Meteorite Care and Preservation
Over the years we have discussed the problems of moisture and corrosion as they relate to the protection of meteorites. I am coming back to that discussion again this month.
As really just custodians of our meteorites during our lives we want to have perfectly preserved specimens to past on when our time on Earth is done. And for most of us that means we must have a proactive attitude about moisture damage. Few of us live where it is really dry enough to never worry about at least some of our specimens. The Earth is out to get our meteorites unless we take action.
I do not over clean my meteorites. I often leave the red or brown patina on irons. I will often leave the caliche on stones. I never personally use anything like CLR or such to remove deposits. I have on rare occasions done some gentle cleaning with modified toothbrushes. I replace the plastic bristle end with a small wire brush. I can get into regmaglyphs and cavities to get some soil and stuff off. I have experimented with abrasive blasting as a learning activity. I do not think it has a value for me as a general practice for preparation of specimens. I leave the surface alone for a reason. This is the coating that has served the specimen very well in the wild for hundreds and certainly in many cases thousands of years. This natural coating especially on irons has developed and become somewhat resistant to further corrosion. I also like the colors of “as found specimens” more then highly cleaned ones.
Henbury with natural patina
Sometimes you just get a specimen that will not behave. It seems like no matter what you do every time you open your sealed dry box to look at your pieces there is that one that shows a little rust or a spot of green-brown liquid. Then it is back to the garage for a soak and a sand and a polish. Often these unstable pieces will finally find themselves confined in a silica gel prison never to see the light of day or smell the wet fresh air of Earth again.
There are some things that can be done to minimize or eliminate the problems of corrosion and moisture damage to our meteorites.
I guess they break down into two categories of solutions. First, eliminate the moisture which allows chemical activity to occur and second to protect the surface of the meteorites with barrier coatings which prevent moisture from reaching the metal.
The first solution of eliminating moisture is in theory easy to do. Get a load of desiccant and suck all the water vapor out of the environment the meteorites are in. In practice it is much harder to do. Meteorites in display cases will be constantly exposed to the changing relative humidity of the room. Day to night changes and seasonal changes, Your desiccant will very quickly become saturated with water vapor and be useless in such an open situation. Desiccants are made to remove the moisture present in a confined and closed container. So they are excellent for well sealed dry boxes that specimens are kept in. But, not really useful in display cases and curio cabinets without constant monitoring. Plastic displays for individual pieces can be made that are sealed and have desiccant hidden in them. They are effective but may not display the pieces quite as you wish. So selection of specimens for open air display presents some choices. Meteorites of lower economic value like unclassified NWAs or perhaps valuable but oil protected irons may be the pieces that are chosen. Desiccants for dry boxes are excellent and require low maintenance. I have used them for years with great success. They only need to change when the indicator cards show increasing relative humidity. But, some specimens and certainly ones on display need more aggressive protection.
The polished face of a rerusting SAU001. This meteorite had bad handling in the beginning maybe as far back as the in the field. It is now stable only after three weeks of soaking in alcohol and regrinding and repolishing dry. After several month I am gaining confidence that its problems are past.
The second solution: barrier coatings, I guess begins with the simplest coating; oil. This works OK for iron individuals that you want to display but it can be messy. And you have to decide what oil to use. The list is long including motor oil, ATF, gun protection oils, silicone sprays, light weight machine oils, and others. There is no doubt that in many cases oil coating will help protect iron meteorites. But, no one wants to dip their stone meteorites in an oil bath. Just the thought send chills up my spine that someone would do that. I have heard of novices cutting stones with oil coolant in their saws and then trying to leach the oil back out with kitty litter clay. I want to scream when I hear things like that. There is another alternative to messy coatings and that is VCI or Volatile Corrosion Inhibitors, also called VPI for Vapor Phase Inhibitors. They do not work on the moisture in the environment. Instead they create a barrier which prevents the chemical or really the electrical activity which is the corrosion process. A thin layer of chemical is deposited as a vapor onto the surface of the metal. This layer may only be a few molecules thick but it will stop the reactions needed for corrosion to occur. The vapors can be released from impregnated papers, plastic sheets, from patches or in other forms. But, regardless of the delivery system the principle is the same. Vapor deposits on the metal surface and is maintained by the VCI product for a long time. Eventually the delivery does run out. After it does the metal will again be subject to the actions of corrosion. Also, VCI material has a range of space that it will be effective in. The vapors do not travel very far. Usually less than two or three feet from the delivery material is all that is protected. This however, is enough to take care of specimens in a display case. VCI products were specifically made for preservation of metals and have found wide use in industrial and military applications where machine parts need to be protected and stored. But, they work very well for iron meteorites and slices of iron meteorites. For specimens that are not on display but that you want closer at hand than your sealed dry box. VCI poly bags provide great protection. Especially the zipper type bags. Interleaving VCI papers between meteorite slices also provides better protection then just drying them and even vacuum sealing them. For those of us that have to keep them in great shape from the time they are cut and etched until they are sold VCI paper or poly bags is the way to go. It prevents reworking and additional lose of material and time.
Here are some other things to consider about corrosion. Avoid corrugated cardboard. It is almost universally acidic. And it is very good as an absorber of moisture. Two strikes against it as a promoter of rust. Therefore, something we should avoid storing meteorites in.
Fingers, should never touch the surface of iron slices or for that matter any meteorite’s prepared surface. Our skin has acids and chemicals that transfer and will over time show up as spots on the surface. I try to be more diligent about using cotton gloves in handling specimens nowadays. I still do not worry too much about older whole stones or older windowed stones. Their rough weathered surfaces seems not to be very bothered by occasional handling. But, fresh fusion crusted stones I handle with cotton gloves. I just do not want to see any rust on them. I still use vacuum storage and specially prepared displays. I am still considering dry nitrogen storage for some rare pieces. But, with each level of protection the costs increase and your accessibility to handle or display your collection goes down. Once again enjoyment is a balancing act, but the returns are worth it I believe.
The pristine surface of NWA 1109 Polymict Eucrite Breccia, this meteorite has rare iron scattered through it. It would be a shame for the meteorite to become stained through rusting and lose its subtle coloration changes.