I recently received an interesting email query from a Coin World reader.

The reader wrote:

“When storing my collection, I am careful to avoid using plastic containers that may contain PVC.

“I recently opened a new safety deposit box at my bank. The new boxes are made from a hard plastic.

“All of my coins are in their own flips and containers. Do I need to be concerned about storing my collection in a plastic safety deposit box?”

The reader is certainly wise to avoid polyvinyl chloride (PVC) near his collection.

Regular readers will remember that PVC suffers from several issues. PVC is available in a plasticized and unplasticized format. Plasticized PVC is unstable with the plasticizer — a phthalate —leaching out over time. The plasticizer causes copper alloy coins to develop the corrosion product that collectors refer to as “green slime.”

Unplasticized PVC can also be problematic. PVC breaks down by a thermal mechanism producing hydrogen chloride gas. This gas is corrosive.

We have clear evidence of this in the conservation literature. Kalvar, a popular microfilm product produced from 1967 to 1970, was found to be unstable. As it broke down it gave off hydrogen chloride that in turn caused corrosion of metal storage cabinets and degraded microfilm storage boxes.

Safe plastics include polyethylene, polypropylene, polystyrene, polyethylene terephthalate (commonly referred to as polyester) and polymethyl methacrylate (e.g., Plexiglas).

My research has shown that plastic safe deposit boxes are made from polypropylene. As such, the box itself should pose no potential harm to the reader’s collection.

Consider carefully what you choose to place in your safe deposit box. Things like poor quality acidic paper and rubber bands can create a polluted environment for your collection, causing coins to corrode and paper notes to degrade.

One drawback of a safe deposit box is that you are sharing the space. Although you can control what you put in your box, you have no control over the neighbors.

Safe deposit boxes have lids, but they are by no means airtight. The air within the vault is shared by all. Some collectors will give their collection a little extra protection by storing them in a food-grade polyethylene bag or container.

This also helps protect collections in the case of a disaster such as a flood or fire. Bank vaults are normally designed to be fireproof but not waterproof. Sprinklers are still the most common form of fire protection.

Susan L. Maltby, Toronto, is a private conservation consultant, with an interest in numismatic preservation.