How to Use the Various Types of Abrasive?

[jdiaz55]

Hi all, I have been searching through posts in the forum about various types of abrasives, and I can't seem to find a comment anywhere where someone breaks down the pros/cons of the different forms of abrasive and their microns.

 

Of course I realize everyone has their own preferences, I am just confused as to the objective benefits of what looks like the three main types of abrasives: dolomite, aluminum oxide, and just plain baking soda (besides the fact that baking soda is more delicate.)

 

The Paasche Air Eraser comes with 240 micron aluminum oxide, but that seems to be a little extreme for fossil preparation, is there an appropriate time and place in which I could use that?

 

What would be the max (or even a general range) micron of either dolomite or aluminum oxide that one would recommend I use for 1. Green River fish, 2. trilobites, 3. ammonites, or 4. just general fossil clean-up?

[Kane]

The medium is contingent upon the hardness of the fossil and the matrix. Ideally, the abrasive should be harder than the matrix, but softer than the fossil. It is not as simple as saying medium x works for, say, all trilobites. Also, some prep requires more than one medium. For instance, I may use a 40 micron dolomite or dolomite/sodium bicarb mix for removing larger matrix, but then switch to bicarb at lower pressure for fine touches. 

 

Sometimes it is less about the abrasive medium, and more about the pressure and the ability to dial down the PSI to avoid blasting off the fossil!

 

Aluminum oxide is among the more powerful cutting agents, but would be too much for what I work on. I've seen it used on some ammonites, but again that is contingent upon the hardness of the matrix and fossil. In terms of the GR fish, that can be done best by hand with pin vice as the bones are quite delicate and flaky. For trilobites, it depends on the trilobite! 

 

A tougher cutting agent can be useful for matrix shaping. I'd put a "guard" between the matrix and the fossil in that case (like a piece of thin cardboard or plastic). I'd be careful with using too strong a cutting agent with your Paasche as it will wear out the little brass fittings inside faster -- and the same can be said in using a softer abrasive at high pressures.

[snolly50]

Kane has provided some thoughtful guidance. I have a Paasche device as well, but have used it only sparingly so my level of experience will be of little help. Except, I can give a strong admonition to protect your lungs, no matter the agent employed or containment system available. Blasting small particles into the air creates conditions that a prudent individual would do well to avoid. Aluminum oxide is classified as a lung irritant. No thanks. No matter the medium, it's best to mask up unless you possess a super tight blast chamber. Good luck, have fun. 

[jpc]

these gentlemen have spoken well.  

 

For Green River fishes, it depends on which layer they come form.  18 inch fish are generally done with dolomite (I do not know know what pressure).  Split fish should not be done with an air abrasive... hand tools only.  

 

I have been doing this work for 20 years and on many many different fossils form many many different beds and continents,and I have only used aluminum oxide once.  Once.  Bicarb and dolomite.  And for the most part it is good to have a supply of both on hand so you can learn with each specimen.  

[Ptychodus04]

I would take @snolly50‘s advice one step further. Wear a respirator, not just a dust mask, even if you have a blast cabinet with a dust collection setup. You only have one set of lungs.

[Sagebrush Steve]

3 hours ago, jpc said:

these gentlemen have spoken well.  

 

For Green River fishes, it depends on which layer they come form.  18 inch fish are generally done with dolomite (I do not know know what pressure).  Split fish should not be done with an air abrasive... hand tools only.  

 

I have been doing this work for 20 years and on many many different fossils form many many different beds and continents,and I have only used aluminum oxide once.  Once.  Bicarb and dolomite.  And for the most part it is good to have a supply of both on hand so you can learn with each specimen.  

 

And just to confuse things further, when I was at Fossil Butte National Monument last fall, the ranger there who was demonstrating how to prep Green River fish from the 18” layer was using iron powder at low pressure.  He told me they rarely use dolomite any more, their abrasive system loaded with dolomite was sitting off in a corner gathering dust.  I think one of the reasons they like iron powder is that you can easily reclaim it using an electromagnet to separate it from the dust.  Here is a link to my trip report:

 

 

[smt126]

Do they use the .3 micron black iron oxide? I was looking online at the different iron powders out there.

[Sagebrush Steve]

7 hours ago, smt126 said:

Do they use the .3 micron black iron oxide? I was looking online at the different iron powders out there.

Good question and I didn’t think to ask.  I don’t think it was iron oxide, just raw iron powder, but I’m not sure. @steelhead9 might know what to recommend.  I could also try emailing the museum curator, Arvid Aase to see if he could share more detail on what they use.

[smt126]

Yea I was just looking online and there are a lot of varying sizes and types of iron powder out there. I’m hoping to try out some air abrading this summer when it gets warmer.

[jpc]

iron powder... NOT iron oxide.  I don't know what size I have, but I tried it and still use dolomite mostly.  Iron powder is more commonly use din Europe.

[Sagebrush Steve]

So I decided to send an email to Arvid Aase, the Museum Curator at Fossil Butte National Monument, to learn more about their use of iron powder abrasive.  He sent me back an entire treatise on the subject, which I have copied and pasted below.  @Vaniman@steelhead9 @Ptychodus04, @aeon.rocks@jpc, let me know what you think.  If anyone gets out to visit Fossil Butte anytime soon, be sure to stop by and thank him.  And if I worked for a company that makes air abrasive systems I would use this info as an excuse to take a company-paid vacation business trip out to Fossil Butte so I could check out the details of what they do, how they have modified their gauges and hoses, etc.

 

Here is what he wrote:

Steve,

Iron powder has been available in Europe as a blasting agent for a number of years. It was only recently imported to the US. As far as I know, the only supplier in the US is Paleo Tools. Mike Eklund, who, to my knowledge, was the first American fossil preparer to recognized the benefits of iron powder, brought a few pounds home with him from Europe. He used it to prepare Green River Formation fossils. It was sometime after his 25th reuse of the powder that he successfully convinced Crystal Mark to import iron powder for sale in the US. I had seen the improved quality of work from using iron powder so we bought some. One of our seasonal staff was half way through preparing a small Diplomystus, a fossil fish, when we switched to iron powder. When finished he showed me the specimen. There was a vertical line through the fish, on the side prepared with dolomite the delicate scales and distal fin tips were nearly all gone and on the side prepared using the iron powder the scales were nearly all present along with the distal fin tips. For more than a year we used solely iron powder. But then we had some fossils in kerogen-rich limestone which is a harder matrix. The iron powder, even at high psi, would not effectively remove the matrix. So, we bought a second abrasive unit which is used for dolomite powder. 

Pros of iron powder: 

1) can be used at very low pressure

2) using at low pressure allows greater control and thus better quality preparation

3) can be reused many times, upper limit has not been reached but well over 50 times

4) the granules are heavy and thus do not become airborne to become a respiratory hazard

5) the granules are heavy enough that low flow dust evacuation will take away the matrix dust but leave behind the iron powder in the bottom of the box

6) iron powder is listed on the supplier label as 80 mic but can be sifted into size fractions for different applications (such as a set of sieves 150um/0.0059in, 106um/0.0041in, 90um/0.0035in, 75um/0.0029in, and 63um/0.0025in). The larger sized sieves remove the matrix residue, you can choose to sieve even smaller sizes if your application requires it.

7) works exceptionally well on fossils with soft matrix

8) reusing the iron powder offsets the initial higher purchase price

9) small amounts of fine matrix powder mixed with reused iron powder doesn't reduce its effectiveness

10) Using proper settings and with proper sized hoses it can be used at pressures in the single psi range without clogging hoses

11) spilled iron powder can be collected using a magnet wrapped in plastic, thus none is wasted even if spilled on the floor (see con regarding use of magnets)

12) the granules are heavy and larger than dolomite powder and not subject to static charge, thus when finished preparing no need to blow air across the specimen to remove residual powder and potentially damage the fossil, simply turn the specimen on edge and the iron powder falls off

13) although on the mohs hardness scale iron is harder than dolomite or baking soda, it is more gentle on the specimens because the iron is in round pellets without the sharp edges of crushed dolomite or baking soda crystals and can be effectively used at lower pressures which translates to lower velocity when impacting the specimen

14) can be used in micro abrasive machines without any modifications to the machine (assuming changeable orifice is sized correctly)

15) powder is rounded pellets and thus less abrasive to hoses and nozzle tips

16) no dust created when refilling the chamber with new product, less dust when filling with reused product

 

 

Cons of iron powder:

1) Initial cost per pound is higher than other blast media

2) The supplier label lists the powder size at 80 mic, but the iron powder is not sifted into specific size ranges, thus if application requires specific size ranges the user must purchase sieves and sift the iron powder.

3) is not effective on harder matrix

4) when preparing very delicate fossils that require pressures in the single psi range, the pressure gauges and hoses on your abrasive machine will need to be changed to provide the user the ability to accurately control air flow and maintain movement of powder through the hoses at very low psi.

5) using magnets to collect powder for reuse has potential to magnetize the iron powder and cause clumping. No formal tests have been undertaken and several users have reported no issues using magnets to collect iron powder for reuse

6) less dust is created (a positive aspect) but don't become complacent, dust collection is still needed to capture the matrix being removed.

 

Product or company names are used only as reference, not as an endorsement.

 

Well, that's what I can think of at the moment. I am sure to have forgotten something, but I think all the most important things are mentioned. I hope this information will help forum members improve the quality of their fossil preparation.

 

Regards,

Arvid

 

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[Vaniman]

Thank you for the heads up @Sagebrush Steve! I am actually well aware of Iron powder as Vaniman has customers in the UK and Germany that specifically deal with this media.  Suffice to say, the people I have spoken to are using Iron Oxide for very hard materials at higher PSI.  Most notably, people on the Jurassic Coast and I believe various areas in Germany like the Messel Pit.  I have not tried the iron myself, however I was given some sample Ammonites to work on using our existing medias of aluminum oxide and sodium bicarbonate.  The results were so-so as the material was extremely hard and likely in the 8.5-9 mohs range.   

 

That being said, his mention of the advantage of "rounded" media is interesting.  Interesting because Glass Beads, which is a very common media, is also rounded and easily purchased pre-sifted.  I have yet to try glass beads at single digit PSI's are our R&D machine is not fitted for anything below 15 PSI.  I have tried prepping Green River specimens using glass beads with great success but I am not as...patient let's say.... as most preppers and was using the media at roughly 40-60 PSI on our machine.  Glass beads are also much lower on hardness scale (maybe 4-5?) so with a lower PSI it may actually be a cheaper, easier solution compared to Iron.  

 

Regarding the potential of sourcing the Iron and selling in the states I can tell you it's all about simple supply and demand.  The demand for that would simply be to low for Vaniman to warrant purchasing and reselling it here unfortunately (something I've looked into previously).  I'm sure you can find several US suppliers though if you look hard enough. 

 

 I'd love to allocate more time to this however I'm afraid this particular issue won't warrant any additional resources from my end of things. Hopefully, some brave soul out there can retrofit a blaster for lower PSI and test glass beads.  Sometimes the easiest solution is the simplest one   

 

 

[Ptychodus04]

@Sagebrush Steve thanks for doing the leg work on this. That is some great info. I can see where it would be a good media to use.

 

I wonder if there's a user bias that hasn't been recognized also. It is quite possible that the simple change of media caused the preparator to be more careful while working, therefor making their results better. Maybe they had become complacent with the dolomite since its relative softness can seem to make it less of a hazard to the specimen.

 

Also, I'm surprised that it would be less effective than dolomite on hard matrices. Iron is so hard that I would think simply increasing the air pressure would make it cut through any matrix.

[jpc]

I did a talk on air abrasion a while back, and in my research I found that iron powder has a hardness of 4, not way up near 9.  I was shocked to see this but I think I got that form a sandblasting supplier.  

 

Mike Eklund says he can prepare Bonanza/Douglas Pass insects with iron powder at 4 psi,  but he does re-rig his Swam Blaster to get controlable airflow at such low psi.   

[Ptychodus04]

7 minutes ago, jpc said:

I did a talk on air abrasion a while back, and in my research I found that iron powder has a hardness of 4, not way up near 9.  I was shocked to see this but I think I got that form a sandblasting supplier.  

 

Mike Eklund says he can prepare Bonanza/Douglas Pass insects with iron powder at 4 psi,  but he does re-rig his Swam Blaster to get controlable airflow at such low psi.   

Now, that hardness information is surprising. What’s the lowest pressure you’ve run dolomite at?

[jpc]

I run mine down to 5 psi and even lower.  The Swam Blaster is better at low psi than the Comco.  

 

I do this for small critters from the  White River Fm... rodents, lizards etc.

[jpc]

I also got a small supply of iron powder fr a European guy a few years ago.  I tried it once and have not converted, but I am impressed with Arvid's tale of the fish.    

[Vaniman]

One last thing.  I wanted to make sure everyone is aware of a major caveat when it comes to using air abrasive.  It is very much a developed skill that improves over time. I personally have experience among several different industries with people who work on various mediums like steel, gems, plastics, etc.  The angle, pressure, and distance all effect the result greatly.  The media is a major factor as well however it's often that I find people using different media for the same application.  Just some food for thought

[oilshale]

3 hours ago, jpc said:

I did a talk on air abrasion a while back, and in my research I found that iron powder has a hardness of 4, not way up near 9.  I was shocked to see this but I think I got that form a sandblasting supplier.  

 

Mike Eklund says he can prepare Bonanza/Douglas Pass insects with iron powder at 4 psi,  but he does re-rig his Swam Blaster to get controlable airflow at such low psi.   

 

Pure Iron powder is very ductile and has a hardness of only 4. Some impurities in the iron such as carbon cause the hardness to increase sharply - the iron becomes steel. The carbon in typical steel alloys may contribute up to around 2% of its weight.  Therefore only very pure iron powder is used for sandblasting - not the normal stuff you get everywhere.

Sandblasting equipment suppliers mainly sell materials that are supposed to produce high abrasion - these iron powders contain a lot of carbon and are not suitable for our purposes.

I operate my DIY sandblaster mostly with iron powder without problems at pressures around 5 psi (or even lower).

Thomas

[oilshale]

Quote from a previous post of mine:

"Air abrasion" is not an especially good term to describe how air blasters really work. The main effect of an air blaster is not grinding away the matix - it is creating small fissures by hard impacts,  hitting away the matrix. Of course, Mohs hardness is also important, but it's not the only important parameter. If it would be only grinding away the matrix, aluminum oxide (often called alumina) with such a high Mohs hardness (9 - diamond has 10!) could not be used at all!
The impact energy of particles onto the substrate is the main effect to create small fissures and to break the matrix apart and hence the density (mass of individual particles) of the "abrasive" is very important. Iron has a much higher density (7.8g/cm³ for iron versus 2.7g/cm³ for carbonate); at the same particle size, the impact of iron powder is more severe (The impact energy is proportional to the mass multiplied by the square of the velocity:  E=mv² ), so you can work at lower air pressures. Soft iron powder (not from carbon rich steel) has a Mohs hardness of only 4 to 5; slightly higher than that of calcite or calcium carbonate (3) and dolomite (4) but much softer than that of aluminium oxide (9). 

The particles are also round, hence the powder will flow quite easily. The particles are ductile and do not burst on impact (unlike calcite or glass beads),  you can recover and clean  the blasting media quite easily with a magnet. 
Do not confuse soft iron powder with ordinary iron powder made out of carbon rich steel which is much harder and will give quite poor results.

Soft iron powder is an extremely good "abrasive" for very delicate Bundenbach fossils such as this Mimetaster:

 

This can explain why even baking soda with a Mohs hardness of 2.5 can remove much harder matrix.

Thomas

 

 

[jpc]

good to know all of this, oilshale.  I will make a note to myself to use the iron powder on my Bundenbach fossils.  

 

[Malcolmt]

you can also use

magnesium carbonate, calcium carbonate, nepheline syenite, silicon carbide, walnut shell, corn starch , alumina .. A wide variety of hardness and shapes. Abrasives generally are either blocky with sharp edges which cut  or rounded which works more by a peening / impact to fracture off tiny bits of matrix. For most fossil prep I tend to use a particle size of 40 microns . I never use anything bigger than about 80 microns and then very rarely when removing bulk from very hard Moroccan fossils. Silicon carbide is very hard about 9 on the mohr scale and is readily available is very small sizes the one I use is 18 to 20 microns but 10 micron is available at reasonable prices.

[Ptychodus04]

@oilshale that added info really helps. It makes a lot of sense. It’s the interaction of mass and velocity.

[Malcolmt]

I tried a very fine green glass bead (powder) many years ago. Got it as a free  1 pound sample from a company that does blasting . Was not as good on Ontario shale or limestone as dolomite so never went back to buy any, Still probably have 1/2 pound somewhere. Particle size was not uniform and probably in the 80 to 100 micron range. You want particles to be uniform in size . If they are not this will promote clogging and clumping of the material.

[smt126]

Lots of information to think about when I try this out this summer. Thank you to all who have been posting.