Stopping Rust & Preserving Iron Meteorites

I had a problem with a 111g Slice of Chinga which was rusting very badly. After posting a message to the Meteorite Central Mailing List, John Gwilliam replied with the following process which works extremely well in stopping and preventing rust in iron meteorites. If you try this process, be extremely careful when handling the chemicals. Sodium Hydroxide (Caustic Soda) is very corrosive and can easily burn skin or cause serious injury to eyes. Always wear gloves and safety glasses. I have left a note after the process description about my personal experience with this method.

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"Many of the Chinga irons they have found have been recovered in stream beds. This means that the irons have been in almost constant contact with water for a long time. Over hundreds of years, rust has been able to migrate into the interior where it can be a chronic problem if not treated properly.

After the meteorite has been sliced, I sand down the flat surfaces starting with a 100 grit or even a fresh 220 grit sanding belt (on a water cooled lapidary sander). Successively finer grits are used until I'm down to a worn 600 grit belt. Remember to wash the iron slice and rinse in anhydrous alcohol in between every grit change so you don't have a stray piece of larger grit mess up your polished surface.

After the worn 600 grit polish, I treat the specimen for 5 - 10 days in the alcohol / sodium hydroxide bath recommended by Steve Schoner. This will attack the rust in the specimen and the sodium in the mix bonds with the chlorides in the rust and becomes common salt.  Believe me, this solution really works. I have several slices of OLD Campos sitting unprotected out in my shop that were treated over 18 months ago and none of them show a hint of rust.

After soaking in the bath, I rinse the specimen thoroughly in FRESH anhydrous alcohol and wipe it dry. Next, I polish the slice with a muslin buff on a jewellers spindle. My polishing unit has dual 6" x 1" stitched cloth buffing wheels that you can mount on any motor with an arbor or spindle. My machine turns at 3,250 RPMs but a slower motor in the 1,720 RPM range will work as well.

The key to the final polish is the compound applied to the moving buff. I use a product manufactured for the jewelry industry called ZAM. I believe it contains chromium oxide as the polishing agent. It is a light green hard stick in a cardboard wrapper and is available from jewellery supply outfits and some lapidary shops. Several minutes of polishing will produce the mirror like polish."

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NOTE: I simply polished my slice by hand with various grade grits which I found worked quite effectively. I have also used this method on larger whole specimens such as Campo del Cielo and Dronino with success. Other collectors and dealers have also mentioned some success using this method on badly rusting chondrites but I would suggest monitoring them carefully if you are to attempt this. As to the mix in the bath, I used a powered form of Sodium Hydroxide (Caustic Soda) and mixed it into methylated spirits (96% Ethanol). I left a slight excess of powder in the solution also. This worked fine for my needs but you could probably alter this mix if desired. 

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