Collected Information on Rock Tumbling

More Detailed Advice on Rotary Tumbling

By Alan Silverstein, ajs@frii.com.
Last update: October 19, 2011 Contents of this section:

Good booklets (and websites):

If you can find this old booklet about rock tumbling, it's pretty good: "Gem Tumbling and Baroque Jewelry Making", by Arthur and Lila Victor of Spokane, Washington, 1957 (!). A new edition is available these days; I saw it for sale at the Denver show in 1998 for $4.50.

The blue-covered instruction booklet that comes with Lortone tumblers ("Gemstone Tumbling Instructions"), or which is available from them for a couple of bucks, is a close second.

In 1995 Jeanne Ridolfi wrote, ...a good book on tumbling... "Secrets of the Pros" by Edward Smith. Covers tumbling from beginning to end, with text and illustrations, step by step. Both rotary and vibratory tumbling. Cost = $14.95...

In 1998 Vance McCollum wrote, I would like to suggest an excellent book on tumbling, whether using a rotary or vibratory tumbler. It is called "How to Tumble Polish Rocks into Gems" by Edward Smith. It will tell you everything you will need to know. It can be ordered through Diamond Pacific Co. Call them at 800-253-2954.

After I started this website I learned about another site, more attractive than this simple text, with similar information.

By request here's a link to Mama's Minerals -- Unique Gifts from Mother Earth to You, a site featuring links to other tumbling information, sales of rock tumblers and supplies, and lots of other good lapidary info. In particular they offer excellent pages called About Rock Tumbling and Tumbling Links.

Rotary Tumbler Types

It's probably clear by now that I have a bias for Lortone rotary tumblers. Their designs are simple but effective and durable. In particular their lid seal is excellent.

I played with a friend's 12 pound Thumler rotary tumbler for a week once but was not as impressed. As I recall it used a large O-ring to seal a rubber lip around a rigid plastic disc, a bit awkward to put on and take off. This didn't leak, but the plastic showed obvious wear after just one week of coarse grinding.

I've seen "toy" tumblers with plastic barrels and small capacities. No one I've talked with has reported satisfaction with these units.

In the fall of 2003 I discovered that Harbor Freight Tools stocked a Chinese-made tumbler that looks like a knock-off of the Lortone 3-pounder. The price was $30, dropped to $25 on January 1, 2004; occasionally on sale for $20. Some folks have expressed contentment with these units except for having to immediately replace the drive belt with a new O-ring. (Check an Ace Hardware Store, they stock a variety.) I studied one myself and it looked solid enough, although not as finely crafted.

Later: My experience with several O-rings from Ace Hardware, used in my Lortone tumblers, is that they don't last very long. Must be something different about different kinds of O-ring materials?

Also, I've heard that newer Harbor Freight tumbler motors have shown motor problems, where they seize up with a burning odor.

In March 2005, Roger Smith wrote:

...I chose to work with the rock tumblers sold by Harbor Freight. These two units look and are worked, pretty much like the Lortone models which have been found to be extremely dependable so I wasn't anticipating any problems with them.

I placed the first two single drum units in service during July of 2004, and have since experienced working with nine of them. I received a dual drum unit as a Christmas gift, and have since worked with three others of this model. I believe I have had ample exposure to these units to form an opinion of these units.

The first two single drum units I purchased are still in service as I type, they have only been shut down for recharging and lubrication in all the time I have owned them. They haven't so much as worn out a belt. Next, I received the first dual drum unit and placed it online. By the first week of March, I had gone through four of these units, wearing out the motors in each within less than a month of use. This model uses the same motor as the single drum unit, and that motor cannot support two drums.

Additionally, there is a design flaw whereby when you place the barrel in position to rotate, the knurled nut that secures the water-proof seal rubs against the base. This causes wear on the knurled nuts and rubs the paint off of the contacted surface on the base. Of course, with the rubbing, "drag" is created which I believe contributed to the early demise of the motor. After much discussion with the Harbor Freight folks, I received a refund for the dual drum unit and purchased two additional single drum units.

While my first two units are valued and dependable, I have three additional units in my possession, one as a back-up. I have replaced each of these newer units at least once in each case, and have not been able to keep one online for more than a couple of weeks without throwing a belt, most times only a couple of days. Harbor Freight says that the newer machines are all experiencing belt problems and they have worked it out, but the new belts are on back-order and won't be available until 15 April 2005. In the meantime, they tell me to take the units back and exchange them if the belts blow. I've had no problems exchanging units, but it sure is a pain having to go to their store a couple times a week to do the exchange.

If they can get the belt issue straight, these units work every bit as well as the Lortone I suspect they were copied from; however, the Dual Drum Harbor Freight model is a piece of garbage until they increase the size of the motor.

In October 2005 I was told that the two-barrel tumbler now has a larger motor than the single-barrel tumbler.

How much stuff?

How much rock with how much grit and how much water for how long?

It depends on the barrel size. As a rule, fill the barrel 2/3 to 3/4 full of rocks. Any less and you get sliding or smashing instead of tumbling. Any more and you don't get enough motion to do the job or consume the grit. After a while you can tell from the sound. It should be loud but not violent, like a waterfall. Play with it.

Rock size matters too. You need at least half the load to be smaller pieces. You can work larger chunks too... The booklets say no bigger than 3/4", but I've had OK results with up to 3-4" across (in the 12 lb tumbler). Naturally as you get to bigger pieces the tumbling action can be less regular or more violent.

Note that the coarser and harder the rocks starts, the longer it will take to be ready to move to fine grit, and the more "evaporation" you get. Each coarse dump and reload is a big remixing of rocks, taking out "done" stuff and adding more "raw" stuff to bring it back up to 3/4 full. With softer rocks, "evaporation" is a problem even after the fine step, so I start with it a little fuller, and sometimes I add more rock (which is sitting around in a box ready to go into fine, prepolish, or polish) to make up a full load.

The booklets says to use 4-6 tablespoons of grit for a 2.5 lb tumbler load, and a full cup for a 12 lb tumbler. I tend to be frugal; the worst that happens is not enough grinding or polishing, and you send the rocks back for some more (another week).

Fill water to touching the bottoms of the top rocks in the barrel.

To be safe, I run each load a week -- or longer if I don't get around to doing a tumbler dump when it's due. You can tell you're tumbling long enough if there's no visible or palpable grit, just fine sludge, when you dump a coarse load.

I gather that bigger barrels give better tumbling?

No, just more capacity and (in my experience) you can polish bigger rocks. If anything the bigger ones are more of a risk for consuming grit faster (can be more of a waste if you do something wrong) and for beating up rocks that don't like to be tumbled (because the fall distance on each rotation is further).

Do you really need grit?

In the natural world you can observe that rocks are often rounded by water action, and nearly polished in some cases. This happens mostly in rivers and streams when water flow is high enough to move the rocks, and/or when the water carries a lot of suspended sand or finer particles. It also happens when waves break on a lakeshore or seashore. Using grinding and polishing grit in a tumbler is more of an accelerator than a necessity. But it's a great accelerator.

In theory you could load a tumbler with rocks and water -- maybe even leave out the water -- and let it run unattended for weeks or months, then open it to find nicely polished rocks. In practice, though, in the closed environment of a tumbler, I would expect the slurry (mud) or dust to build up to a point where grinding and polishing action nearly ceased. Furthermore, the grit or polish compounds are harder than most rocks and cause them to smooth and polish a lot faster than merely going rock-on-rock with without grit or polish compound.

After a long time I finally tried this experiment myself in my tire tumbler. Sure enough, while slurry did form and the rocks did round off a little, it was only 20-25% as fast as with grit.

Buying grit:

Tumbling grit is available in a wide variety of sizes, types, and prices. For volume work, say with 12 lb or larger tumbers, it's worth locating an industrial supply house and buying 50 lb or 100 lb boxes of coarse and fine grit for about $1.50/lb (or higher); otherwise expect $2-3/lb in smaller quantities at lapidary, hobby, and similar shops, sometimes even more. It might hurt to spend so much up front, but remember that the net cost still works out to about just $1 worth of grit (all grades) per pound of finished rocks -- it's a cheap hobby.

By shopping around I am usually able to buy silicon carbide grit (coarse or fine) in 50-pound or 100-pound units, on sale for about $1.50/pound delivered to my doorstep (typically by UPS, in the United States).

Over time I've bought grit from a number of places and heard or inquired of others. I've also noticed that contact information, even companies, come and go. Rather than fully list sources here, I'll just provide the names (key words) that should allow you to search for possible candidates. My most recent favorites are first.

November 2003: I bought another 100 pounds of straight 30-grade silicon carbide grit for $1.485/lb delivered, much cheaper than any other sources even on sale, from Jesco Products.

"You know you are seriously into a hobby when you buy supplies in hundred pound lots and congratulate yourself for saving money."

Handling grit:

When you buy grit in bulk, you might dole it out directly from the shipping container, or if that's inconvenient, repackage it into a 5 gallon bucket with a lid.

Another possibility is to spread out a plastic dropcloth and decant the grit through a funnel into clean, dry, 2-liter pop bottles. The result is quite manageable... The bottles are tidy, visible, easy to pour, and weigh about 8 lbs each.

I carefully measured grit by the tablespoon into a cheap plastic cup and observed that 20 tbsp just filled it. Now the cup is a handy measure for loading my 12 lb tumbler. Also I measured 4 tbsp into the same cup and marked the outside so I can use it for the 3 lb tumbler too.

Disposing of slurry:

Expanding on what I wrote earlier about not pouring slurry down the drain...

From my big truck tire tumbler I get a lot of slurry. (I call it "slurry in a hurry.") I fill a five-gallon bucket with gray "mud" (slime) and a little muddy water about every month or two and then float-filter off the water as much as I can. The float filter is a cheap freestanding strainer with the outside covered with old denim fabric literally stitched onto it. I float it in the bucket, usually adding a rock for a little more weight, and about twice a day sponge out the clear water that collects, until there's no more. Then the bucket goes outside.

In winter I turn over the bucket onto a plastic tarp after it freezes, and the next warm day I get to take the bucket off a frozen slurry block that can then dry out as weather permits. In summer I set the mostly dewatered slurry bucket out in the sun to finish drying. I let the slurry get rather dry, turn over the bucket the same way, wait again a day or two, and remove the bucket. In both cases if there's much crud stuck to the bottom of the bucket, I let it dry out a while longer, whack and shake, until it's cleaned out enough.

November 2005: After busting one bucket in the winter when the slurry froze, I had a better idea. I dug a hole in the side yard, and from now on I'll pour the slurry in there to dry or freeze before busting it up to dispose of in the trash. So far, works great.

The dried slurry block is not as hard as concrete, but it does have cohesion. If I wait until it dries completely, I have to whack it pretty hard with a shovel to bust it up so I can get about half at a time into the trash (so the pail's not too heavy).

When slurry hardens on a surface, it can get resistant to being hosed off. The key is to avoid letting the slurry dry before you're ready, and to avoid letting it settle (even if it stays wet) anyplace you can't reach to clean, like inside your plumbing. When I dump the tire tumbler, I try to get the tub of rocks and slurry into the sink and rinsed off without undue delay. If it dries for even a couple of minutes (here in dry Colorado), some of the rocks get a sticky grey coating that won't spray off later, so I must rub it off each rock as I sort them.

I've noticed something very interesting about my slurry. Sometimes when I'm scooping it out of the spooge bucket it sticks together in a really bizarre way, as if it were soft Silly Putty. I suspect that trace amounts of organics from lichen on the rocks cause this effect. It's weird stuff, hard to extract into the drying bucket. Someone said that what I'm seeing could be thixotropy.

Reusing prepolish and polish compounds:

The late owner of Dick's Rock Shop in Estes Park, Colorado did me a huge favor years ago when I was up there buying polish compounds from him. He clued me that I could reuse the relatively expensive prepolish and polish oxides (typically $3-12/pound). He was right, with care the same batch is useful for years.

The trick is to catch the slurry in a clean container without contaminating it. Use the minimum amount of water to wash as much of it as possible off the rocks. I suspend a heavy duty round plastic collander in a 1.5 gallon plastic mixing bowl, both obtained cheap at a "dollar store". After rinsing the barrel over the rocks, I hold the collander up over the bowl and rinse a bit more water through it while circulating the rocks a little. Usually I'm able to separate and capture the slurry from a 12-pound tumbler load with less than one gallon of total fluid resulting. But sometimes you dilute it more. Over time I've ended up with three one-gallon bottles for each type (prepolish and polish).

Pour the prepolish or polish fluid through a clean funnel into a plastic gallon jug designated and marked for that material. (I marked the lids "PRE" and "POL".) Let the bottle settle until you need it again, usually weeks later. At that time, carefully pour off the mostly clear water until lumps of slurry appear, then stop. Be careful not to shake up the bottle while doing this. After separating as much nearly clear water as you can from the reused slurry, cap the bottle, shake up the remainder, and pour the (hopefully soupy-thick) slurry into a tumbler load of new stones ready for prepolish or polish.

Note that you don't dry out the slurry, you just let gravity condense it back to a suitable density.

Over time you lose some of the polish compound, even as the slurry grows in volume due to rock dust. You can add a bit of fresh compound to the mix as needed to freshen it. Eventually, though, it ceases to polish well. Then it's time to dump both the prepolish and polish bottles, just like coarse and fine slurry, and start over.

Separating types of rocks:

Professional rock tumblers say you should never mix different types of rocks in a load. I find this hard to do, and not that important anyway. Some rocks, like petrified wood, naturally vary in hardness between pieces, or even across a single piece. Also, not even I operate on batches large enough to take one type of stone all the way from coarse grinding to polishing. This is particularly true since rocks "evaporate" during coarse grinding -- a load of stones that starts out filling a tumbler on coarse grinding has not enough volume when you get to final polish, if you try to just take one load all the way through.

I don't worry about mixing rock types during coarse grinding. Everything goes in my big tumbler with the 31-inch tire for a barrel.

After dumping each coarse load I sort the rocks by type: Hard and ready to polish, soft and ready to polish, not yet ready to polish, and junk to throw away. "Soft" means anything that needs buffering (pellets), including quartz, feldspar, calcite, etc. The sorting takes time, but it's relaxing and I enjoy it.

There are a few exceptions. I don't mix hematite with anything else because of the incredibly messy blood-red slurry hematite produces. (Sometimes I get a surprise when I tumble or hand-grind a rock that doesn't look red or metallic because the slurry from it is distinctively red (rusty). To me that just means there's a lot of iron in the rock.) Very soft material does not go in my big tumbler, instead I coarse grind it in a smaller one. Also agate slabs that were never polished, or which have broken, don't survive coarse grinding well in my big tumbler, so they go in a small one.

When filling a barrel for a "fine" run from my "hard, ready to polish" or "soft, ready to polish" buckets, I overfill the barrel a little, then that load goes through fine, prepolish, and polish runs without adding or subtracting rocks. For "soft" runs I leave room and add plastic pellets.

Buffering agents:

Softer stones like calcite, and some harder stones like quartz, do not take a good polish in a tumbler because their surface erodes or chips too fast. These materials need a buffering agent to cushion them. Apache tears (obsidian) are particularly hard to shine. Buffering agents range all over the map from sugar solutions to walnut shells to bits of leather.

I have gotten a very nice shine on quartz; also on monocrystal pink feldspar, but being a soft stone, it needs a buffering agent during prepolish and polish, or else it "mattes". Bear in mind that amazonite, moonstone, and many other types of stones are actually feldspars.

I suppose you could also turn down the tumbler speed -- but this is hard to do on commercial rotaries, and dysfunctional on vibratories even if you could? They must shake to function.

My favorite buffer is plastic pellets. These are not cheap, maybe $2-4/pound, but very reusable. It's more of a pain to deal with them than just grit, but you can float them off the rocks and slurry after dumping the barrel into a big enough bucket, and catch them in a strainer. Rinse the pellets well, shake the water off them as much as possible, and dump them into an open container for a while until the water evaporates and the pellets no longer stick together, then store them in a sealed container to keep them clean.

I store and use two "grades" of pellets: Some only go with fine grit, and the others with prepolish and polish. The "fine" pellets turn black and slowly wear away. The "polish" pellets I simply wash thoroughly when going from prepolish to polish compounds, and then I reuse them on the polish run.

In 1998 Geri Arms wrote (and I edited),

I have found that if you run two tablespoons of Tide (flake style) (per 12 lb tumbler) in between the different grits it will clean them nicely and takes off a lot of excess grit that is left on the stones... Even when you think you have it all off. I also use rubber bands in all of my tumbling as a cushion. Easier to clean and to remove from the stones.

There is a new polish on the market for Obsidian, called M-5, available thru [update Oct 2011:] Sister's Rocks, Westminster, Colorado; email or website.

Burnishing polished stones:

Some say that to get a very high gloss shine you should "burnish" your stones by running them beyond final polish with a soap solution. I never bothered to try this. I guess I'm happy enough with what comes out of a polish run.

Later I did try it once, using Boraxo as the "soap". The stones didn't look any shinier, but little bits of black material came out with the load (oops). I wouldn't try that chemical again.

In 1996 Roger Pabian wrote (and I edited),

I have had excellent results using Palmolive dish washing detergent (for hand dish washing, not automatic dishwashers). About 2 tablespoons or 1/4 cup does an excellent job in about a week's time where agate, jasper, and petrified wood are concerned. The above measure applies to a small tumbler of 6 to 8 pound capacity.

I have utilized Spic and Span and liquid Tide, but find the Palmolive to be about the easiest to use, and easy on hands too. All you use is the detergent, no other compounds, and no water.

I have used this method for about 10 years.

Some Rocks Won't Polish:

Many rocks grind fast but won't polish. Sandstone might be an extreme example. Gneiss and schist, even calcite, similarly, are too irregular or soft. Quartz, feldspar, quartzite, maybe others, can tumble-shine but only with pellets after the coarse grind, or else they frost.

If I like the shape of a rock and how it looks wet, even if it won't shine, I spray it with cheap, clear acrylic enamel, and that comes out nice too. The finish won't stand up to water or sunlight, but it's very durable indoors and doesn't mind human handling.

Polished pebbles:

Coarse grinding naturally produces a lot of small rock chips, especially from my tire tumbler. Sorting the rocks gets less productive (and less fun) as the sizes of the rocks remaining to be sorted from a batch gets smaller. Nowadays when I get down to a certain point I sift the remaining rocks through a screen so I don't have to hand-sort down to pinhead size. This screen is 1/4" wire mesh stapled over a square frame made of 2x2's.

The pebbles that fall through the screen into my catch sink get scooped up, rinsed, and set aside in a pebbles bucket. Once in a while I load a small tumbler with this material and shine it up (fine, prepolish, polish) to make a nice "polished gravel" for filling clear jars, etc.

Note, the metal screen leaves marks on the rocks, but these marks come off during fine tumbling.

Oiling the motor on a small Lortone:

After a few years my 3-pound Lortone tumbler started to make squeaking noises. A support person at Lortone told me I could fix this as follows: Disassemble the unit all the way to the motor, then take the motor apart too. (Note well the steps you go through, and save all the parts.) Soak the motor bushings (only) in motor oil overnight. Wipe the parts dry and reassemble.

I've done this a couple of times now, and it works well. As of this writing my unit is over 10 years of mostly continuous operation with the original barrel, and I've replaced the lid liner ("boot") once and the drive belt (O-ring) once, but no other parts.

FYI here's the email I got from Lortone in 1996:

From: "LORTONE, inc." <equipment@seanet.com> (Doug Guthrie)
Subject: Tumbler motors

Most of the small tumbler motors last longer than a couple years in service. The mean time between failures is approximately 40,000 hours or 4.5 years of 24 hour/day, 365 day/year use. Of course this is just an average.

It doesn't take much shaft wear for the bearings to become noisy, but when it happens the best solution is to reoil it as you have done. Unfortunately, using a lighter weight oil won't help the problem. Too thin an oil actually causes the bearing to run dry faster as the oil wicks out during running. Remove the bearing plates and soak them overnight in a 20 wt oil. This should help. If it doesn't, consider replacing the motor.

PS: Don't run the belt too tight as this can put a lot of side pressure on the bearing and cause early failure. The belt should be just tight enough so that it doesn't slip.

Extending the life of the rubber barrels:

The rubber barrels are moderately expensive, about $20 for the 3-pound Lortone and $50 for the 12-pound unit. So you don't want them to wear out if you can help it.

I observe that most of the wear occurs on the ends of the barrels, not on the round walls. My theory on this is that it happens because the rocks and grit can't help but rub against the barrel ends. However they "stick" a little to the round parts of the barrel, tumbling rather than sliding, so almost no wear occurs on those surfaces.

You can replace Lortone lid liners ("boots") for about $5. In ten years my 3-pound unit has only needed one replacement boot, but the 12-pound unit was going through them every year or two. Also, the bottoms of the barrels were getting noticeably thinner. I developed several hacks that seem to help with these problems.