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Polishing Rocks With A Tumbler
A Beginner's Guide
by Rick Stinson
I called this article a beginners guide for several reasons. It is intended for people who are just starting out with their first tumbler. In addition, it is for those who may be interested in polishing rocks and don't have an idea on how to go about it. For those reasons, many of the ideas that are presented here are very basic, but, once understood, help to solve some of the mysteries. You will find that they apply to other lapidary techniques or procedures as well. Tumbling is the starting place for many lapidaries and can lead to many new avenues of education, enjoyment and profit.
This is based on my personal experience, observations and methods. That is to say, it is most certainly NOT the only way to do things. It is the way I have done it, and I give full credit to the manufacturers, authors, and fellow lapidaries who have presented their ideas before me. The thing to remember about cutting rocks is, there is no teacher like experience. You can study this to death, but the way to learn is to jump in there and get started!! Remember, the door is wide open for you to come up with your own methods.
Beginning Rockhounds -- General Resources
Getting started involves a little research, some sort of budget for equipment, and supplies like grit and rocks. There is a lot of equipment that can be researched on the web. Visit a rock shop in your area, find a gem club - there are hundreds of them all over the US. Visit a local university and find the geology department. Go to the local library and check out some books on gems & minerals etc. Buy a lapidary magazine like Lapidary Journal, they have a listing of clubs and shows. There are gem and mineral shows all over the country the year around. Go to a show and you'll usually find a variety of dealers selling rough, finished goods, equipment, fossils, findings, and you name it. Clubs usually meet monthly or bimonthly and have programs to teach, any many conduct field trips for collecting. It's a great way to learn and meet other rockhounds too. Put a want ad in the paper for equipment. Keep an eye out at garage sales. If you need supplies, most rock shops carry everything you need, equipment, tumbling supplies and would be happy to sell you some rocks too!
Tumbling in general - what is it?
Tumbling is a way of machine polishing a quantity of rocks all at the same time. Rocks are processed in a rubber-lined barrel or bowl, using a series of grits and water, with careful cleaning in between steps. And finally, a polishing compound is used to bring out the final polished surface. The quantity depends on the capacity of the equipment, which is simply called a tumbler. The shape of the rocks can be natural, water worn pebbles, busted up pieces, pre-ground or sawed out geometric shapes, or slabs - just about anything that is hard and tough enough to take a polish. How do you know what to start with? First of all, just look at the rock. Is it chalky? Does it rub off on your hands? Does it flake off or can you break off a piece easily? Is it full of tiny pits or holes? Those types of rocks probably will not make good pieces to tumble. So what rocks would be good? A good starting place is the quartz family. There is enough available quartz in the form of crystal, amethyst, citrine, agates of all kind, jasper, chert, and flint to keep you busy for years. Most tumbling grade quartz is inexpensive to buy and agates and jaspers can usually be found without too much trouble. Find a sand bar, look along a stream bank, a road cut, a washed out area in a field.. You can usually find quartz as it is probably the most plentiful mineral on the planet.
The size of the rocks you can use for tumbling can be very small - just a few millimeters, up to fist size, depending on the size of the tumbler. Some rock shops carry a mixed tumbling assortment already broken to consistent sizes and pretty much ready to go. Some tumblers only hold a hand full and would naturally be suited more for small sizes. Some are big enough to process several hundred pounds of rock. (Tumblers are also used to clean, deburr, polish or burnish metal.) Some tumblers are in the shape of a barrel or cylinder that rotates and some tumblers are in the shape of a bowl or bucket that vibrates. That rotary or oscillating/vibrating action is what works to process the rocks inside the tumbler along with silicon carbide grit and water.
Silicon carbide grit is used because it is harder than most rocks, and therefore will scratch most rock and gem material. Use good quality silicon carbide grit in your lapidary work. Aluminum oxide, garnet and other abrasives will probably work, but the time it takes verses the cost savings aren't worth your time or the extra time on motors/electric bill.
Tumbling is done in several stages starting with a very coarse 60 to 90 size grit which cuts, grinds off sharp edges, and shapes. Then everything has to be washed clean. Barrels, bowls and rocks. Everything. Next, a finer 220 grit is used to smooth away the scratches left by the coarse grit. Then when that stage is through, the equipment and load are again washed clean. And so on, until a very fine matte prepolished surface is produced. All scratches from coarser grits must be gone. I use 1200 grit for the prepolish, but this can vary depending on individual tastes. I think I get a quicker and better polish by taking down the surface to that fine of finish. Some lapidaries use 600 or 800 mesh as a prepolish. So, it's a matter of choice. Do what works for you. Then, usually as a final step, an oxide polishing compound, such as tin oxide or cerium oxide, is added in place of the grit. In addition, plastic pellets are sometimes used to cushion the stones against dings as well as taking up volume from the amount of material that has been ground away. The oxides bring out the polished surface. Some lapidaries use soap flakes as a burnishing step for final polish.
That's really about it in a nutshell! Working down a stone's surface until no scratches can be seen with a 10 power loupe, is the way stones are polished. Other lapidary cutting techniques such as cutting cabochons or faceting, only do it one stone at a time, and use a variety of different supplies and equipment.
Equipment and Technique
There are two basic kinds of tumblers: barrels that rotate and bowls that vibrate. Each causes the stones inside to move from bottom to top, and in the process carry grit and water, which works with the friction of the stones as they slide over one another. Actually the idea of "tumbling" the rocks is not really accurate because that implies being thrown around. What happens, or what you want to happen, is a sliding action. This sliding is what causes trapped grit between two surfaces to scratch away material from each other, thus wearing down both stones. With the proper amount of rock, as a barrel rotates, rocks are carried up the side of the barrel and reach a "flop" point at which they flop over and slide back to the bottom, grinding themselves and the rocks beneath them as they go. That is ideal. In a barrel tumbler, too much speed, and centrifugal force carries the stones too far to the top and they just fall -crashing into and probably making tiny cracks in the stones at the bottom. If the rotation is too slow or there is not enough material, or sometimes too much water, they simply slide back down the side and only part of the load is working. Manufactured equipment is already designed for the proper speed and will give satisfactory results with a proper load. The best way to tell the proper load in a barrel tumbler is simply to listen to the load. If it sounds like stones are falling from the top and crashing into the stones at the bottom, try and adjust the speed or the quantity of rocks or water in the barrel. A vibrating tumbler does not have quite the same flop over problem, because the entire load is vibrating as well as rotating turning over. But there is still a "flop point" and sliding action as well. It's just compounded by the vibrating action. That is what makes a vibrating tumbler work faster. As this happens over and over and over, surfaces are worn (scratched) smooth. The water in the tumbler keeps the mud washed out of the scratches, but eventually will slow the action as mud builds up. It may be necessary in coarse grinds especially, to wash out the mud rather than just adding a little more water.
So, how do you judge when to change grit? Take a few stones out and wash the mud off of them. Look at the shape. Look at the surface. I use a 10X loupe (which is standard equipment for rockhounds) to see if the scratches from the previous grind are being removed or are already gone. That's how to tell when it's time to move on to the next grit. You have to look at what you are doing. Charts and time tables are just the starting place, but nevertheless very good guides.
Both types of tumblers will do a good job. Much depends on the operator and how determined they are to produce a quality polish. The big difference is working time. Vibrators work about four times faster than barrels. A vibrating tumbler will do a load in about 7 to 10 days and a barrel will sometimes take 4 to five weeks, or about one week per grit size. Price is also generally higher for a vibrator.
I did a quick web search and found a variety of barrel tumblers from tiny ones that cost around $40 to $50 and only produce a handful of stones, to heavy duty jobs that would hold about 15 lbs and were running around $225 to $250. A call to a local rock shop found vibrator tumblers from around $90 for something small up to the neighborhood of $1100 for a 50 pound capacity.
Home Made Equipment
One of the first tumblers I had was made from the rollers out of an old printing press. The rollers ran in brass bearings that were supported by a wooden frame. Pulleys connected the rollers and were turned by a regular quarter horse motor. The "barrel" was nothing more than one gallon paint cans! Not real whippy, but it worked and it didn't cost much. I have seen and heard of many home made tumblers. I remember hearing of one that used a car tire which was the "barrel", and it hung on an old washing machine wringer cylinder which became the friction drive. Stones were put inside the tire with grit and water and the tire was hung on the roller. Weight created enough friction to keep the tire rotating and tumble the load. Not many of these old washers around outside of museums, but an idea that's pretty inexpensive outside of the electricity to run one. The other thing I've read or heard of somewhere, was to use a car or truck tire and let it ride on a set of rollers (instead of hanging from the inside). This might be a little less messy, but both ideas would be hard to clean I would think. Also the run time was even more time than a regular barrel type. Don't know why - maybe the way it was geared . If you are going to try out your ideas, remember, the cost of electricity can really add up fast for tumblers because the motor must run constantly for extended periods. Just another reason to strive for quality in your ideas and equipment too. Remember, you can produce something of solid quality and a beautiful polish for about what it costs to crank out something junky. I think that the motivation of the lapidary to come up with something different, is one of the strongest forces on the planet. What ideas do you have? Don't forget the material resources you may have such as old motors, steel or aluminum scrap, or friends who are good at mechanics to help you design and build something. Get your family involved. It's something that everyone can enjoy doing. For homemade power think of solar power, or the power of a wind mill, a water wheel. Wave action. For grit there is absolutely nothing like silicon carbide - unless you're doing corundum or something nearly as hard as the silicon carbide.
You may find garnet on the market and you could always use sand to grind rocks softer than quartz. But those will take extra time, then there is the problem of sizing/grading. In theory you could just use one coarse grit to start and eventually that would break down to a grain size that would produce a polish. But, that could take a very long time. It seems that home made equipment has always been part of rockhounding, -- a lot of it simply due to not being available. But those times are past. for the most part. Just don't get so hung- up that you can't get anything produced. I like to conserve grit as well as the next guy, but about all I ever try to save and reuse is the very rough 80 to 100 mesh, and that goes back in the first grind of course.
The neat thing about tumbling is that there are almost always rocks everywhere you go to pick up and tumble. Once you have the items you need, load the tumbler with grit, rock and water. Turn on the switch. So what if you don't get everything perfect the first time or two. This is tumbling rocks -- not a Mars mission. If they don't turn out the first time, look at your log book. See what you did and try something different. If your batch comes out a little less polished than you want - look for your mistakes and simply re-grind if you have to. You're learning something. Don't give up! You probably haven't wasted much except a little time and grit. One thing I just mentioned, is a good log book to record what stones, grit, measurements, time and sequence you have used. It can be a great reference. When you get zeroed in on what works, it's nice to refer back to it, just in case you can't remember exactly what you did etc. One final very important note. Have fun.
One caution about barrel tumblers. They might build up gas as they are working, so play it safe and stop the tumbler in order to relieve any pressure build-up inside the barrel. Once every day or two simply crack open the barrel lid enough to relieve any pressure. Some recipes call for a teaspoon of baking soda to be added for this reason. Besides, it doesn't hurt anything to stop for a few minutes to inspect a few stones and see how the batch is doing.
However this brings up general safety in any lapidary project. That is, inspect your equipment and be aware of potential hazards in your process. Most lapidary processes use water and electricity. So it seems obvious to keep wiring, plugs and motors in good repair. There are moving parts, wheels, pulleys, sometimes high speeds, dust, oils, noise, chemical compounds and all sorts of potential hazards in a lapidary shop. Even some rocks and organic materials will make you sick if you breath their dust particles. Protect yourself - think safety!! Protect your eyes, ears and lungs. If you've got motors and belts running, always watch out to be sure your hair and clothes don't get caught.
There are a couple of things you can do to prepare your stones for tumbling. A lot of this involves inspecting the stones. Clean off dirt, and wash out any debris. Look at what you are trying to polish. If a stone is chalky or porous it probably will not be worth your time. Stones that are too soft will simply be ground up by harder rocks and grit. Stones that are full of big cracks and a lot of holes will trap the silicon carbide grit and make it very hard to wash out when it is time to change grit size. Stones that are sharp slivers or pieces that have sharp edges or points, or stones that are very thin, will all grind down until something breaks off. When that happens toward the end of the process - as it usually does, it will not polish. So it is best to try and break (or better yet grind if you have grinding wheels) off these areas to begin with. You will get a much better quality polish in the end. Also stones of the same size generally will polish better. (See variables below). The other thing you can do is prepare a good area to run the tumbler. A small shelf for storing grit and supplies will come in very handy. It is best to work where it is easy to clean up some spilled grit or water. One other consideration is noise. It's nice to have the equipment handy, but larger tumblers can also be very noisy. So consider others family members before you set up in the living room and proceed to drive everyone crackers. Also a tablespoon to measure grit and one pound coffee cans to keep grit and polish stored, will also come in handy. Keep the grit separated and labeled. A few particles of the coarse grains accidentally mixed in with a finer mesh grit or polish can make a difference between a water-wet professional looking polish and disappointment.
So here's a rule of thumb which is true for most lapidary projects: Keep grit separated. Keep your lapidary clean. You will produce much better quality.
Silicon Carbide Grit
Silicon carbide grit is used to grind most rocks. It is graded into various mesh sizes. The lower the number, the coarser the particle size. 80 grit is very coarse and 800 grit is much finer. I generally use 1200 grit for a prepolish for tumbling. Lapidary grinding wheels are also made of silicon carbide grit . Grit is defined by "mesh" sizes or inches, or in the metric system by micron size. For example 100 grit is .0068 inches in diameter or 173 microns. You can buy a tumbling "kit" at rock shops that has already has grits and polish in premeasured amounts. If you want to order your own separate grit sizes (which may be cheaper) just ask for "100 grit" or "400 grit" for example. Microns are not usually specified unless you are using extremely fine diamond powder and even then grit size is generally called out.
Suggestions for Sequence and Quantities
There are probably as many recommendations for grit sequence in tumbling as there are lapidaries. Here's the sizes I most often use for tumbling:
How much to use? How long to run?
|1st grind 60 to 80 grit.
||Grading (grit size) is not real important here. The idea is to start
rounding and smoothing rough surfaces and edges.
|2nd grind 220 grit
||This removes the scratches left by the first grind.
|3rd grind 600 to 800
||Just depends on what I have on hand. Removes 220 scratches.
|4th grind 1200
||Very fine powder. Prepolish.
||Depends on the type of stones. Quartz rocks polish well with Cerium Oxide.
In my vibrating tumbler bowl I pour in water until there is ¼ inch in the bottom. Then I fill the bowl about 2/3 full of wet rocks. I add grit and turn on the motor. After only a few minutes I may add a little more water or grit. I am judging just by eye and by sound, until a good slurry is formed. A slurry is just a mix that coats the stones and doesn't wash off from too much water in the bottom, as they rotate. Put on the lid and that's it. After running for an hour or two, I check to make sure the mix hasn't dried out and slowed down the action. As mud builds up, I add water accordingly. With a barrel tumbler that is not practical so many formulas have been developed to try and make sure a good ratio of grit and water are used. A general rule of thumb that many lapidaries use is: one pound of grit per 10 pounds of rock. A good way to estimate grit, is to fill the barrel about 2/3 full of rock and then empty it out and weigh it to determine how much grit to use. For example, if the rock weighed 5 pounds, then you would use ½ pound of grit. Then reload rock, add the grit and then add water to approximately the top of the stones. Put in a teaspoon to baking soda, put on the lid and start the motor. How long does this take? First grind is the most important to make sure stones are well rounded. I sometimes go for two days or even more if necessary in the vibrator. In a barrel sometimes the charts/manufacturer call for four to six days and sometimes seven to ten days. With either type of equipment, you have got to be the judge on how satisfied you are with the way the rounded off shapes look. If you go ahead and decide to stop and then after clean up, find a few stones that need more grinding, you can just set them aside for the next batch. Be sure and record what measurements you've used. After you're through with the coarse grind its time to wash. This is probably the most important part of tumbling. See Cleaning below.
The second grind will use 200 grit and may need a little less grit than the first. Due to the amount of material that has been ground away, the rocks may not fill the barrel or bowl to the level you started with. In that case you can add a filler such as little rounded off, clean, pea gravel, marbles, or crushed nut shells for examples. There are lots of things that are used. Running time for this grind depends on how rounded and smooth the first grind was. Also remember that this step needs to grind away the scratches made by the first grit. When you inspect the stones from this step, use a loupe (magnifier with 10 power) and check to see what the surface looks like.
When you are sure the first grind coarse scratches are gone, it's time to stop and wash. If you use nut shells for a filler, they may be impregnated with 220 grit and should be thrown out or saved. Definitely do not use them in the next grind. Same general time frame usually applies to the second stage for barrel tumblers. Usually a day and maybe a few extra hours are required for a vibrator.
The third grind starts getting to a very fine mesh of silicon carbide. The surface of the rocks is also starting to smooth considerably. Finer grit is harder to wash out of cracks and/or holes. Inspect & clean carefully! Also be sure to prevent the stones from chipping by handling them carefully. Also use a filler described in step two for this purpose and to make the right volume in the barrel. Add grit in about the same quantity as previous step - maybe a little less depending on coverage. At this point some are using 600 to 800 grit and they polish after this step. I generally use one of these grits, but then I go on with another grind using 1200 grit which produces a smoother surface for final polishing.
Before polishing, as with each step above, be very critical of each stone. Make absolutely sure there is no grit trapped in a small crack or hole. Everything must be completely clean. Remove any stone that has a knife sharp edge or that is chipped or broken. Take extra care in cleaning up the barrel or bowl. Some lapidaries like to keep a separate bowl and use it just for this polishing step. Not a bad idea if you can afford it. Operate in the same sequence as before, handle carefully, add new clean filler and use only about one quarter to one half pound of oxide per 10 lbs of rock. Use the oxide polish to suit the type of rock you are going to polish. Usually a cerium oxide or tin oxide is successful on quartz gems. Other oxides that are used are titanium oxide and linde A
If you burnish, use soap as a final burnishing step. Ivory flakes are probably used as much as anything. Run time may be reduced by as much as half. This depends on how carefully you have cleaned and the surface condition between steps. Did you really get those scratches out? If you did, you will wind up with a real knock-out water wet polish. Clean up the same as before and you're done.
Quality of material
The quality of the material that you are tumbling obviously will make a difference in the outcome. It's the GIGO principle. Garbage In, Garbage Out. You will have much more success if you have solid rock with attractive colors to start with. Stones that are full of holes, cracks and so forth, are much harder to clean between grit changes and also to polish.
Shape of material
Look at the shape of the stones you intend to tumble. A nice rounded surface either concave or convex , is more likely to polish than something with ridges or sharp edges. Why? As the sharp edge rolls and tumbles and grinds into other stones, it is more likely to chip. And a chipped edge is going to be rough and noticeable. So if you find boulders and bust them up, or buy rough material already sized. Look at the shapes you are trying to tumble. One way to improve the finish of your tumbled stones and also to reduce tumbling time is to grind off sharp points and edges before hand on a grinding wheel. If you have one of course. If not., this is something to watch for in the first grind. I usually just bite the bullet and take a couple of hours and inspect for quality and grind off sharp stuff, get rid of stones with holes and so on, as the first step. This saves a lot of grief later on when it comes to cleaning up between grits.
Washing and cleaning between grits is probably the most boring and very time consuming of producing a nice batch of stones. At least until the end when the beauty of the stones start to show. However it is one of the most important things to do properly. It is the one thing that will help insure your efforts pay off. You must clean all of the grit off of each stone and out of the barrel before proceeding to the next finer grit. If you don't you will prolong the time it takes to tumble. The little piece of grit you miss will keep on working -- scratching until it eventually breaks down to a smaller and smaller size. One important note about cleaning grit: Do not clean out your tumbler in the sink. Do it outside if you possibly can. This can be a nuisance in the cold months, but going outside is really the best way to clean. Washing silicon carbide and rock dust down your drains will lead to one thing. That is, you'll be spending time unclogging drains instead of tumbling. I run a hose and hand stir until the water runs clean. Even then, there is always grit in the bottom of the barrel. Then go and find some place comfortable (inside) to sit. Use another clean bucket and fill it about half full of water and a little dishwashing soap. Get a toothbrush and an old towel. You guessed it. Inspect each rock and you will usually find some that have a little mud or grit that didn't wash off. Scrub these with the toothbrush to loosen grit, rinse in the water bucket, and then put the finished stones on the towel. When you are through, then do the barrel. Be sure and empty out the grit and tiny chips that are sometimes left and then thoroughly scrub the barrel to prepare for the next grit size. It's a pain, but I don't know of any easy way to do this part of the job. After you've spent some time cleaning, you'll soon realize why it was important to let the first coarse grind work to round and smooth.
Different kinds of rocks in one batch
Sometimes the various types of rock in a batch can cause problems that won't show up until you try to polish. These may be due to a range of different hardness. It's a good idea to separate stones by hardness before you begin. This can be done with a simple scratch test. Use a piece of agate and try and scratch another stone. Quartz will scratch anything as hard as it is and anything softer. Or to put it another way, anything softer will not scratch the quartz. Separate the stones that will not scratch the quartz. These are stones that may be too soft for tumbling. Even so, some stones may polish quickly and others seem not to polish. Some different brittle qualities may be present depending on what you are tumbling. Very brittle stones like obsidian are prone to a very minute fracture when they impact another stone, thus causing a surface that is frosted and will not polish very well.
The amount of stones that are loaded in a tumbler can also make a difference - especially in a barrel tumbler. If the barrel to too full the stones will not slide or flop over and slide properly. That's why it's important to follow the manufacturer's instruction for quantities. See suggestions above.
There are may things people have come up with to add to a tumbler to help cushion stones in the final polishing step. This is especially true for brittle stones such as obsidian. Plastic pellets, small pieces of hardwood, ground nut shells, have all been used to soften the action and cushion the rocks. Also soap such as Ivory flakes are sometimes added for this purpose. It's hard to tell how many different techniques there are concerning the few items I have mentioned here. Experience and perseverance are still the keys to success.
There are many uses for tumbled stones. Some cabochon cutters cut preforms and with careful grinding and attention to proportions, are able to finish a batch of cabs in a tumbler. Many metal manufacturers make costume jewelry findings suitable for mounting a baroque tumbled stone. You can easily find pins, bracelets, and earrings. Buttons and beads also lend themselves to tumble production. Really, there is no limit what can be done with a tumbler with exception of size and cavities, undercuts or recessed areas that can not be scoured out by smaller stones and grit. There are some beautiful wire tree sculptures that use small tumbled stones for leaves and blossoms. I have used tumbled stones for trade and have sold tumbled stones to help finance other equipment or tools. Many dealers who are on the road doing shows and don't have time to tumble, are eager to trade or will buy quality tumbled stones that they can sell. One of the fun things to enjoy with tumbled stones is simply to give them away. The sheer pleasure of offering someone a chance to pick out a beautifully polished baroque gem is most enjoyable, and makes it easy to talk with someone and establish a friendship. Whatever path you travel in your pursuit of the pleasure that comes from the gem world,
I wish you good luck and success.