The Oddy test

If a material is going to sit inside a sealed enclosure with an object for twenty years, somebody has to establish that it will not corrode it. The Oddy test, developed at the British Museum in the 1970s, remains the standard way of doing so, and it has survived because nothing faster has proved as reliable.

How it works

A sample of the test material is sealed in a vessel with three metal coupons, silver, copper and lead, together with a small quantity of water to maintain a high humidity. A control vessel contains the coupons and water without any test material. Both are held at elevated temperature, conventionally around 60 °C, for four weeks.

The elevated temperature and humidity accelerate any reaction that would occur slowly at room conditions. After four weeks the coupons are examined and compared against the control, and the material is classified according to how much corrosion appeared.

The classification is usually three-way: suitable for permanent use, suitable for temporary use, and unsuitable. The middle category exists because a material that produces slight corrosion over four accelerated weeks may be acceptable in an exhibition lasting months but not in a store lasting decades.

What each coupon detects

Lead responds to organic acids, chiefly acetic and formic. This is the coupon that catches wood, wood-based boards, and many adhesives and coatings, and it is the one that fails most often in practice.

Silver responds to reduced sulphur compounds and to carbonyl sulphide. Wool, some rubbers, certain adhesives and some proteinaceous materials show up here.

Copper responds to chlorides, oxides and sulphur compounds, and acts as a broad indicator.

Between them the three cover the corrosive gas hazards that matter most in enclosed collection environments. What they do not cover is equally important to know.

What the test does not tell you

It does not detect substances that damage plastics, dyes or photographic emulsions rather than metals. It does not assess physical properties such as abrasion or shedding of fibres. It does not measure quantity, so it cannot tell you whether a small amount of a marginal material would be acceptable. And it is a comparative visual assessment, which means results from different laboratories are not always directly comparable.

For photographic material the relevant test is a different one entirely, the Photographic Activity Test, and enclosures for photographs should be specified against that instead. Photographs covers this.

Using published results

Several institutions maintain databases of tested materials, and a number of suppliers publish test reports for their products. For most collections this is the practical route: rather than commissioning tests, specify materials that already have results.

Two cautions apply. Formulations change without the product name changing, so a result from ten years ago may not describe what is currently in the box. And a result for one product in a range does not transfer to the others.

The materials that generally do not need testing

Some materials are established as inert by long use and by chemistry, and testing them is usually unnecessary: glass, ceramic, powder-coated steel with a properly cured coating, anodised aluminium, stainless steel, polyester, polyethylene and polypropylene.

The list of usual offenders is equally stable: wood and wood-based boards of all kinds, many adhesives and sealants, PVC, wool felt, some rubbers, and oil-based paints that have not fully cured. A sealed case built from any of these is the most common self-inflicted pollutant problem in collections, and air quality covers what to do when replacement is not an option.

Where it fits in a case project

Test, or specify from published results, before the case is built rather than after. Retro-fixing a pollutant problem in a completed sealed case means either replacing the material, sealing it behind a barrier, or running pollutant sorbents permanently as a maintenance item. All three are more expensive than getting the specification right at the drawing stage.