ARMEX Blast media opinions

[FossilSloth]

I've had pretty good results with the ARMEX Maintenance formula sodium bicarbonate blast media. I have to run it at the highest PSI my compressor can give (about 120 psi) in order for it to remove matrix relatively quickly. Still, that's only good with soft matrix. Anything hard, and it's almost worthless. I switch over to glass beads at about 30-50 psi, but you can definitely destroy a fossil with it. It's too abrasive. I'm wondering if anyone has used the ARMEX Maintenance XL, which is supposed to be 270 microns/70 grit, as opposed to the regular formula which is 170 microns/100 grit. Then again the glass beads are supposed to be 80 grit...

[Malcolmt]

270 microns is far to large to be using blasting a fossil. The media should at most be 1/2 the size of the smallest detail you are trying to preserve. Go to a pottery supply store and buy a bag of 40 micron dolomite. It will work fine for any limestone like matrix.A 50 pound bag should cost you 25 to 30 dollars. 40 micron is around 320 mesh. for my best material I use a 600 mesh powder which is down in the 20 micron range.

[Vaniman]

@FossilSloth I am currently doing a bit of a review on abrasive media in relation to matrix removal.  I'll try to loop you in when complete.

 

[FossilSloth]

I just got some dolomite from Crystal Mark, Inc. and I wish I had been using this stuff all along. Using an air eraser now finally makes sense. Just get dolomite, any new guys reading this. Don't try to make blast media from your local hardware store work. 

[Sagebrush Steve]

1 hour ago, FossilSloth said:

I just got some dolomite from Crystal Mark, Inc. and I wish I had been using this stuff all along. Using an air eraser now finally makes sense. Just get dolomite, any new guys reading this. Don't try to make blast media from your local hardware store work. 

Can you describe or show us the difference?

[FossilSloth]

Steve, I was using 2 different kind of blast media. The first was "80 grit" glass beads from Harbor Freight. They fed through my eraser fairly well, but they were just way too aggressive. I unfortunately destroyed a few fossils figuring this out. I then switched to ARMEX Maintenance (after trying to use just plain grocery store baking soda... totally not worth the hassle of trying to filter out all the large chunks.) ARMEX does feed well through my eraser as well, and it did work fairly well on really soft matrix, but anything that wasn't super-soft, it was completely ineffective on, no matter what the air pressure. I ended up having to do a lot of preparing by hand, with an X-acto knife or a needle and pin vise. 

 

But this dolomite stuff, it seems to be able to remove the harder matrix without destroying the fossil. It is also much much more fine, so it seems to be able to get into the little cracks and dimples better. It also means it makes more fine dust, so watch out for that and make sure your filter or respirator setup is ready for it.