I get asked surprisingly often about how to care for a Roman dodecahedron -- because not all of them are in museums under expert care! What I have written below applies to all small bronze things, so I've just said 'your bronze artefact' throughout the guidance below. Professional conservation care is always best, so be sure to read the final section Finding help.

Archaeological bronzes come in many flavours, but all include copper and tin. Some will also include zinc and/or lead. Preservation through the centuries depends on the conditions under which the artefact was existing, and this usually means buried in soil or underwater. Some artefacts will be found in remarkably good condition while others are highly corroded and damaged. To preserve your object, you need to understand corrosion.

All metals, except gold and titantium, will oxidise in air and humidity (the products of this oxidation are known as corrosion). This can be mild (silver tarnishing) or drastic (iron turning to rust). Bronze is somewhere in between, and the corrosion result is highly dependent on which metals the copper was alloyed with, plus the soil chemistry, moisture level/drainage, and exposure to oxygen over the centuries. Corrosion may start and stop under changing conditions, and is attracted to irregularities and damage in the surface of the object.

In the present, the artefact can and will continue to oxidise. To repeat, corrosion requires two elements: water and oxygen; this is why you should never wash something metal that just came out of the ground! You could be kicking off a destructive corrosion process by mistake.

Basics

Your job is to minimise the oxygen and moisture around your bronze artefact.

• Wear gloves when handling it - and handle with care

• Do not let your bronze artefact get wet

• Store your bronze artefact in an airtight container

Minimise the humidity around the bronze artefact as much as possible. This includes controlling temperatures.

• Under 40% humidity at all times

• Under 35% humidity if possible

• Monitor the humidity with humidity test strips or a hygrometer (moisture meter) inside the container

  • Check the humidity levels at different times of the day, because they can change over time

• Silica gel packets can reduce humidity, but, if you don't check/change them on a strict schedule, you will just be making a 'wet sponge' near the artefact, which is worse than not having it in the first place

• Keep the artefact out of direct sunlight and away from heat sources, as this can change the humidity levels around it

• Keep the surrounding heat levels steady and temperate

• More excellent information about heat and humidty is available here.

• Keep your artefact in an airtight plastic container or at least under a bell jar

  • The kind of clear plastic containers that 'click' shut and seal are best

  • Use clear plastic bubble wrap for cushioning

  • Do not store your bronze artefact in paper, cardboard, or wooden boxes. Wood fibers emit gases that can promote corrosion, especially for iron. Even acid-free paper is best avoided.

  • Remember to keep the humidity test strip or hygrometer inside the container (but not touching the bronze)

  • Pro tip: if your bronze artefact has any damage or iffy-looking areas, fight the urge to turn it around so that bit is hidden. This is how bronze disease gets ahold - because you're not seeing the ugly truth. Visibility is a key tool.

Museums spend a great deal of their time, effort, and budget on controlling for the very conditions described on this page to care for their collections. 

It's always best to consult a conservator-restorer professional to clean, preserve or repair a bronze artefact. 

Especially if you suspect bronze disease, do not hesitate; get help ASAP! Contact a museum or find a qualified conservator as soon as possible.