Causes of 'spoiled' slabs?
logger7
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Here is a type II gold dollar that I sent to NGC for their "appearance review", which they honored running it by their main finalizer and putting it through conservation: 
Is it so hard to catch these issues which develop later?
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Send it back to get fixed.
Absolutely, they need to do it right. Let us know the outcome. Peace Roy
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I was asking about how they would have missed it to begin with as substance doesn't just "happen". M.S. agreed there was an issue and had it conserved.
Do you have “after” photos?
He who knows he has enough is rich.
I have a Saint like that in a PCGS slab.
It looks like there is mold growing on the coin when held at an low angle.
I'll see if I can get a picture.
My guess is water/humidity.
My Saint Set
It was shipped out yesterday, image isn't up yet.
Here is my conservation candidate...Can't see it much from straight on.
My Saint Set
I would be interested to know if the substance was identified...That would give good indication if it could be detected earlier. Did you ask for this information? Cheers, RickO
Correct, "it doesn't just happen." However, putty, PVC, and other contaminants are often not obvious when a coin is first graded. Appearance changes over time.
Witness the slime we see when a dipped coin is not properly rinsed. It looked fine originally. As PVC contamination ages it becomes more obvious. Putty changes color and texture over time. Even verdigris grows larger and brighter in color.
No doubt the TPG's occasionally miss something they shouldn't have. But my guess is the great majority of these issues could not be perceived at grading.
Lance.
With the large volume of certification they do, what are probably regarded as minor issues to the well trained human eye can develop over long periods of time even without environmental deterioration (humidity, light, temps). I have not had a presumably doctored coin not finally dip out with a straight grade, which shows that they do catch virtually all real problems.
Not sure if they still use it but a while ago PCGS announced they were using a "sniffer" to identify doctored coins. I thought it made sense, it has to be next to impossible for the human eye to pick up everything while grading, especially if putty or or chemicals were applied by a person who knows what they are doing. I know this is coin is in an older NGC older.
I googled it the coin PCGS coin sniffer was announced in 2010, doesn't seem that long ago, I'm getting old.
I found some spoiled slabs in the back of my refrigerator -- next to 3-year old green yogurt and what might have been a cucumber.
Add some eggs plus ham and you’ll have yourself a meal.
Yum! Just like Grandma used to make !
Big Secret: Many types of chemically altered surface alterations can be detected using florescent light (and others). I've used this type of light at every place I've worked. At NGC I was the only authenticator/grader allowed to use it in combination with my stereo scope.
Good move by them!
That looks like PVC to me.
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I got the coin back, better luster than before:
Can we get images similar to those in the OP for comparison?
Lance.