What to pad specimens with, and what not to use
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For long storage, pad specimens with plain polyethylene foam, the kind with no color or additives, and put a layer of washed cotton fabric or unbuffered acid-free tissue between foam and specimen if the surface is delicate [1][2]. Do not use polyurethane foam for anything you plan to keep: conservation sources say it turns brittle, yellows and can give off acids [3][4]. Polyurethane is acceptable for a short trip. This page explains which padding materials last, how to tell them apart, and what to do with the padding a specimen arrived in.
Storage padding and shipping padding are different jobs
Most bad padding in a collection was never chosen for storage. It came in the box. Dealers ship specimens in whatever cushions well and is cheap, which is often polyurethane foam, bubble wrap or newspaper. That is reasonable for a week in transit. It is a problem when the same padding stays in the box for ten years.
The Canadian Conservation Institute (CCI) draws the line clearly. Low-cost polyurethane foam "is also problematic at the end of its life cycle," it may be used "for short-term loan programs," but it is not suitable for long-term use [3]. So the first rule is a routine: when a specimen arrives, take it out of its shipping padding and rehouse it before it goes into a drawer. How to pack outgoing parcels is a separate subject, covered on packing and shipping specimens.
Padding materials compared
| Material | For long storage? | What the sources say |
|---|---|---|
| Polyethylene foam, plain (no color, no additives) | Yes | COG 11/2: "Sheets of polyethylene microfoam in different thicknesses can be used as bumpers"; "Only use the type of foam that has no coloring or other additives" [1]. Northern States Conservation Center: polyethylene foams "are still the best material we have for their purpose" [2] |
| Closed-cell polyethylene foams (sold under names such as Plastazote and Volara) | Yes | "Probably have better aging properties" than open-cell types [2] |
| Polypropylene foam | Yes | CCI: "similar properties to polyethylene foam with the potential for a longer lifetime" [3] |
| Polystyrene foam | Usable, second choice | CCAHA: polyethylene foam "is preferable to polystyrene as it does not yellow as it ages" [4] |
| Polyurethane foam (the soft sponge foam in many shipping boxes) | No | Becomes "brittle, no longer maintaining its original cushioning ability after only a few months"; "progressively turns a yellowish-brown colour, which could stain an object by contact and can emit corrosive acids" [3]. CCAHA: "Do not use polyurethane foams as they become acidic and crumble overtime" [4] |
| Bubble wrap (low-density polyethylene) | Short term only | CCI: "the cells tend to rupture and deflate with time" [3] |
| Washed, undyed cotton fabric | Yes, as a liner | CCAHA: unsized, unbleached, undyed cotton is suitable "for storage enclosures, padding, and in exhibition cases," washed first with a phosphate-free detergent [4] |
| Unbuffered acid-free tissue | Yes, as a liner | Suggested as a separator between object and polyethylene foam [2] |
| Newspaper and colored paper | No | Not a storage material in any of the sources above; replace it on arrival |
Polyethylene foam is good, not perfect
Polyethylene foam is the standard, but CCI records one bad episode worth knowing about: "A significant problem occurred in the 1990s when polyethylene foams degraded and crumbled after only a few years," and the faulty foam "had an acrid smell" [3]. CCI also reports "waxy residues" on the skin of some polyethylene foam planks, with "no damages" reported from them, and warns that recycled foam may contain polyethylene "that is already degraded" [3]. The practical upshot: buy plain, virgin foam from a conservation or packaging supplier who can tell you what it is, and record the product and date you bought it. The Northern States Conservation Center makes the same point, noting that foams vary "depending on manufacturer, product, brand and date of manufacture" [2].
How to tell which foam you have
Padding that arrived with a specimen rarely comes with a name. You can sort most of it by eye and feel.
| What you see and feel | Likely material | What to do |
|---|---|---|
| Soft, open sponge that springs back; may be yellow, tan or crumbly at the edges | Polyurethane | Remove and discard; check the specimen for staining [3] |
| Firm, smooth, closed bubbles, white or gray, slightly waxy | Polyethylene foam | Keep if clean, uncolored and not crumbling [1] |
| Stiff, white, beaded or brittle, snaps rather than bends | Expanded polystyrene | Usable as a spacer; replace with polyethylene when convenient [4] |
| Plastic film with air pockets | Bubble wrap | Fine for the trip home; replace for storage [3] |
| Any foam that smells sharp or acrid, or leaves crumbs | Degraded foam of any kind | Remove at once [3] |
When in doubt, replace it. Plain polyethylene foam is inexpensive compared with a stained or acid-etched specimen.
Lining foam where the specimen touches it
Foam is cut, and cut foam has a rougher surface than the skin it came with. CCAHA describes extruded polyethylene and polystyrene foams as "inert and non-abrasive prior to cutting" and recommends a lining once the foam is shaped [4]. For a delicate surface (fine crystal points, a druse, a soft species), line the cavity or tray with washed cotton fabric or unbuffered tissue [2][4]. For a robust specimen in a box, a plain foam bottom is usually enough.
Fitting padding to the specimen
Padding is there to stop movement. A specimen that cannot slide or tip will not chip.
- Choose the box first. The box should fit the specimen's largest dimension with a little room; the sizes that match each size class are on what size box.
- Line the bottom with a pad of plain polyethylene foam [1].
- Seat the specimen on its most stable face, crystals up. For a piece that rocks, cut a shallow hollow in the foam rather than wedging it with loose material.
- Add bumpers of polyethylene microfoam at the sides if the piece can slide [1].
- Leave the label room in the box, beside the specimen rather than underneath it, so it can be read without lifting the piece.
For fragile or heavy specimens, COG 11/2 describes "form-fitted mounts of plaster of Paris and stable foam" for large pieces stored on shelves [1]. Building one is museum work; for a home collection, a carved polyethylene cradle does the same job at small scale.
What padding does not fix
Padding protects against knocks. It does nothing for humidity, light or acid vapors. A sulfide sitting on perfect foam in a damp room will still decay. Those problems, and the RH bounds that matter, are on humidity and acid vapors.
From padding to the box around it
Padding is only as good as the box it sits in, and the box's plastic matters too. Which resins conservation sources consider stable, including polystyrene and PET, is on which plastic is safe for specimens. For loose stones and small lots, where foam inserts in jars are common, see gem jars and small lots.
References
- National Park Service, Kathy Hall, *Storage Concerns for Geological Collections*, Conserve O Gram 11/2, April 1998. https://www.nps.gov/subjects/museums/upload/11-02_508.pdf
- Northern States Conservation Center, Helen Alten, *Collections Caretaker* e-newsletter (polyethylene foam article), August 28, 2013. https://www.collectioncare.org/pubs/Aug282013.html
- Canadian Conservation Institute, Jean Tétreault, *Products Used in Preventive Conservation*, Technical Bulletin 32, issued February 7, 2018, modified February 19, 2021. https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/technical-bulletins/products-used-preventive-conservation.html
- Conservation Center for Art and Historic Artifacts (CCAHA), *Selecting Materials for Storage and Display*, undated. https://sustainableheritagenetwork.org/system/files/atoms/file/Selecting%20Materials%20for%20Storage%20and%20Display_CCAHA.pdf
