Choosing storage enclosures

An enclosure does four jobs at once: it buffers humidity, excludes light completely, forms a barrier to dust and insects, and lets the object be moved without being touched. It also, if specified badly, becomes a source of acid sitting in contact with the object for decades.

The difficulty is that the vocabulary used to sell enclosures is looser than the vocabulary needed to specify them.

Three claims that are not the same

Acid-free describes pH at the time of manufacture. It is a snapshot. A board made from wood pulp containing lignin can be acid-free when it leaves the factory and measurably acidic ten years later, because lignin degrades and generates acid as it does.

Lignin-free describes composition, and it is the property that predicts long-term behaviour. This is the more meaningful claim of the two.

Buffered means the board contains an alkaline reserve, typically calcium carbonate, which neutralises acid as it forms, whether from the board itself or migrating from the object. This is a genuine benefit for paper and other plant-fibre material, and it is a problem for protein material.

The last point is the one most often got wrong. Wool, silk, leather, parchment, feather and hair are damaged by alkaline conditions, and some dyes shift colour in an alkaline environment. Several photographic processes have the same issue. For these, the correct specification is unbuffered, neutral pH.

Plastics

For plastic enclosures the question is simply which polymer. Polyester, polyethylene and polypropylene are stable, chemically inert and the established choices. PVC is not: it contains plasticisers that migrate and it releases acidic degradation products.

Trade names do not answer the question. A supplier who cannot say which polymer a sleeve is made from is a supplier to avoid.

One general caution about plastic enclosures: they are impermeable, so anything sealed inside with moisture stays with it. For material that is not fully dry, or in an environment where the object could be brought in cold and warmed, a permeable paper enclosure is safer.

Photographs need their own test

Photographic emulsions react with enclosure materials in ways that the general paper tests do not predict. The relevant assessment is the Photographic Activity Test, which exposes the enclosure material to a detector under accelerated conditions and looks for image fading and staining.

Enclosures for photographic material should state a PAT pass. Absence of any statement should be read as untested rather than as passed. Photographs covers the wider question, including which processes need unbuffered enclosures.

Practical fit and construction

Beyond chemistry, the mundane things decide whether an enclosure protects anything.

A box that is too large lets the contents shift; one that is too small forces the object in. Both cause more damage than the chemistry ever will. Drop-front and clamshell designs allow an object to be lifted out without being dragged over a lip, which matters a great deal for anything flat or fragile.

Metal staples and fasteners corrode and stain, and adhesives vary enormously in quality. A box with well-made corner construction and no metal in contact with the interior is worth the difference.

Size to what the store can actually hold and to what a person can lift down safely from the shelf it will live on. Shelving and racking covers the other half of that decision.

Tissue and interleaving

Acid-free tissue is the general-purpose padding, interleaving and wrapping material. Unbuffered is the safer default in a mixed collection, because it is acceptable everywhere while buffered tissue is not.

Use it to support rather than to fill. Rolled tissue supports a fold so a textile is not creased; crumpled tissue packed tight around an object exerts pressure and can distort it.