Making Friends With Your Metals

The Ubiquity of Metals

Most of us will have metals in our collections in one form or another, whether it’s giant ship anchors and machinery or staples and paper clips on documents. Metals are everywhere, and small wonder! Humans have been using these materials for at least 10,000 years so we have a long, long relationship.

Busting Some Myths

If there’s one thing I’d like people to know it’s that metals aren’t as robust as we think they are. Often we recall that metal is used for durable things like ships and beams and industry - they’re strong and tough materials, after all! While this is true, it’s also true that metals need looking after just as much as other materials in our collections. They can be finicky, they can be downright fussy, and they can disappear altogether if we don’t care for them well. (On the bright side, it’s one of the few things we collect that doesn’t attract pests, so that’s at least a bonus!)

The Big Risks

Our metal collections are at risk from physical forces (being damaged by us), fire (getting melted), thieves and vandals (who love to nab valuable metals), and dissociation (or the loss of information about their use or value), but the one thing I’m going to focus on in this blog entry is this: corrosion! It’s the bane of a conservator’s life sometimes.

Corrosion is the deterioration of a metal as a result of a reaction with its environment. The effects can vary from a minor surface change to a complete loss of material. Depending on what metal you’ve got and what the cause is, corrosion can look really different: from a change in colour through to cracking and pitting and all the way to the point where you’ve just got a pile of rust.

Corrosion: Active vs Inactive

Corrosion can be active or inactive. Active corrosion is easy to spot: it’s often a bright colour, often voluminous, and can be powdery or flaky in nature. It might cover a whole surface or appear only in isolated areas. Inactive or stable corrosion is duller in appearance (e.g. stable rust on iron tends to be brown rather than orange), appears compact and adherent, and can be either smooth or rough to the touch. It’s also worth saying that not all corrosion is bad: some patinas are highly sought after as long as they’re stable!

Causes of Corrosion

• High relative humidity (too much moisture in the air)
• Pollutants in the air (e.g. from outside traffic or industry) or from storage/display materials (which can off-gas)
• Contact with salts or acids
• Exposure to oxygen
• Being next to another metal
• An old conservation treatment gone wrong
• Old polish or cleaning paste left in crevices
• Dust attracting moisture to the surface

Steps We Can Take

1. Control the Environment

We can control the environment in our galleries or storage areas to prevent further deterioration: we do this by lowering the relative humidity and keeping it as stable as possible. Run dehumidifiers and empty them regularly, monitor your spaces somehow, and keep an eye on your metals for signs of corrosion. If you’ve got rooms in your store rather than one massive space it may be worth putting your metals in one room so that you can keep that one area extra dry. We can also try to make sure there are as few pollutants as possible in the air by using good quality storage furniture and running air purifiers. Keeping our metal objects dust free will also help.

2. Box or Enclose Sensitive Metals

We can box or enclose our most sensitive friends: this is standard practice for archaeological metals but it works a treat for any metal that’s feeling sad. Pop your metal object in an air-tight box (currently that means a plastic box made from petrochemicals, because we need a tight seal so air doesn’t leak out). Personally I like a box that clips shut because the seal is usually really snug. If we’re worried about the relative humidity, then we pop silica gel and a humidity indicator (a colour-indicating card or a logger) in there too. There are many silica gel calculators available out there, for example this one from CCI (which you can download as a nifty spreadsheet), so you know how much to put in the box - it’s more than you think!

3. Apply a Coating

We can apply a coating or protective layer to our metal friends: if our friends are too big to be enclosed or it’s impossible to control the environment then coating an object may be the way to go. It makes a huge difference: here are two sets of beautiful old keys. One set has been coated with Renaissance wax, the other has been left uncoated, and I popped both of these in a humid environment of 70-80% RH for 2 weeks - spot the difference! Wax is quite a nice, easy one to apply, but there are other types of coatings like lacquers or paints that may be suitable depending on the metal you’re trying to look after. Always seek advice if you’re unsure, but coatings can do an awful lot of good, particularly for objects on the larger side.

Conclusion

So to recap, metals have many enemies, but we don’t have to be one of them. Corrosion is the main villain in my book, and we can combat corrosion by looking after our metal friends by: controlling their environment, enclosing them in a micro-climate, or giving them a protective coating. That’s how we keep our metals happy!

As always, ask a conservator for help if you find something unexpected in your collection or a problem you don’t know how to fix. It’s better to ask for help than ruin something all by yourself!