Preserving Your Meteorite Collection
By Alan Sailer

With a few rare exceptions, every meteorite that lands on Earth has successfully survived for over four billion years, orbiting countless times in the cold, dark storage room of outer space. The conditions for survival in space are nearly perfect: no oxygen, no water, none of the elements that start tearing meteorites apart from the moment they touch down on the surface of planet Earth. These destructive processes do no stop after recovery, but continue as our meteorites sit in display cabinets at home. I find it sad to think that something that has lasted four billion years can weather away after a few years here on Earth. More to the point, some specimens can rust away in weeks or months after collection. How can you prevent this process of decay?

You can slow down (but never stop) the disintegration of your meteorite collection by putting them inside a drybox. The villain of meteoritic corrosion was once thought to be the mineral lawrencite, a compound containing iron and chlorine. More recent research (1) has shown that another chlorine containing compound, akaganeite, is responsible for corrosion in iron containing meteorites. The common feature of either compound is its catalytic action, where water creates a cycle of corrosion, renewal of the compound and another cycle of corrosion. To interrupt the cycle, you must either remove the akaganeite or prevent water vapor from reaching the meteorite. A drybox attempts the latter. Inside a drybox, the moisture levels will be closer to a desert, rather than the humid climate many of us live in. This will prevent the rust that causes so much damage to stony, stone-iron, and iron meteorites.

The amount of water vapor in the air is described by the term relative humidity. 100% relative humidity (or RH) means that the air is completely saturated with water vapor and can hold no more. At normal pressure and temperature one cubic foot of air can hold about 8 grams of water. 0% RH means completely dry air. A dry environment would have a RH of less than 20%.

The simplest drybox is a common polyethylene food storage container which can be found at most grocery stores. The snap on lid forms a good moisture seal and by adding a suitable desiccant you can have an instant drybox. My only problem with this solution is that it does not look very good. I want to be able to see the samples easily and the milky, translucent plastic does not allow this.

I built this drybox in response to a small tragedy in my own meteorite collection. A small Brenham pallasite slice, only three months old, almost completely disintegrated from rust. The relative humidity in my area is about 40 to 60%, not unusually moist. After this incident, I put all my remaining specimens in polyethylene food containers and started building a display quality drybox. Now, every meteorite rests in a 10 to 15% RH environment and I have seen no new rust or corrosion in the last six months of storage.

This drybox was made using a table saw, drill and a jigsaw. If you have access to other tools, you might be able to build a better box. The sides and front of the box are made from sheets of one-half inch thick PVC (or a similar non-porous plastic) cut to size using the table saw. The opening in the front piece was cut using a jigsaw. I attached the steel drawer slides to the side pieces using woodscrews, about 3/8 inch long to keep from breaking through the plastic. All the holes for the woodscrews were pre-drilled and tapped for easy assembly. The two plastic sides were attached to the back with woodscrews. Run a bead of clear silicone seal along all joints to keep out moist air.

When the sides and back are done (silicone seal takes about one day to cure), install the Plexiglas drawer to the steel slides using the brass angle. I made these from brass shim stock, about 0.25 to 0.30 inch thick, cut to size, drilled and finally bent at right angles. The meteorites will rest on the Plexiglas surface.

The next step involves running a bead of the silicone seal around the top of this assembly. Carefully place the top piece of glass onto the silicone bead and press it down. The front of the glass plate should stick out about 1/4 inch and the back and sides should be flush with the plastic. You should see a continuous band of sealant around the top of the box. Let it cure and repeat for the bottom glass. Any excess sealant can be carefully removed using sharp razor blade.

Now install the PVC front piece. Apply silicone seal to all the joints and attach the front to the rest of the box using four woodscrews. Let the sealant cure. The last step is to glue the rubber gasket to the front of the box. Thread the 1/4" x 20 studs into the plastic front door. Run another bead of silicone seal around the front and put on the rubber gasket. Clamp it down tight using the aluminum door and the four finger nuts. When the silicone cures, the drybox is finished. Clean up any excess silicone using the razor blade.

There are several types of desiccant that can be sued in this drybox. My favorite is silica gel which comes in the form of small (1/8 inch) beads. The beads come in several types of containers, such as Mylar or metal. The beads have an indicator built in. When new and ready to use, the beads are a deep blue. When full of moisture, the beads turn light pink.

To recycle the desiccant, put the beads in a 300 °F oven for about three hours. This drives out the moisture and turns the beads bright blue again. The price is about twenty dollars a pound. Another type is anhydrous calcium sulfate, which looks like fine gravel. This also comes with an indicator (blue/pink) and is renewable in an oven. The price is about the same. One desiccant that I would avoid would be calcium chloride. It might be good at removing water vapor, but the addition of calcium ions to the drybox might offset the good of a low humidity environment. A source for this and other desiccant products is Cole-Parmer, phone number (800) 323-4340. In fact, if you are willing to pay about $600, Cole-Parmer carries a complete desiccating cabinet, with several shelves and an electric powered de-humidifier. This unit will hold the relative humidity at about 20 - 30%, with no need to ever change the desiccant.

As I implied in the previous paragraph, this drybox, like any other, is not a perfect moisture seal. You will have to change or recycle the desiccant periodically. For example, my drybox, with about one pound of desiccant inside, needs a change of the silica gel about once every four months. I open the box to examine a specimen about once a week, which lets in a blast of moist air. I change the desiccant when the RH goes higher than about 20%.

To conclude, I would like to mention an improvement that can be made to this drybox. Another element that degrades meteorite specimens is oxygen. I do not have any information on how much damage can be caused by atmospheric oxygen, but the priceless lunar and Antarctic samples stored at NASA's Johnson Space Center are all in a completely dry nitrogen atmosphere. It would be easy, but expensive, to add a nitrogen purge to this drybox. At this time, I don't see the need.

1. Buchwald and Clarke (1989), American Mineralogist, 74, p. 656-667.