Storage materials: acid-free, Ethafoam, no oak

The materials that touch specimens should be chemically inert: polyethylene and polypropylene (boxes, foam, bags), polyester film (Mylar/Melinex sleeves), archival board that is lignin-free and, for calcareous specimens, buffered with calcium carbonate, and cotton rag paper. Ethafoam (closed-cell polyethylene foam) cut to cradle a specimen is the museum standard support; Plastazote is the same chemistry in a finer grade.

The materials to avoid are the ones that off-gas acids or plasticisers: oak and other tannin-rich hardwoods (acetic and formic acid → Byne's disease), plywood, MDF and particleboard (formaldehyde and acids), rubber and rubber bands (sulphur), PVC (hydrochloric acid, plasticisers that migrate into plastics), polyurethane foam (crumbles to a sticky powder in a decade), cotton wool (fibres catch in sculpture), and newspaper (acidic, ink transfers).

Wooden cabinets that came with a collection can be used if they are old enough to have finished off-gassing and are lined: seal the interior with two coats of a water-based acrylic or a polyurethane varnish, wait a month, and line the drawers with polyethylene foam or archival board. Powder-coated steel is the safe purchase.

Specimens live in individual trays or boxes — polystyrene “cardboard” trays are fine if unpainted, archival board better — with the label inside the tray and the number on the specimen. Nothing sits on anything else. Heavy pieces go low; pyrite goes in its own sealed micro-climate; amber stays out of the desiccated box.

Relative humidity of 40–55 %, stable, is the target for a mixed collection; temperature matters less than stability. A cheap datalogger in the cabinet tells you more than any amount of inspection.