Air Abrasion Media

[erose]

Can anyone point me at a list of the different materials I can shoot thru a micro air abrader? Maybe in order of hardness and with some ideas about sources and qualities to look for.

I am finally getting set up for this and have purchased some dolomite and aluminum oxide. Found those in a fine form at the local ceramics supplier.

I have been reading thru the old threads and piecing together advice but really want to make sure I am on the right track.

For instance....can I use Arm and Hammer baking soda or do I need something more industrial?

[silcrete]

Hello erose,

I am not aware of a comprehensive list of the materials that can be shot thorugh a micro air abrader.

After a couple of years using an eco-blaster and an accuflo there are various qualities that I would suggest that you consider when selecting your powders, including:

• Particle size distribution (PSD) - 40 micron dolomite powder bought from a supplier of microabrasive blasters will have a very limited PSD (ie the vast majority of the powder particles will be close to 40 microns in size). However 40 micron powder bought from a ceramics supplier will have a wide PSD (ie the particles are all less than 40 micron, but range from 1 to 40 microns). The benefits of powder with a limited PSD include:
  
  • Better control over abrasive flow rates.
    
  • Less dust in the sand blasting cabinet.
    
  • Less dust, especially fine dust, clogging up the dust collector.
    

  [*]Particle size - Relatively coarse particles will remove matrix far more aggressively than fine particles. I have a preference for 40 to 50 micron particles with fine work such as cleaning coral fossils in limestone or exposing opalised fossils in claystone. For coarse work I have about 500 micron particles for removing grunge from larger lumps of petrified wood.

  [*]Hardness - It is critical to consider the relative hardness of the matrix and the fossil before selecting the powder that you will be using. Under ideal conditions the fossil is distincly harder than the matrix and you can use a powder that is significantly softer than the fossil to remove the matrix. Often the matrix and fossil have a similar hardness, which means that you have to use an abrasive with sufficient hardness to also abrade the fossil. Good technique is critical in this situation, as outlined in this very useful document - http://vertpaleo.org...9a332e370b3.pdf

  [*]Toxicity - Are there any significant health risks with a given powder?

  [*]Cost & availability.

  [*]The type of microabrasive blaster that it will be used in - The effectiveness of a particular powder can range from good to poor depending on the type of blaster that it is used in. For example, diatomite powder (swimming pool filtration medium) will work well in an eco-blaster but in an accuflo it generally hangs up in the powder tank and will not flow out. This appears to be the result of a wide PSD and/or the 'clingy' shape of the particles. An eco-blaster moves powder via a relatively simple air jet within the powder tank which is not as vulnerable to hang ups (however it does consume lots more powder and has less control options).

  [*]Recycling - Unless you have a compelling reason, I would not recommend recycling, especially for softer powders (Moh's hardness 7 or less). .

The ideal powders come from the suppliers of microabrasive blasters, but their high cost forces people to use other powders. Examples that I have tried include:

• <100?? micron soda bicarbonate (Moh's hardness 2.5-3),), food grade from supermarket. I have been unable to get it to flow in an accuflo probably because of wide PSD and lengthy particle shape.
  
• 100-200?? micron soda bicarbonate, from bulk cleaning chemical supplier. Works well and seems to have a limited PSD.
  
• 200-400?? micron soda bicarbonate, from sandblasting supplier. Works well and has a limited psd. Too coarse for fine work under a microscope.
  
• '40' micron limestone (Moh's hardness 3-4), from ceramics supplier. Creates significant dust in sand blasting cabinet and clogs up dust collector beacuse of wide PSD. Requires more frequent cleaning of cabinet and collector than most other powders. With accuflo the powder tank must be >50% full to facilitate powder flow. It is still a good powder for many tasks.
  
• 40?? micron potash feldspar (Moh's hardness 6), from ceramics supplier. Similar attributes to 40 micron limestone, only harder.
  
• Fine ilmenite, from ceramics supplier, hung up in accuflo and dark colour impaired view of item being prepared.
  

I would suggest that you try a range of powders on various fossils and find information from someone who has prepared similar fossils. Keep notes on what works and what does not. You will probably also have to fiddle with any variables that can be adjusted on your blaster to get the best results (eg nozzle sizes, air jet direction, orifice size, etc).

[erose]

Thanks, that certainly adds to my knowledge base.

[Uncle Siphuncle]

95% of my blast work is grocery store baking soda shot thru a .060 inch orifice at cretaceous echinoids. occasionally i'll throw dolomite at echs. and once in a blue moon i'll mask an ordovician crinoid or trilobite with silicone caulk, then get aggressive on surrounding shale with iron filings or aluminum oxide.

i don't like to throw hard media at fossils, even when trying to blend scribe marks on ammonites. instead i scribe matrix away, blend marks as much as possible with the scribe stylus turned sideways, then hit the ammo with a ten second muriatic acid scrub followed by a good hose rinse. this process tends to accent sutures while blending scribe marks without obliterating other detail if timed properly.

all the above plus KOH for certain echinoid applications...now you have my my entire prep playbook. good prep is part experience, part skill, and part artful eye. i don't fancy myself as very artful nor skilled so i lean more on an experienced hand.

[erose]

Thanks guys. Very helpful.

I have avoided detailed prep work for years because I have a history of doing damage. The crazy part is that I can be extremely meticulous in the field, crawling across yards of matrix, picking up micros and finding the odd bits that others miss. And for many years I was a serious model builder who prided himself on re-creating a very high level of detail. My model railroad work won awards and some of my armor dioramas are still on display at Smitty's Hobby Shop in Ohio. BUT for fossils I have shown no patience. Oh those poor ammonites...

I'm hoping better tools and a higher need (tighter IDs) for clean specimens will prove I can do this and well.

Edited November 1, 2012 by erose

[jpc]

erose-

what are you prepping with the air abrasive unit? I use a lot of dolomite and sodium bicarb. My dolomite supplier has recently become hard to find, so i/we are going to limestone. Don't know how it wioll act compared to the dolomite. Al Oxide is generally toorough for fossil work. I have some, but I have nly used it on one fossil. I will also suggest have look at the powerpoint silcrete linked to... heck, I worte it.. its full of GREAT info . I also just got some iron filings to work on German ammonites... had to order that one from Germany. A few things silcrete did not mention... make sure you have dry air. Use at least a few air driers in your comressed air lines. And the soil sieves are rgreat for sieving the less expensive dolomite and bicarb. I use a finer grade bicarb than store boyughtt Arm and Hammer, but I also doa lot of microscopic sand blasting.

feel free to ask more q's.

[Brian1701]

Hello Erose, you should look into getting some calcium carb. This is what I like to use the most. Works well on hard rock. I will find out more for on a supplier for you.

[erose]

Thanks for the further suggestions. Right now I have a dime store (=Harbor Freight) air eraser. I have to get the air lines set up with drying filters and a glove box would help. I did a little experimenting in my open shop wearing full mask and goggles and the dust certainly spread it's way around. Without the filters the pen would shoot for a few minutes, clog, clear, clog, clear....

I purchased media of the correct size but that eraser comes with only one nozzle and from what I have read on the forum it is small.

I've been busy with work, home life, chores, etc. but will get back to this soon.

[Malcolmt]

The nozzle on the air erasers is actually from my point of view quite large and impresice. Read my posts on how you can use blunt hyperdermic needs in different gages to get more precision from it.

[Brian1701]

Sounds like you need an air dryer. I built two of my own for what it cost to buy one! If you would like I could give you spec's to build your own. I also have built a dust collector if you want info on that.

[erose]

The nozzle on the air erasers is actually from my point of view quite large and impresice. Read my posts on how you can use blunt hyperdermic needs in different gages to get more precision from it.

Thanks Malcolm. All I really know is it is not the ideal size. I have a lot to get straightened out in my head on this subject.

Sounds like you need an air dryer. I built two of my own for what it cost to buy one! If you would like I could give you spec's to build your own. I also have built a dust collector if you want info on that.

I'm game for DIY. Send me a PM or post it here. I know others would be interested as well.

[Grenzton]

hello erose.

since any years i use only ironpowder in my "cherrybomb". depend of the airpressure, it is soft enough to save any shale-materials like calcitic shells of seaurchins, but it is hard enough to work properly against the matrix.

After the use of the iron you can seperate it from rockpowder very easy. So you can recycle the iron endless times. all of my gernan fossilfriends work with ironpowder

greets, Karl

Edited November 29, 2012 by Grenzton

[jpc]

I just bought some iron powder... imported from Deutchland, and am looking forward to using it... at least on German ammonites. I am scared of what it might do to a lot of the bones I work on.

[jpc]

Sounds like you need an air dryer. I built two of my own for what it cost to buy one! If you would like I could give you spec's to build your own. I also have built a dust collector if you want info on that.

I, as well, would love to see your DIY projects.

[Nandomas]

I just bought some iron powder... imported from Deutchland, and am looking forward to using it... at least on German ammonites. I am scared of what it might do to a lot of the bones I work on.

Just curious: did you buy the abrasive from our German common friend Martin?

I am experiencing a lot of clogging problems with my new dental air abrasion machine. I am using baking soda, I am working at 70/80 psi with 1,2 mm orifice. The problem is that most of the time the baking soda doesn't go out from the reservoir

Edited December 9, 2012 by Nandomas

[Malcolmt]

Empty your unit, Bake the media in the oven at 250 fareinheight or 110 centigrade (just over boiling point) for 20 minutes on a cookie sheet, let it cool right down put it back in the unit. If it works then you have a moisture issue. dolomite and sodium bicarb tend to absorb moisture from the air. If it stops working after about 10 minutes then you likely also have mositure in your air from the compressor. The mosture problem gets better in the winter over here in Canada because the air is much drier in the winter because the outside temperature gets below the dew point. My compressor is in the garage and has a large tank so it essentially gets outside air not humidified air. Our furnaces have humidifiers on them here to put moisture back into the air. Before I got a compressor in the garage I was having major moisture problems. I now run about 50 feet of air hose from the garage to my basement laundry room where I have a wall mounted large desication unit. (cost about $100 at Princess auto (like Harbour in the states) costs about $30 to change out the dessicant every 6 months or so.In the summer I get tons of moisture in the drop line in the winter I get virtually none.

Edited December 10, 2012 by Malcolmt

[Nandomas]

Thanks for your help. Please tell me more about those moisture traps. If I understood good, they need to be on the air line from the compressor to the sand blaster unit.

my sand blaster unit already has two sinterized filters, one in each the unit

[jpc]

nando-

what is a sinterized filter?. There are several types of air drying filters, and I am also sure that this s where your problem is. Unless the powder is perhaps too big for your orifice in your sandblaster. Some air driers work with silica gel and some by centifugal spinning of the air. I have one of each in my system. I live in a dry place, too, and with two filters I never have a moisture issue.

malcomt... I also have baked my dolomite, but now I fnd that a small ziplock of silica gel in the 5 gallon bucket of powder works well... at least in my dry climate. I have heard that sodium bicarb does not stand up well to heating it enough to dry it. Any comments on that?

[Malcolmt]

I have not had an issue with bicarb or dolomite at 250 F.

Sodium bicarbonate does dissacociate with heat but my understanding is that it is a fairly slow process, the 20 minutes that I bake does not seem to do much to it. I believe that Dolomite attracts water much more readily than bicarb that certainly is consistent with my experience with the two products

On heating bicarb you get the following

2 NaHCO₃ ==> Na₂CO₃ + H₂O + CO₂

The water and CO₂ are driven off in the oven. I suspect that the sodium carbonate that remains mixed in with the remaining bicarb works just fine as an abrasive

I bake a quantity of media I put it in sealed glass containers, they seem to keep the mosture out. The issue I have is if I leave the powder in my COMCO air abarasion unit for 3 or 4 days without usinng it it definately attracts mositure and has clumping issues. I typically spend an hour or two sieving and baking ending up with about 2 litres worth of media that I store in 4 500 ml glass containers with tight fitting lids. I grade them as the following sizes

< 320 mesh (<44 micron) (like a very fine talc use for final prepping of delicate features with .018 nozzle)

>350 mesh and < 200 mesh (< 75 micron) (a good general purpose size with .018 nozzle)

> 200 mesh and < 120 mesh (Good with a .03 nozzle)

> 120 mesh and < 100 mesh (good for rough blasting with a .046 or . 06 nozzle)

Edited December 11, 2012 by Malcolmt

[jpc]

thanks for the input malcolmt. Hs another question. My dolomite supplier seems to be out of stock more often than they actually have any. Where do you get your dolomite? I just tried some limestone powder for the first time, and it is not as good as dolomite.

[Malcolmt]

I get my 44 micron dolomite from Paletools in a 22 pound box for about $32 plus shipping. I screened some when I got it and virtually 100% goes through a 320 mesh sieve. Fortunately I have a US address (Rochester) that my son goes to all the time that I can get heavy things shipped to otherwise shipping to Canada would be about $100. I also get a large 40 kilogram bag of dolomite from a local air abrasion company that does industrial soda blasting. ( He will sell me bicarb, dolomite, aluminum oxide and glass powder). The dolomite was very cheap $20 had to go pick it up from their shop. When I screen it I get about 40 % is bigger than 100 mesh, (will go through a 80 mesh but not 100 mesh). This I just throw away. The rest is pretty much in the 100 to 200 mesh range. Based on what I throw away it is about 1/4 of the price that the Paleotools media costs me after shipping. The operator of the blast shop uses large industrial nozzels so particle size is pretty much irrellevant to him. Unfortunately in the cheap bag of dolomite I get very little that is smaller than 320 mesh (44micron) so I still need to buy the good stuff from Paleotools. I tried the glass powder (a free sample) but I did not like it. Left tiny pits in the fossil, I suspect because the particles are somewhat round and all the energy is focused to one tiny impact point.

When I bake the dolomite I do about a pound at a time. I do recycle my dolomite. I clean the blast cabinet out if I am switching between bicarb and dolomite. I use dolomite for 90% of what I do. I also hate screening powder but if I did not do it then the costs of prepping would go through the roof....... I also have a 2 canister dental unit with a built in blast chamber, I keep one of its canisters permanantly loaded with bicarb. Don't use it much any more as it is no where near as precise as the COMCO.

Edited December 11, 2012 by Malcolmt

[squalicorax]

I have not ordered here from Continental Clay but i will soon and report back. Paleotools is pretty expensive just for some dolomite powder. http://www.continentalclay.com/detail.php?PID=664&cat_id=197&sub_categoryID=4 is less than a dollar a pound.

[Malcolmt]

Squali, I don't like that the data sheet indicates that it can contain up to 1% quartz. Silica is very bad news....

[squalicorax]

I noticed that. Does the paleo tools say the same thing? I would be there would be some minerals other than dolomite in there. At less than one percent I think its reasonable to try out.

[sward]

erose,

Thanks for starting this thread.

I'm in the same boat as you. I had picked up a Paasche air eraser earlier, but only recently obtained the air compressor. I've only had the chance to try it out for very little time so far, so I'm still trying to figure it out.

The air eraser came with a container of "fast cutting compound" (ingredient list...aluminum oxide). I also ordered a container of "slow cutting compound" (ingredient list...food starch). I'm wondering if the "slow cutting compound" is an excellent marketing scheme that I fell for. :doh!:

I have tried operating the eraser anywhere from approximately 10-65 psi. So far, I haven't had much luck, particularly with some of the limestone from the area. I have found that if I can remove most/all of the limestone by mechanical means, the eraser does a pretty good job of "cleaning up" the fossil. I've had the best luck so far with the aluminum oxide at higher pressures.

I was hoping that this would also help me remove some of the limestone so I wouldn't have to do it my mechanical means. Some of the stuff I planned on prepping with the eraser is somewhat fragile, therefore, not always a good candidate for mechanical prep.

I'm learning alot from this thread. Keep us posted on how you're progressing with your experience. I'm anxious to hear how it's working for you. Maybe I'm still figuring it out...maybe I was expecting too much from the eraser.