This is the category where the storage mistake is usually something somebody bought specifically to protect the coin.
The PVC problem
Soft plastic flips and sleeves made with PVC break down over years, and what comes off them attacks the coin — the result is a sticky green film that is well known in the trade and that removing is a specialist job.
Use holders described as PVC-free, or inert flips, or card-and-mylar holders. If you have coins in soft flips of unknown age, move them.
What else destroys them
Humidity, which corrodes and spots, particularly copper and bronze. Handling, because skin leaves salts and oils. Hold by the edge. Rubber bands and newspaper, which give off sulphur and tarnish silver. Cleaning, which is not storage but destroys more value than any of the above. Common mistakes.
For notes
Flat, in inert sleeves, out of light, with nothing stacked on them. Folds are graded, and a new fold made in storage is a permanent loss. Never laminate.
What to do
- PVC-free holders, stored upright in a box
- Silica gel, replaced when spent
- Inside wall, out of sunlight, off the floor
- Handle by the edges, and not often
What it costs
Very little for the holders. The expensive part is replacing bad holders you already own — and that is still cheaper than one spoiled coin.
Questions people ask
How do I know if a flip is PVC?
If it is soft and flexible, treat it as suspect. Rigid holders are generally the safer choice.
Is a green film removable?
Sometimes, by a specialist. It is not a job for a kitchen.
Does toning matter?
Natural toning is normal and often desirable. It is corrosion that is the problem, and the difference is worth learning.