With a few notable exceptions meteorites have iron in metallic form as part of their composition. On Earth native iron is truly rare, the moisture and oxygen in the atmosphere quickly convert metallic iron into iron oxide. Well let's make it simple: rust. So meteorites with metallic iron need to be protected from moisture. In most cases we can not prevent them from being exposed to the air. Keeping them dry will break the evil rust causing duo of oxygen and water vapor.
Lowering the relative humidity that the meteorites are stored in is perhaps the most important part of protecting them. But we will get to some other items too. Of all the desiccants and drying agents available by far the most convenient and reliable is silica gel. It is inexpensive and widely sold. It can even be found in pet stores. Silica gel can hold a large portion of its dry weight in water. But it will reach its maximum capacity and need to be baked out to recharge. This can be done in a kitchen oven. Place the silica gel on a cookie sheet and "cook" for several hours at around 265 degrees F.
How will you know if the silica gel is doing its job. You will need to measure the relative humidity in the sealed storage box or other containers the meteorites are kept in. There are expensive gauges for this but an indicator card is just as good and very inexpensive. The indicator card will have several spots of a chemical that is blue when it is drier than a certain relative humidity level and turns red when that level is exceeded. By maintaining a level of low relative humidity no rust can form on or in the meteorites. Vigilance is the word to remember; you must keep an eye on the dryness. If you are using a very well sealed container and have put in a lot of silica gel, then you may be safe for a long time if the box is not opened often. However, if you have the meteorites on display or in an unsealed container your risks of rust and corrosion are greatly increased.
Beyond the dangers of moisture are the problems generated by handling of the meteorites. We love to show guests and friends our collections. But humans have oils in their skin that not only make fingerprints, but can leave traces that over time may promote discoloration and rusting. I recommend the use of clean white cotton gloves for handling meteorites just to be safe.
For iron meteorites that are left on display it is possible to coat them with a thin covering of an oil that does not contain water or chemical cleansers. Just plain old oil is fine. Some of the gun protecting oils and preservers have been used by meteorites collectors with success. These oils and treatments should not be used on stone meteorites, since they will be absorbed into the matrix and change its color and possible ruin the stone. But, on irons they can make it possible to share the wonder of meteorites with all who visit your home.
Sometimes we receive a meteorite into our collection that is not cleaned. We want to be free of the sand and clay of the region it was found. The key word to remember here is gentle. Use just enough force to remove the dirt. Under that covering of dirt may be the remains of the original surface of the meteorite. Its fusion crust may still be there hiding. If you use too aggressive a cleaning method you may damage or remove important characteristics of the meteorite. I begin with toothbrushes and make my way to small brass and steel wire brushes if necessary. Dental picks and small jewelry files can be used to remove small stubborn pieces of encrusting dirt. I never resort personally to electric drill powered wire brushes. Though on large irons I know they have their application. Never rush into cleaning a meteorite. Take the time to carefully examine the stone before beginning to clean it.
If you should see rust on a stone or iron, of if you see an ooze of green liquid, it is time to do some maintenance. The first and easiest step is to remove the spot of rust with a knife blade scrapping it off and immersing the stone or iron in 99.9% alcohol. Let the stone sit in the bath completely covered for several days to a week. If there is a lot of color change to the alcohol bath you may want to change the bath and let the soaking continue. After the soaking you may be able to return the meteorite to storage, but observe it. It may be cured of its rusting problem. But, if the rusting continues and you know it is in a dry location, you have a meteorite with some kind of chemical problem that needs more intensive treatment then a cleansing soak. If it is an iron meteorite that will not stop rusting. You may need to soak it in a lye solution and then neutralize and soak in alcohol. Complete instructions on this procedure are beyond the scope of this article and are available in many places on the internet. Use of chemicals for intensive treatment of rusting meteorites can be dangerous to the meteorite as well as presenting a personal danger to the one doing the procedure. Extreme care should be observed.
Most meteorites kept dry will be happy and rust free. If handled carefully and stored well a collection should last for generation after generation. If you live near the ocean or in a very humid climate remember to be vigilant about keeping you collection dry. Dehumidifiers can be obtained for display cases if you want to show some meteorites. Many of the Northwest Africa stones are so stable that they can be displayed with confidence under most condition with little care being needed. But, they should be looked at occasionally for signs of rust. Carbonaceous Chondrites, Lunar and Martian meteorites, and most of the achondrites have little metallic iron in them. They can be displayed without worry of rusting but they are also the more expensive varieties, so maintain care in handling. Above all enjoy your collection.