Alternative grits for tumbling???

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.

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

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.

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.

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.

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!)

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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
...

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!

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.

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