Alternative grits for tumbling???

[chaosdsm]

So, I've been looking at cheaper alternatives for tumbling grits.   In this thread I'll only be referencing stage 1 grits and pricing is to my house including tax & shipping as applicable.  

Silicon Carbide (or SiC) @ 9.5 Mohs is great for really hard rocks, and probably the best option for rocks over 8.5 Mohs, but shouldn't be used on anything too soft. It's really too hard for anything under Mohs 7 unless you need some very aggressive cut in the first stage.  Every online company I've found wants to charge more for shipping than for the actual product in the bulk bags.  I have an industrial mine & mill supply store near me where I can buy it in store. Best price per 50lb is about $189 locally for 60/90 grit.

1> Aluminum Oxide (or AO) is marginally softer @ 9.0 Mohs, and typically half the cost of SiC, but still very hard for soft rocks.  Best price per 50lb is about $85 for 60 grit

2> Garnet:  7.0 to 8.5 Mohs depending on variety of garnet, it may be a better option for some of the softer rocks.  Best price per 55lb is about $65 for 30/60 grit.

3> Coal Slag:  6-6.5 Mohs This stuff can often be picked up quite cheaply.  With its relatively soft nature, this may be an ideal stage 1 grit for Labradorite, Obsidian, and other softer stones.  Unlike the first three (including SiC), it doesn't fracture into new sharp (but smaller) abrasives, instead it crumbles as is considered non-recoverable.   I have two 50lb unopened bags of 60 grit that I bought for $7 per bag, but current pricing is about $15 per 50lb bag.  

I've used all the above in sandblasting with great results.  I've of course use SiC in stage 1 & 2 tumbling, and I currently have a batch of Labradorite running with Coal Slag just to see how it does.   

4> Sand: common washed play sand 6 Mohs. Widely available and cheap, but you have to be sure to get washed and filtered sand, and do not run a dry tumble with it.  Sand will break more like Garnet, AO, & SiC than how coal slag breaks, but is also a non-recoverable product.  I'm thinking that sand may be an ideal media for tumbling shells and other delicate materials as there are not a lot of sharp edges unlike the other materials listed.   Best price per 50lb = $5 typically 60 to 120 grit depending on where it was processed....

5> Superoxalloy:  7-8 Mohs.  EDIT: 90%+ has smooth rounded sides / edges, or is even spherical - not of much use for tumbling    I've been sandblasting stuff for a few years, but this one is new to me.   Superoxalloy is an engineered alloy of oxide minerals that's created through a process of controlled formulation and tempering, which involves high heat and rapid cooling. This process creates non-crystalline particles that are resistant to breakage under high-speed mechanical impact.  I've found 2 varieties of this product, one called EpiX, available as 60 or 80 grit.  Manufacturer claims it can be reused 8 to 15 times in sandblasting.  Other variety is called KinetiX, available as 36 or 46 grit & is supposed to be reusable up to 6 times in sandblasting.   EpiX is about $85 per 50lb bag, KinetiX is about $60 per 50lb bag.

I may try play sand in a vibratory with sea shells. 

I am intrigued with the Superoxalloy, about the same price as Garnet, and supposedly resists breakage in sandblasting, might work well for tumbling.  I'm seriously considering buying a bag next payday!  

Anyone use any other alternative abrasives???  

[dillonf]

I use Tripoli powder as a pre-polish and polish in the vibe, but it could be used as a "coarse" grind for softer material. Not sure what it would do to shells? Might work pretty good.

[chris1956]

Have you tried the garnet?   

[titaniumkid]

If you try new things, please provide updates for us boring, unadventurous types so we can live vicariously through you

[chaosdsm]

Apr 26, 2024 20:06:50 GMT -5 chris1956 said:

Have you tried the garnet?   

Not for tumbling, I'm out of it right now.  All I have left from sand blasting is about 5lbs of 120 grit AO, and 100lbs of coal slag. 

 

[Starguy]

chaosdsm

The best thing about SiC is that it’s fast. Also, when it breaks down, it breaks into sharp pieces. Most other grits will round off and become less effective pretty fast.

[chaosdsm]

Apr 27, 2024 0:41:01 GMT -5 Starguy said:

chaosdsm

The best thing about SiC is that it’s fast. Also, when it breaks down, it breaks into sharp pieces. Most other grits will round off and become less effective pretty fast.

Garnet & AO do the same thing, however, they do tend to round off a tiny bit more than SiC though.   I have a harder time recovering them too even though both are slightly denser than SiC. 

I love SiC, just don't love the price these days...  Consumer pricing in the US for SiC has steadily risen faster than production pricing, about 15%/yr vs 11%/yr respectively.   Fortunately, I can capture used grit down to about 800 grit / 25 microns.  However, my screens are too small for processing a lot of grit at a time, and get clogged from all the rock flakes, chips, & dust once I get below 96 microns if I try to process too much.

[rockbrain]

Johnson Brothers has 60-90 SIC(and other grades too) shipped for $109 for 44 pounds. JB can be notoriously slow and unresponsive for orders but I've ordered grit a few times and it's always been real quick for this size. It's all prepackaged. You get a whole 20 kilo bag that just fits in a flat rate box.

[Fossilman]

Sand is worthless, it's been tried... Anymore I buy all my grit from the rock shed in SD, he sells good grit and has fair prices and fast delivery...

[pebblesky]

Rockshed ships really fast. A 25 pound bag of 60-90 is $70+$20 shipping for me (as of April 2024). I might try Johnson Brothers 44 pound bag next time, once I finish using that 25 pound bag.

[chaosdsm]

So... I got some of the Superoxalloy....  definitely for sand blasting NOT for tumbling: 

Here's 120grit SiC:
 
Plenty of sharp fractured edges there to cut & scrape rocks....


Superoxalloy 40/70 medium mix?  I thought I ordered the 20/70 coarse mix.... anyway:


This stuff is more rounded than sand    There's a few odd sharp pieces in there but I'd say 90% has rounded edges, or is spherical.  At least I can use it in my sandblasting cabinet!  

[advancedlemon]

Hi, I'm basically a lurker on here but I have a handy tip for you.

I mostly do agates, carnelians, jaspers, flint, volcanics and other relatively stubborn stuff. I can't speak for tumbling in the USA but here in the UK SiC is very expensive, electricity is also very expensive.

Have you tried boron carbide? Here, boron carbide is about twice as expensive as silicon carbide, but works at least twice as fast, cuts for at least twice as long, and requires 1/2 of the amount of grit per load. Overall I find that one tub lasts for ages, saving time, electricity and grit. Win win win!

Watch out though, you will have to add lots more water because that slurry gets THICK! I have my theory about this too which runs quite contrary to all the wisdom on this forum: Rocks do not tumble about in air, being cushioned by the slurry, or by the grit in the slurry. They tumble about in water, or slurry, where they have a lower relative density, which 'floats' the rocks a little and prevents them from crashing about. If you think about it, this is exactly what happens on a beach and is the reason that water can move such large rocks with relative ease. Now, the grit and slurry mixture has a 'golden ratio' where most of the grinding happens. It starts out too thin to mix the grit in with the rocks well, after a few days starts to thicken and eventually passes through the golden point where almost all the grinding work gets done very quickly, resulting in a slurry which becomes too thick with rock dust for the grit to cut effectively. You can hear what stage you are at from the sound the barrel makes. Somewhere during this last stage is the point to stop the process and start over with fresh water and grit. (You can filter the slurry to save the grit for later use if you are able.) If you leave it too long the slurry gets so thick that it sticks to the rocks - then they do crash about in 'air' and very soon get ruined!

So, it's basically a bell curve which is affected by several starting variables. There is therefore a limit to how fast you can grind rocks inside a closed space with no way for the slurry to escape, no matter what grit you use. Time spent mucking out barrels in between runs should be factored into the equation too. More water at the starts means more room for more rock dust, which effectively means more work gets done, but time taken increases too. You can use less grit to use less water which takes less time but less work gets done as a result.

With SiC I have found that the grit itself breaks down too easily and the dust in the water is mostly spent (in this case wasted) abrasive. With boron carbide, the grit is very resistant and will produce a lot more rock dust as a result, so you have to use lots more water and allow a bit more time, but overall you get fast results, using less grit. I often have a little unspent boron carbide at the end of a run, but lots of rock dust forming the slurry. (I'm still experimenting - perhaps I should use even less grit to cut down on unspent abrasive?) Now I imagine that diamond powder would be even more resistant, and it's hardness would get you a very thick slurry very fast, so again you would use even more water and even less grit and maybe even more time, to get marginally faster results, most likely. Possibly for a great deal more money but I have never tried it or done the maths since I am not doing this commercially. You could of course reuse the unspent diamond if you were able to recover it from the slurry. Also, there comes a point where you have to start reducing the amount of rocks in the barrel to allow for the extra water. I have not tried that since my gut feeling tells me that the result would surely lean towards becoming less economical again.

So yes, none of this use virtually no water stuff you hear about, and in the end it's about how economical it all is when everything is put together. Personally though I haven't time for silicon carbide even if it _were_ much cheaper.

Basically my barrel starts out very full, maybe 90-95% (See, I even get far more rocks in than with SiC!), with lots of water almost to the top, not too much grit, runs for up to 2 weeks, and the result? Half a barrel full of rocks swimming in a thick black goo. This is using boron carbide.

Usually I have a good handful or two ready to move on (my starting material is sadly often very flawed) which results in 1/4 to 1/3 barrel full of rocks to run again. In fact I have trouble keeping up and the machine ends up being idle some of the time waiting for enough new material to be added. This isn't really a problem as it runs in my den within the house and I get fed up with the noise after a while anyway.

I went back to SiC once to see if I could make it work effectively. I got through a whole tub of the stuff and wasted a lot of time doing in 4-6 weeks what a few tablespoons of boron carbide will achieve in 10-14 days! It was a very frustrating experience which I don't want to repeat.

Needless to say I will not be using SiC in coarse again for this type of material, however for the later stages I do use SiC since all I'm doing is removing the scratches from the previous runs and this isn't as difficult to achieve so doesn't justify the need for a tougher grit. In fact boron carbide will cut for too long and it would actually take longer than with silicon carbide which breaks down quicker to a finer and finer abrasive - unless you use a lot less, but then surely there is a point where your grit is spread too thin to do a good job of removing all the scratches?

I have heard it said that some commercial tumblers use green silicon carbide. For me, this would cost about the same as boron carbide and may be similarly tough, depending on the product, so this could also be an option worth exploring depending on what's available.

Anyway, these are just my experiences here in the UK, yours may differ. I should probably add that my machine runs rather fast and cannot be adjusted without major work. It's not a toy but it's not a huge machine either. It's rather old and venerable and was originally made in Scotland. It came to me secondhand with no frills whatsoever so I don't really know what kind of barrel it was designed to run. I have two 6" diameter rubber barrels which I put on it and it runs them at around 60rpm. (it could easily handle way more rocks with another or larger barrel but I simply have no need - also it will aways be too fast whatever size barrel is put on since sfpm is independent of barrel size) A slower machine, or differently sized barrels, might have a different 'golden ratio' which would change the way the bell curve sits, and clearly this could ultimately go so far as to affect your choice of grit as well as the amounts of water and rocks you put it with it! It may be that my machine doesn't work so well with SiC because it runs too rough, breaking down the grit too fast compared to a machine with a more gentle action. Or perhaps the SiC sold in the UK has an inferior toughness to your USA products? Or perhaps I am simply impatient, who knows!?

I tried brown fused aluminium oxide several years ago and the grit hardly wore down at all. (probably it rounded off rather than fracturing) The resulting rocks were terribly undercut. I have also had very bad (and quite similar) experiences using too much volcanic gravel as a filler for other hard but delicate rocks. To avoid undercutting you definitely want some kind of grit that fractures into sharp pieces for tumbling purposes. I have not tried the white aluminium oxide yet though I believe it is supposed to be somewhat friable so perhaps it would work. It isn't easy to obtain in the UK but I have managed to procure some of rather uncertain but comparatively fine grade which I plan to try as a prepolish.

I have not yet found anything really suitable for the softest rocks. Crushed glass would probably be a good bet but I cannot find a supplier of this in a graded form. Sand is normally too variable, could still be too hard, and is not friable enough. There is a good reason the beach is full of sand - stones and chips off of rocks basically get smaller and smaller through abrasion but once a critical size is reached they have not much weight and therefore don't really get any smaller except by fracturing into nothingness. Those grains that still are left after this process of testing become the sand that you see on the beach while the rest is washed away as silt to the ocean depths and becomes mudstone, so the sand tends to be the sort of stuff that gets rounded rather than fractures into usable grit. If you want friable sand to use as grit then it obviously needs to start out as sharp builders sand but this is still no guarantee of its behaviour when tumbled. That said there are a variety of different types of speciality 'sand' which could be tried. Also iron silicate (copper furnace slag?) is cheap and may be worth a go, but it's still relatively hard?

Now if only I could get the polish stage figured out! (I can get a wonderful polish using a very cheap vibratory machine which however still shows up as hazy under the LED spotlights in my house, which are extremely unforgiving - I have a handful of stones from various commercial sources, and all have the exact same problem, so it's not just me, but I want perfection! A bit of cerium oxide on a rag easily goes a long way to improving this result but obviously I want to be able to achieve this by machine rather than by hand!)

---

Blimey I've just realised this could even be my first post and already I am trying to teach my grandmother to suck eggs! But there you go, dare to be different haha! 🐗 Too many rumours on here of people wishing to try alternatives to silicon carbide but having never done so. Well, I've been using boron carbide for ages! 😎

Actually to tell the truth I've been keeping my methods quiet so I don't inadvertently find the price of alternatives going up due to increased demand! However I want to put the tumbling to bed for a while soon as my house is becoming unnecessarily full of tumbled stones, and not all of them finished sadly! It's all UK material - mostly glacially derived, from boulder clay and suchlike - a right mixture - if a stone is hard, has nice patterns, and it's from the north of the UK then there's a pretty good chance that I've tried tumbling one! (though I currently live in the southeast where just about the only suitable material one can find for tumbling is flint in mostly boring shades)

Anyway, I hope some of the above is useful to somebody! Probably I'll stay as a lurker I'm afraid...

*dives back down to the fathomless depths
...

[advancedlemon]

Sorry came out ultra long! Typical lurker eh?

[advancedlemon]

I want to add that thick *dark* goo might have been a more accurate description of my results. It is still grey, not actually black! The boron carbide itself is quite dark in colour and I put it down mostly to this, but it is possible that I will see increased barrel wear from the harder abrasive and yes, the rubber is black - I haven't noticed it getting any thinner however.

Another thing about the slurry that is different is that it never seems to dry up. Must be the boron in it. This can make disposal more difficult! I had a frog living in it once, it absolutely loved the stuff as it made the perfect gooey diguise! Must've caught loads as only it's eyes were showing. It wasn't terribly happy when I decided to dispose of that container and had to turf it out. Pity as my garden is full of slugs and often mosquitoes too, which cause me a real problem. I really wanted to keep the frog! I tried giving it somewhere more natural to live but I couldn't convince it to stay. There was no way our light sandy soil could compete with that dark sticky goo!

[chris1956]

I would be curious as to what you pay for the boron carbide grit and silicon carbide grit.   There is a thread on this forum about boron carbide from 2008 where the boron carbide cost was about 10 times the cost of silicon carbide.  You mentioned the boron carbide was twice the cost of silicon carbide.  I didn't look extensively but here is what a quick search found.   Not sure what might include shipping unless noted otherwise.

Boron Carbide $34 per pound, 10 pound minium.
Boron Carbide 5 pounds for $127.49

Rock Shed SiC 60/90 grit 50 pounds for $128
Kingsley North SiC 60/90 grit 45 pounds for $128.85 (this includes flat rate shipping)
Kingsley North SiC 60/90 grit 5 pounds for $13.55

It looks like the ratio is still about 10 to 1 maybe a little less.  There may be cheaper compainies if you look deeper. 

Also, is there any concern about the toxicity of boron carbide?  Coming from someone who sometimes uses borax in polishing stages.  

Thanks.

[jamesp]

Look into green silicon carbide. It is a bit more aggressive but not worth the extra cost. Hard to beat common black grade SIC.
You can speed your barrel up and use premium colloidal slurry additives and do way more coarse abrasion doses to speed step 1.

[chaosdsm]

Boron Carbide is harder (~ 9.7 - 9.8 Mohs) and about 30% more resistant to breakage @ 3.5 MPa·m1/2  -Vs-  SiC @ 2.5 MPa·m1/2, so makes sense that it could work faster and longer than SiC.

However, here in the US, cheapest I see Boron Carbide is about $24.50 per pound (if ordering at least 10lbs) + shipping, Vs about $2.50 per pound shipped for SiC.  So unless your coarse to polished processing time is of extreme importance, and you have a good system for grit recovery in place, it's probably not gonna be worth it, unless 10lbs BC would last as long as 100lbs SiC.  

RE: Toxicity, somewhat less than SiC  

In general, SiC is classified as toxic, BC is classified as non-toxic - the human body is poorly equipped to deal with silicon.

The human body can get rid of Boron (to an extent) but not Silicon, but both have extreme long term effects with SiC being a bit worse:   Both will exacerbate pre-existing conditions if inhaled.

Silicon carbide fibers (can be formed from dry sanding) can cause lung fibrosis, lung cancer, and possibly mesothelioma. 

Boron Carbide causes irritation to upper respiratory tract, and nasopharynx, but these effects are usually reversible.  However long term continuous exposure can also cause permanent lung damage, but has not so far been shown to cause fibrosis, cancer, or mesothelioma.  

wet grinding (tumbling) will reduce chances of any of these from happening.  SiC also seems to be more of a skin irritant than BC:

SiC: Can cause immediate or delayed skin irritations, dermatitis, &/or skin infection.
BC: Prolonged contact with large amounts of dust can cause mechanical irritation (like you get with beach sand in the wrong places), especially in skin folds or when worn with tight clothing. 

[brybry]

Apr 27, 2024 9:39:04 GMT -5 rockbrain said:

Johnson Brothers has 60-90 SIC(and other grades too) shipped for $109 for 44 pounds. JB can be notoriously slow and unresponsive for orders but I've ordered grit a few times and it's always been real quick for this size. It's all prepackaged. You get a whole 20 kilo bag that just fits in a flat rate box.

Just bought a 44 lb bag of 46 grit from them. Ordered on Friday and FedEx dropped it off the following g Tuesday.

[advancedlemon]

I think the problem may be that here in the UK silicon carbide is ridiculously expensive!

The boron carbide I buy is imported from China and yes it does seem a bit on the cheap side! Supposedly it's 98-99% pure or something but I wonder what the 1-2% is exactly? (Maybe I should test it with a geiger counter just to be on the safe side...) It costs me almost exactly twice as much as ordinary grey silicon carbide, delivered. It would cost about the same for green silicon carbide which I've never bothered to try since boron carbide has higher specs and should work better.

Also I don't buy huge quantities. No space to store it. That could affect things.

Boron carbide is pretty inert stuff from a chemical point of view. I personally wouldn't want to breathe in any kind of abrasive since I kind of suspect it could cause lung cancer, even if nobody's really tried it yet! But that's not to mention the known threat from breathing in crystalline silica dust that surely makes up a large proportion of my tumbling slurry - I definitely don't want to be breathing that in either!

The only thing I worry about is that the part of my garden I do my clean outs over might eventually get too rich in boron for the plants, but it's just me being paranoid! It is totally unfounded. From what I could find on the stability of boron carbide, I think that over a very, very long period of time a little boron may dissolve into the rain falling on the soil, where I suppose it would wash away into the groundwater, into the river, and eventually into the sea, which if it dries up in the geological timescales involved here, will form borate nodules. In fact I don't really think this would happen. Too stable. In these timescales it would probably just end up as grains in some sedimentary rock somewhere!

The slurry does seem to stay sticky 'forever' though which I thought may be due to some boron being present which is not fully bound after the structure it is in has eventually been smashed into smithereens by the truly stubborn pieces of agate in my machine! Or there may be another reason? I don't know.

[advancedlemon]

It's difficult to judge the various parameters of different abrasives since there's absolutely conflicting information online and I think that every product is slightly different. In my experience the boron carbide works a lot better for me but that's about all I can definitively say. I've used two different boron carbides and there was a notable difference in the way that the grit wore out, which I assume is related to fracture toughness. The tougher grit lasts longer but at the same time less new cutting surfaces are exposed so it takes a bit longer to cut as well. I feel that there must be an ideal balance of parameters one could wish for but ultimately I just buy what is available to me and try to hone in the parameters I can change, which, simply put, are barrel loading and time, to match those that I can't, such as the specifics of hardness and fracture toughness, so that I can maximise my results.

I didn't notice as much variability with the grey silicon carbide I used when I first started tumbling but there was still one supplier who sold grit that worked slightly better. I never could get the silicon carbide to come close to my results with the boron carbide no matter what I tried.

I think that if I couldn't get the boron carbide relatively cheaply I would look into the green silicon carbide instead, or try to find another alternative.