Metals

Metals reverse the usual advice. For organic material, stability matters more than the number. For metals, the absolute level matters, because corrosion reactions have thresholds below which they effectively do not proceed and above which they accelerate sharply.

This is what makes a mixed store difficult, and why the answer is nearly always an enclosure rather than a room specification.

The main mechanisms

Silver tarnish. Reduced sulphur compounds react with silver to form a dark surface layer. It is superficial, but every removal takes a little metal with it, so an object polished repeatedly over a century is measurably diminished. The source is almost always inside the enclosure: wool felt linings, some rubbers, certain adhesives and fabrics. Anti-tarnish sorbents are the practical control, alongside removing the source.

Copper alloy corrosion. Stable patina is a protective surface and should be left alone. Bronze disease is not: it is active chloride corrosion, visible as pale green powdery spots that recur after cleaning, and it consumes the object. It requires moisture, so dry storage arrests it, and it needs specialist treatment rather than an environmental fix alone.

Iron corrosion. Ordinary rusting requires moisture and accelerates with humidity. Archaeological iron with chloride contamination is a much more serious case: it corrodes at humidity levels that would be considered dry for anything else, and it does so continuously until stabilised.

Lead corrosion. Lead is highly sensitive to organic acids, and it is the metal that most reliably reveals a pollutant problem in an enclosure. Lead objects developing a white powdery corrosion are pointing directly at the case material. This is exactly why lead is one of the three coupons in the Oddy test.

Humidity thresholds in practice

Situation	What is needed
General metal collections	Drier than organic material tolerates, and stable
Copper alloy with chloride activity	Well below the level at which the reaction proceeds
Archaeological iron	Very dry, in sealed containers with desiccant
Mixed metal and organic store	Room set for the organic material, metals in sealed enclosures

Rather than quote figures with more precision than the evidence supports, the useful generalisation is this: metals want it drier than wood and textiles do, archaeological metal wants it drier still, and none of that is achievable in a room shared with organic collections.

Sealed containers with desiccant

For unstable metal the standard answer is a sealed container, an appropriate quantity of desiccant, and a means of checking that it is working.

Note the difference from every other page in this section. Elsewhere, conditioning-grade silica gel is used to hold a mid-range value, and desiccant grade is explicitly the wrong product. For archaeological iron it is the right one, because here the aim genuinely is to drive humidity as low as possible.

The container needs an indicator or a small logger, because a desiccant that has saturated leaves the object in a sealed box at the humidity that saturated it, which is worse than an open shelf. This is a maintenance item with a checking schedule.

The enclosure is usually the problem

The recurring theme with metals is that damage comes from what the object is stored or displayed in rather than from the room.

Wool felt, which is traditional for silver, is a sulphur source. Untested wood, board and adhesives are acid sources. Rubber gaskets and some sealants emit sulphur compounds. And the tighter the case, the more concentrated all of it becomes, so improving the seal of a case built from the wrong material makes things worse.

Before changing a room specification because of tarnish or corrosion, open the case and find out what it is made of. Display cases covers what a specification should say, and air quality covers the mitigation options when replacement is not possible.

Handling

Skin salts and oils corrode metal, and a fingerprint will etch permanently into a polished surface. Metals are the clearest case for gloves in the whole collection, and nitrile is the usual choice. Handling and gloves covers the wider question.