To Clean or Not to Clean
For a number of years I have been intrigued by the prospect of cleaning meteorites with fine abrasive blasting. I had never done it. I had checked out the equipment at Tucson and other places. I had found it to be expensive. Way more than the amount I wanted to spend for the small number times I might use it. Still I thought it would be interesting to try.
A few month ago my old Harris offset press was finally sent off as scrap metal. Before the old press left I took off the Ortman-McCain powder spray unit that had served me so well for 25 years; preventing offset of printing jobs. I wondered if it could not be adapted to spray fine abrasive powder instead of vegetable starch powder.
I am sort of known for making interesting machines out of old junk. This
would be just one more project of many. I started modifying the fifty year old
unit at 9 am and was done by 10:30. My regular blow gun worked for a test.
However, it would be way too large for the type of work that I wanted to do on
meteorites. It did tell me that I was on the right track with the Ortman-McCain
unit. Printing equipment powder spray units are not designed to operate at high
pressure. They have gauged regulators and secondary gauges to show the pulsed
working pressure. This gauge topped out at only 30 PSI. I set it to that
pressure and pressed the trigger. I got a spray of offset powder to blow out and
that was encouraging. Now I needed a much finer tip and to connect some more
flexible smaller hose. I chose ¼ inch clear plastic hose that I had around the
house. I needed some barbed hose connectors too. I knew just the store about
five minutes away that would have what I needed. A $3.99 kit that contained two
barged connectors with normal air equipment threads on the other end, four hose
clamps and one two ended barbed connector for joining hoses. Perfect, just what
I needed. A tip of steel tubing only about 1/16 of an inch diameter and this
project would be done. That was no problem either. Another half hour work and I
was pouring out the old offset powder and replacing it with 220 grit aluminum
oxide for a real test on a meteorite.
This kind of equipment
needs to be used with a sealed box so the operator never breathes the dust. This
test would be very short. I found a nice NWA 869 that had a good coat of scale.
It had some nice rust spots also and brown spots in the scale. I took the unit
out to the backyard in the fresh air, popped on the quick disconnect of my
compressor hose and charged the unit up. I opened up the volume control for the
air, set the needle valve control for the powder delivery, pushed the trigger
and saw a burst of abrasive powder. It settled instantly to a fine stream. There
must have been a little spilled into the hose from carrying the unit outside.
Now to try it on the meteorite. Boy did it work great. I was surprised. It swept
away the scale and dirt and rust like crazy. But it was soon totally
ineffective. After the scale was gone it did nothing else to the meteorite. Or
so I thought at first. Later when I examined the stone under a good light with
magnification I noticed that there was a little polish to the surface that could
not have been there under the scale. I think the abrasive blasting created that
I should say at some point here that I personally like my meteorites in a
clean, but as found condition. This entire project was more for the experience
of doing it than to use the process very often. I only brush meteorites with an
old toothbrush usually to clean off the remaining dirt. I might use the spray
unit on an occasional low cost NWA stone with stubborn scale. It was fast and if
used conservatively the results looked good. Clearly continued use in one area
of the stone results in a little alteration of the surface. But, even with this
experimental stone there is a great improvement in the detail visible and the
hidden fusion crust was greatly improved.
I will probably get some fine glass beads and try the unit with that instead of the really aggressive aluminum oxide. Not a bad couple hours work and a few dollars. Learned a lot and got some interesting results. Now the harder part of building the dust proof glove box. It has to have a clear lid so I can watch the work obviously. I think it ought to have a light inside too. Maybe next weekend I will start doing that project.