Whay aren't slabs airtight?
TheLiberator
Posts: 1,023 ✭✭✭
I have heard many times that they are not. How difficult is it to make them airtight? It doesn't seem that hard and indeed, the allready LOOK airtight. Doesn anyone have any info on this?
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<< <i>Perhaps because plastic is porous? >>
True..but aren't there many types of plastic containers out there? Hmmm...I dunno! It is something I have always wondered.
<< <i>
<< <i>Perhaps because plastic is porous? >>
True..but aren't there many types of plastic containers out there? Hmmm...I dunno! It is something I have always wondered. >>
I asked the same thing too. Apparently due to the sealing process, that uses sonic sealing (Sound waves to seal the slab), does not make it airtight. I think you would need to have a more airtight material for the slab, perhaps, glass on all sides, then sealed somehow. But if you did that, it would break very easily. For the most part the coins seem to be safe from some enviormental conditions. If you store a slab in a humid and hot place, it can actually tarnish in the slab. (For example in a hot safety desposit safe at the bank that isn't properly "equipped"). Instead, you can use a type of oxygen absorber, you put the coin or slab into this box like champer and it keeps it dry and safe from the destructiveness of humidity.
Oh, and also. I believe in order for something to be airtight all the air has to be sucked out of it; like a vacuum.
<< <i>It might not be a good idea to make slabs airtight. They could trap moisture inside, and if condensation occurs due to temperature changes, the moisture would have no way to get out. >>
Agreed...
42/92
I suspect many slabs are airtight, but that would be more the exception as opposed to design.
It had something to do with a coffee cup.
<< <i>I think you would need to have a more airtight material for the slab, perhaps, glass on all sides, then sealed somehow. But if you did that, it would break very easily. For the most part the coins seem to be safe from some enviormental conditions. If you store a slab in a humid and hot place, it can actually tarnish in the slab. (For example in a hot safety desposit safe at the bank that isn't properly "equipped"). Instead, you can use a type of oxygen absorber, you put the coin or slab into this box like champer and it keeps it dry and safe from the destructiveness of humidity. >>
Darn! Maybe you are listening.
What you've described is the shell of a vacuum flourescent display. Like a lightbulb, they can only operate in a vacuum which would be sealed with a getter (metal cap over the evacuation hole) after the coin is in place. However, you would probably have problems with the rubber material which holds the coin deforming when the vacuum is applied. Moisture? It's a vacuum.....not a problem. The problem would be assembling such a creature to hold the coin.....too expensive. You would also potentially have fine glass particles on the inside of the enclosure unless it was run with 'clean room' precision.
And, when you dropped it, it would shatter and your coin would go bouncing across the concrete bourse floor.
The new fangled slab, vacuum sealed to lock freshness in!
<< <i>Oh, and also. I believe in order for something to be airtight all the air has to be sucked out of it; like a vacuum. >>
Not necessarily so. As long as no more air could get in then the small amount trapped inside after sealing would be of little consequence. To set the thing up in a vacuum or with a dry nitrogen purge would add a lot to the cost.
<< <i>To set the thing up in a vacuum or with a dry nitrogen purge would add a lot to the cost. >>
Exactly!!
Some people ask Whay?
I ask Whay not?
My posts viewed
since 8/1/6
<< <i>The permeability of the slab material to atmospheric contaminants would have little to no importance if the slab was effectively airtight. >>
Sorry don't understand what your saying here.
Actually the answer is because air pressure at sea level is 14.7 pounds per square inch.
My posts viewed
since 8/1/6
<< <i><< The permeability of the slab material to atmospheric contaminants would have little to no importance if the slab was effectively airtight. >> >>
Translation. The ultrasonic weld could be perfect, i.e. uniform wall thickness and no pinholes around the entire periphery, but the plastic halves themselves are gas permeable.
So, it doesn't matter if the weld is perfect.....plus there will always be "air" on the inside of the slab of any TPG slab.
I don't know if this could be adapted for use on coins. I imagine it would have to be a one piece flat square bottle with one end open (like a container that Tic-Tac candy comes in). A coin could be put into an NGC type insert, the insert could be slid into the flat square bottle, then a foil lined cap with the meltable sealing stuff would have to be somehow attached with enough pressure to hold it firmly in place (on a drug bottle the cap is screwed on with enough torque to accomplish this) and it could be induction sealed. Then some kind of tamper evident seal would have to be put on the cap.
Or something along those lines.
Sorry don't understand what your saying here.
The point is that if the weld between the two halves was a 100% seal then there would be no danger of air or airborne contaminants entering the slab through the slab surfaces. The slab material is too thick to allow free passage of gases through it; its NOT a membrane which is normally thin, pliable and elastic. So the main ingress of contaminants into the slab is through the seal and NOT the slab itself.
He did several tests on various slabs. Results showed that the old rattler PCGS holders were far from airtight. The ANACS slabs are also not airtight while the older NGC olders were better. As for the new slabs (experiments were conducted in 1996) the newer PCGS and NGC slabs were much better at protecting the coins from toning with NGC's slab being slightly better.
These test were conducted 8 years ago under extreme conditions, ie., accelerated toning was applied, and for the most part the newer (then) slabs worked quite well. Since then I am sure the technology has improved even further. I think NGC has really tried to make their slabs as airtight as possible.
Interesting side note, I'm trying to recreate White's experiment with the old rattler older, a Capitol Plastics holder, an airtight holder and a aftermarket mint set holder from the 60s to see how each will react to accelerated toning conditions. So far (2 days) and there's no toning on any coin, in each of the holders. If there's no change after 10 days I plan on changing the conditions a bit and try to add some more sulphides to see if I can speed up the process. And of course I will post pics, details and results.
Michael
<< <i>Frattlaw- it sounds like you're becoming quite the man of science!! >>
I kinda always have been, but now that I get to combine it with coins only makes it more interesting. The science of toning is very interesting to me. I have always believed that with todays technology there should be a definitive method to be able to detect AT vs. NT.
Michael
Not true!! Airtight means gases won't pass in and out of the slab thru the seams. When the slab is sealed whats in it at the time of sealing stays in; nothing else gets in OR out. There is nothing that says that the slab must be evacuated before sealing to be airtight. The small amount of air remaining in the slab would be expected to be relatively harmless if the slab could be sealed so as to be airtight.