What the plastic does to the coin

Biaxially-oriented polyethylene terephthalate contains no plasticizer and is chemically stable at room temperature. It is the film archives standardised on for document enclosures.

In the store: Mylar-Window 2x2, Double-Pocket 2x2.

Hard, unplasticized shells with no plasticizer to migrate. Manufacturers state PVC-free composition rather than naming the resin, so that is how we state it.

In the store: A-16, A-18, A-19, A-21, A-22, A-24.3 and 13 more.

Simple hydrocarbon polymers, unplasticized and inert toward coin metal. Inert is not protective: a tube is inert and still lets coins rub together.

In the store: Slab Box, Rechargeable Silica Gel, Square Tube.

A polymer loaded with finely divided copper, developed at Bell Laboratories to protect electronics. The copper reacts preferentially with sulfur and chlorine compounds before they reach the coin. It is consumable and has a stated service life.

In the store: Album, Box.

Paper made for coins is manufactured low in sulfur and free of lignin. That is a specification, not a property of cardboard: ordinary kraft board and manila envelopes are sulfur-bearing and will tone silver. Buy board sold for coins.

In the store: Classic Folder, 6108, 6110, 6120, 6121, 6122 and 37 more.

A proud staple leg scratches the coin below it in the box, and the classic injury happens on removal — prying a stapled 2x2 open with the coin still inside. Flatten every staple; cut flips open rather than tearing them.

In the store: Mylar-Window 2x2.

Soft vinyl is only soft because it is loaded with plasticizer, and plasticizer migrates out of the film over years. On the coin it degrades, liberating hydrogen chloride, which in the presence of atmospheric moisture becomes hydrochloric acid and attacks the surface. The green residue is the visible stage; the etching underneath is permanent, and on copper it cannot be reversed.

Untreated paper carries residual sulfur from pulping. Sulfur plus silver makes silver sulfide — toning. Even toning can be attractive; contact toning from an envelope seam is a stripe.

Adhesives cure, cross-link, and cannot be removed later without solvents that also remove original surface.