxide, which forms very slowly underground, results from a bond between oxygen and copper in the +1 state. In this state the copper atoms contribute a white color to the mix. Copper carbonate results from copper in the +2 state when the copper atoms contribute a blue color. The precise tones will vary due to other materials that may get into the mix.

The natural patina cannot be replaced once it has been stripped off, for the simple reason that some of the metal of the original state of the coin has been lost. Any kind of replacement is a remake of the coin. Toning the outside surface is not a true "repatination" since it does nothing to replace the lost copper atoms.

Those are the basics of what goes into the formation of patina. But what is going on when a coin with a nice coating of copper oxide patina starts to form a greenish colored accretion on it? Spray a mist of vinegar on a nicely patinated coin each day for a week and watch what happens. Azure colored dots and spots will form. Doesn't the patina protect it? The protection a fully patinated coin is going to get from the chemicals around it is dependent upon the activation energy potential of the contaminants. The acetic acid in vinegar has a high enough activation energy so as to interact with copper atoms in the patina and change their valence states from +1 to +2. That will create an azure colored accretion on the coin surface at the outermost level of the patina. For that accretion, and only the accretion, I reserve the term "verdigris." Since it is formed on the surface of the patina it can be removed without deep damage to the coin surface, but because it is bonded to the surface it may take effort and care must be taken not to go too deep. This is particularly true of mechanical methods of removal. I am still looking for a good chemical solution to that problem.

There is one more topic to mention: accretions. These are contaminants which adhere to the coin surface due to chemical fusion. Two nails which overlap in a box, if allowed to get wet, may fuse together as they rust (oxidize). One is an accretion with respect to the other. Foreign matter in the matrix within which a coin has lain for centuries, is often found bonded to the coin. The subject is wide and varied, so my comments are limited to the one dimension of removable accretions versus non-removable. If a patina had formed before the contact was made, then usually the accretion can be removed without too much damage to the coin surface. However, contaminants touching the coin surface before it became fully patinated may find that the accretion cannot easily be removed without damage to the coin. Professional conservators frequently resort to using a hand held grinder tool (e.g. Dremel) to remove bonded accretions or to reduce surface ones.

The formation of stable copper salts on the surface of a coin has nothing to do with bronze disease. "Bronze disease is the irreversible and nearly inexorable corrosion process occurring when chlorides come into contact with bronze or other copper-bearing alloys." - Wikipedia, for verification and further information see: https://en.wikipedia.org/wiki/Bronze_disease

I don't want to be a nuisance (I can hear some wag saying, "Too late.") but more can be said and if you ask me to explain something, I will respond.

 

OK, here's some slipshod science...

There was a thread that concerned cleaning penn...uh cents with vinegar and table salt (NaCl). I decided why not, so I selected two cents (95% copper and plated) in three different groups:


I made a solution of table salt in vinegar and immersed four of these coins. After soaking them about an hour, I removed them and rinsed two of them and let the other two dry (the third two are the control...I did nothing to them):

For some reason I changed their orientation, but the first two to the left are the ones I let dry, the middle two are the control, and the last two got rinsed. This was taken after about a month. I then forgot about them for probably 6 more months, but then decided to see what they looked like:

The controls, of course, haven't changed, and the 95% cent doesn't show any damage, but the ones that were just allowed to dry show some interesting "patina".