Slab Vs Air-Tites Vs Sealed Heavy Vinyl, So Why Then Does My Silver Tarnish ?

They haven't and the only one that even has advertised water resistance is the current PCGS slab.

 

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Kentucky, you are almost right. The chlorine atoms are bound by sigma bonds to carbon atoms in the molecular skeleton. BUT the carbon-chlorine bonds are "photochemically labile", which means that they can be dissociated by ultraviolet light to liberate free chlorine atoms to a measurable degree. In fluorescent lights and otherwise normal white light there is enough of an ultraviolet component to initiate that process. The carbon-chlorine bond can immediately reform, of course, but for a finite amount of time there will be a free chlorine atom or three in the material. These can do things other than reform a C-Cl bond. They can pull off a hydrogen atom from the carbon atom next door to produce HCl (aka hydrochloric acid) or another chlorine atom to produce molecular chlorine (Cl2), both of which are anathema to metals. That, btw, is one of the actual processes by which PVC degrades. If you study material structure at the molecular level, you will find that there is a lot of empty space between the atoms and molecules in what we tend to think of as solid materials, so there is ample room for atoms to move about and do things like tarnish coins, discolour the casing material, and so on. Maybe I should write up an ebook for coin aficionados explaining the whole chemistry thing...

 

Do you collect coins?

 

I do, and have described them on our 'Welcome' conversation thread.

 

Yup...senior moment

 

I think I am going to enjoy conversing with you...

 

The covalently bound chlorine is not the threat to metals but it certainly is once the PVC starts to breakdown. We use various chlorinated oils in metalworking. If they caused corrosion on customer parts, we wouldn't sell much of those type of fluids.

I as recall from previous readings, the plasticizer is the problem with coin holders. I'm not the organic chemist - but apparently it somehow accelerates the PVC degradation to form HCl. Which is extremely damaging to metal. The root of the problem is storage conditions. I've seen very old PVC holders that were fine. I wouldn't keep using them but the coins were still fine. I suspect those owners did a good job in their storage.

 

Correct me if I'm wrong about this, but those cutting fluids are not typically in contact with the metal for any significant length of time, are they? I would expect that the metal pieces are cleaned of residual cutting fluid before being sent on. Do you know if there have been any tests or studies done to examine the effect of long-term contact? Just curious.

I can see the plasticizer used to render PVC flexible being a problem in regard to promoting corrosion. It's often a diester such as di-isooctyl terephthalate and can diffuse out of the PVC rather easily. I'm sure we've seen coins taken out of PVC holders to have an oily coating on them.

 

The DOP (dioctyl phthalate) causes problems in that it loosens up the matrix of the polymer and helps the HCl (hydrogen chloride, however with water, hydrochloric acid) from decomposition to escape and then holds it in the vicinity of the coin. Vinyl holders can be stable for many years, but when they decompose, it is a world of hurt. I think most slabs and the 2x2 flips are usually made of polyester and don't have any plasticizer.

 

Neither PCGS or NGC will disclose exactly what they're made of but on the PCGS website it says theirs is a proprietary polymer formula tested to be 99.99% chemically-inert. The modern NGC ones are pretty inert as well. The major companies have been pretty good with having inert material for quite some time, the ones that do cause toning was more than likely a label and/or ink issue. Even with those they toned some coins a lot, some a little, and some didn't tone at all. I guess theoretically it should have been more even but in practice it really just seems like some coins will tone and others just really won't.

 

I think most materials for storing coins is going to have some kind of negative
affect, on Silver in general, main focus is actually on 10 OZ bars but i do have
A nice set of Silver world coins which i would like to protect

 

The other side of the composition coin (ba-boom-shee!) is that 99.99% inert also means 0.01% chemically active. It doesn't take much.

 

The copper clad on late cents is very active, in fact it begins to tarnish very shortly after striking, but the tarnish in itself is protective as fresh copper isn't exposed directly to the surface gases, the previously tarnished layer is, and it is not as reactive. The same for silver, although different metal and a slightly less reactivity. One need not get paranoid about the holders unless they live in the worse of environmental situations. IMO. Jim

 

The formation of that 'protective' layer is called 'passivation', and it is different for different metals. Aluminum forms a layer of aluminum oxide that effectively seals the metal from contact with air. Otherwise, aluminum would burn as well and as bright;y as magnesium. Copper forms a green-blue oxide coating called verdigris, which is the stuff that makes copper clad roofs turn green. But the extent to which such a layer is able to protect the metal underneath from contact with atmospheric gases and liquids depends very much on the crystal lattice structure of the oxide layer. Iron and steel readily form an oxide layer (rust) but it offers no protection whatsoever to the metal underneath because the crystal lattice of the material is wide open and easily allows oxygen and water to go through it. Copper and aluminum oxide passivation layers, on the other hand, have very compact crystal lattice structures that do not allow oxygen, water, etc. to pass through. A little coinage metal science for you...

 

Thank you Richard, but if I may offer a suggestion. Many on the board have advanced degrees in chemistry also, but 99+% do not. I could have used the word passivation also, but it would be of no value to most. My point was that verdigris/ toning, outside of liquid chemical solutions, does not progress at light speed, but to be visible, even minutely, takes time. Some people spend more than the coin is worth as they think it can be immediate and terminal. The forum is to educate on numismatics, and chemistry is a minor part of that. We want all to feel they can enter any discussion whether they have the extended education or not. I am not implying any thing is wrong with your post, just saying that one must know and respect their audience if they seriously wish to transfer the information. Jim.

 

I don't apologize for trying to add to the knowledge that others have. It is how I make my living, after all. It's a discussion for another place and time.

 

Sometimes yes and sometimes no. There's are thousands of different metalworking operations using thousands of different types of fluid. In general, most machined metal parts are not cleaned. It's adds too much production expense. Depending on what's being made, the fluid contact can range to minimal over a brief time to being flooded with fluid for hours.

Many studies have been performed on the long-term stability of metals machined with fluids. In fact, most manufacturers must validate their fluid and their ability to prevent corrosion during storage, sometimes even outdoors. Afterall, most companies making various metal parts are selling those parts and not manufacturing anything with them. If a customer opened a crate and found corroded parts, they would most certainly be returned. We design and sell many fluids that have enhanced long-term protection via specialty corrosion inhibitors and unique chemistry.

Good chatting with you Richard! It's great to have another chemist on the forums!

 

I love that! I preach it all the time when someone makes the "purity claim" (nothing more than a marketing term). That seemingly tiny bit left in a percentage claim can be HUGELY detrimental - of course, depending on what exactly that impurity is.

For example, to say something is "99.99% carcinogen free". Well, that 0.01% carcinogen is enough to cause cancer in a repeated exposure scenario. Would anyone knowingly use such a product themselves if you flipped the statement to "Only contains 0.01% carcinogens"? My guess is probably not.

 

Reminds me of that TV commercial where at a "Water Bar" the customers are told "This water contains an acceptable level of lead"

 

Probably actually, that describes really everything in life.