Rubber band toning a coin through mylar

Several years ago I bought a coin lot that included a silver proof Washington 25C. When I received it, the coin stack was rubber banded, and when I slid the rubber band off the stack I saw a band of dark toning on this silver proof, exactly where the rubber band had been.



The seller replaced the proof as soon as I sent the photo I took, but was shocked that this could happen through the mylar at all, let alone in such a short time span. Since then I have been avoiding rubber banding coin stacks like the plague, to the extent where I have made all my TPG submissions at shows to avoid using rubber bands. I'm sure there are brands that are safe to use, but I don't recall ever seeing "safe" rubber bands sold by coin supply companies.

To those who use special rubber bands to avoid this, what do you use and where do you get them?

 

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Just a stab in the dark but possibly some gasses from the rubber were able to leach through the mylar? physicsfan might know better though.

 

Most flexible plastics are somewhat permeable. For hydrogen sulfide, at room temp, normal atm ~ H2S is 10cc/100 sq. inches/24hrs/mil thickness. For 2x2 like shown, assuming that there are no microscopic tears, I would suspect the thickness of the mylar. I haven't seen any comparison of mylar thickness of 2x2, but I have noticed some are thinner than others. That and staples are why I prefer the fold over mylar or similar flips

http://usa.dupontteijinfilms.com/informationcenter/downloads/Chemical_Properties.pdf

 

My guess is that it wasn't directly the rubber band that did the toning but the pressure and the closer contact of the mylar at the site where the rubber band held the mylar closest to the coin.

 

Both of the above are quite plausible.

 

I'm completely with you on this one, but in this case, the seller said she only put the rubber band on when packaging it for shipping. It was only on the flip for about 2-3 days, though there may have been substantial pressure and/or heat on the package during shipping.

TPG instructions typically say to use rubber bands to secure each order's stack of flips. With damage to the surfaces clearly being possible after only a couple days, I have to assume TPGs don't have regular problems with this. In that case, perhaps the difference in thickness between the mylar in fold over flips and in cardboard flips with mylar windows makes the difference, or possibly the combination of humidity/heat/pressure has to be very specific for this to occur. Either way, as I don't know for sure, I have not been using them on any kind of flips, for any reason.

 

I would agreed, if I understood what they said.

 

So then it's your contention that the composition of the rubber band is irrelevant and it was simply the pressure? Wouldn't this happen a lot more often if this was the case? By what process would this tone the coin over a span of a couple days? Isn't much of the point of using mylar that it is inert?

 

Yes, it is the composition of the rubber band. Natural rubber has to be "vulcanized" which involves treating it with sulfur to cross-link the molecules and make it useable. It doesn't even have to be a gas. If you slap a polished piece of lead together with a polished piece of gold, you can detect gold that has migrated into the lead and vice-versa. A superior material for flips might be the plastic "saran", but I don't know if anyone makes them.

 

The plastic in the flips is much thicker than the mylar in the 2X2 so it is safe for the short period of time the coins are in transit. Eventually though if the rubber bands are left on the flips, eventually the sulfur will tone the coins.

The plastic in saran wrap is (or at least was) PVC.

 

I'm not going to contend anything LOL...like I said it was my guess.

I had read somewhere that even the mylar in these flips have to have PVC, or some other softener, used in them to soften the film, that they were not a good long term storage solution, and that even the mylar will tone coins. This is what I based my guess on. I've seen mylar flips where they were stapled shut all catty-whompus which caused the mylar film to wrinkle or wave...the places where that film was closer to the coin's surface received toning while the places in the film that were higher off the surface of the coin had completely different or no coloring at all.

I hadn't thought about rubber bands having to have sulfur used in them..in fact all these folks around here who know this stuff blow my mind. I only have enough room in this mind for one thing...coins...trying to learn the physics and science of a rubber band out gassing sulfur through the mylar film on a coin flip by which it tones a coin in less than two weeks is beyond the capacity of my memory banks.

Sheesh, it took enough brain power to type that out that I probably just lost memory of otherwise more important coin info.

Anyway, i got a cigar from @GDJMSP for my educated guess, so I'm happy

 

PVC isn't a softener it is a type of plastic. Mylar, in the thin sheets used in 2X2's is thin enough to be flexible on its own. It is also an inert plastic.

 

PVC=polyvinyl chloride a synthetic plastic polymer and the softener used in it to make flips is called phthalates. There's another mind bender for you phankins11! LOL.

 

Mylar doesn't have the chemical shape to produce static charges, or you would not easily be able to pull the sheets from each other if you fold and then unfold a paper 2x2. Mylar because of its strength/thickness ratio is used in electrostatic sound panels, loudspeakers, and earphones~ but they are aluminized or have a chargeable material added.

It is easy to see if the chemical structure of the rubber band contributes or not by taking take 2 unused mylar 2x2, put a rubber band tightly around the middle of one, and no rubber band around the other and put on a wood shelf and before dusting the house, compare the amount of dust around the 2. I would recommend someone who dusts very seldom

 

That's right, I didn't say Saran Wrap, I mentioned the plastic saran. http://en.wikipedia.org/wiki/Saran_(plastic)

 

Not sure about contaminants being attracted, but the Mylar would not present much of a barrier.

 

Mylar is actually polyester and Mylar is simply a trade marked brand of Dupont. In it's pure form (type D) it is chemically inert. However, when plastisizers are added to soften it, it becomes less stable.