How To Polish Stones with a Rotary or Vibratory Tumbler

Preparations:

Before you start tumbling the first time, you'll want to think about where to place your tumbler. Even tumblers with rubber barrels (such as the Lortone) produce some noise, though much less than metal barrels do. There's also always the possibility of a barrel leaking. Therefore, the best location for your tumbler will be where leaks won't matter, and it won't drive you crazy by rumbling gently through the night. Someplace that also has a utility sink and room to store your materials is ideal. (However, DON'T USE THE SINK TO DISPOSE OF SLURRY!) And of course you'll need an electrical outlet handy.

Some guidelines to choosing appropriate rocks to tumble are:

• Pick rocks that are all approximately the same level of hardness. If you put soft rocks in with harder ones, the softer ones may get worn away to nothing. Rotary tumblers work best for materials in the range of hardness between Mohs 5 and Mohs 7.
• Rocks should have a similar surface condition, and shouldn't have deep pits or cracks.
• A mix of sizes between 1/2" and 1" -- more small than large – is best for proper tumbling action.
• Your rocks should look good when wet (which approximates what they will look like polished).

Keeping a record noting the kind of rocks for each batch, the abrasives and media (plastic pellets, etc.), the time required for each step, and the results you obtained can be useful for future reference. This doesn't need to be elaborate.

Step One:

Load your barrel 2/3 to 3/4 full with the rocks you've chosen. The larger amount is generally better because the load will get smaller as the steps proceed. If you don't have enough rocks you'll need to use plastic pellets to make up the load. If your rocks are on the softer end of the scale you will do well to make some of the load plastic pellets for their cushioning effect. For more information on cushioning soft loads, see How to Tumble-Polish Glass.

In most cases you will start with 80 grit silicon carbide, though if your rocks are already fairly smooth or Mohs 6 and under in hardness you can try 120-180 grit. The amount will vary with the size of your barrel. If you don't have the chart that came with your tumbler, a rule of thumb is to use 2 to 2-1/2 tablespoons per pound of load. Sprinkle this over your load and then slowly add water until it is just below your top layer of rocks. Check that the sealing surfaces of your barrel and lid are clean, and then seal the barrel. Once the tumbling starts watch for a few minutes to be sure there are no leaks.

Check daily on the progress of your load. If the slurry seems thick enough to impede the smooth tumbling of your rocks, thin it with a small amount of water. If the barrel shows any sign of swelling, open it to vent the buildup of gas, and then add a pinch of baking soda.

The goal of this step is for all surfaces of your rocks to be well-rounded. If this is not achieved in 7 to 10 days, repeat this step with fresh grit.

When the surfaces are rounded to your satisfaction, clean your rocks and the barrel thoroughly. Scrub each rock with soap and water and an old toothbrush. Likewise scrub out the interior of the barrel. Any bits of coarse grit left will damage the surface of your rocks in the next step. DON'T DUMP THE OLD SLURRY DOWN THE DRAIN! It will clog your plumbing and make you very unpopular in your household -- even if you live alone!

Plastic pellets used in this step can also be cleaned and set aside for use in another batch, though only for Step One. The easiest way to separate them from the rest of the mix is to float them off by adding water to the barrel before cleaning it.

 

Step Two:

Put your cleaned rocks in the barrel again, and add fresh (or recycled from an earlier Step Two) plastic pellets to make a full (3/4 of barrel) load. With most rocks you will use 120 silicon carbide grit. The proportions are the same as with Step One – 2 to 2-1/2 tablespoons per pound of load. Add water up to the bottom of the top layer of rocks and, if you had any problem with gas buildup last time, add a pinch of baking soda. Seal and tumble as before. Your slurry should be a little thinner than in the last step unless you are processing soft or fragile materials.

Your goal in this step is to see that all the scratches from the first step are ground away, producing a smooth, matte finish. A  10x loupe can help in determining this. Step Two usually takes 7 to 10 days. Reclaim any plastic pellets and clean the rocks and barrel thoroughly as described in Step One.

 

Step Three (Prepolish):

Start as with Steps One and Two: put cleaned rocks and fresh plastic pellets in the barrel to make a full load. Use 600 silicon carbide grit for this step, about 2-1/2 tablespoons per pound of load, and add water up to the bottom of the top layer of rock. Tumble for about a week, checking slurry consistency and general progress every day or two.

At the end of the week, test one of your rocks by rubbing it on a slightly damp piece of felt, rough toweling, or leather with a bit of the polish compound. If the surface doesn't get a definite shine on it, continue processing, checking every day until you get the right effect. With softer rocks, you may need to do a second prepolish step. See How to Tumble-Polish Glass for more information on this.

Reclaim any plastic pellets and clean the rocks and barrel as described above, only be twice as careful! The finer the grit, the harder it is to get rid of, and thorough cleaning is crucial to a good polish.

 

Step Four (Polish):

Be very gentle when putting your rocks into the barrel this time. If you have not yet used plastic pellets, now is the time to do it, as they will hold the polish compound, acting as little polishing cloths, as well as cushioning the load and bringing it up to the proper level. Polish compounds vary, but for the most common materials CPP polish is good. Follow the directions on amount that come with your tumbler or with the polish, or use about 2-1/2 tablespoons per pound of load. Add water to the usual level.

This step should take between 5 to 7 days. When your (cleaned) stones look the same dry as they do wet, you are done with this step. Clean them and your equipment thoroughly.

 

Optional Step Five (Burnish):

If the surface of your rocks shows a slight haze or film, you can try burnishing them. There are various techniques for this, most of them involving tumbling your rocks for an additional period of time with powdered soap (NOT liquid detergents or dishwasher powder) such as Ivory Snow or the equivalent. Use 1/2 ounce per pound of load or less, along with the usual amount of water. You can reuse the plastic pellets from Step Four for this; cushioning your rocks is important. Some people add lump sugar as well, 1 to 2 lumps per pound of load.

Run this for about 4 days, checking frequently, then clean and reclaim the plastic pellets.

And then you're done, and ready to gather up materials for a new batch!

Vibratory Tumblers

Preparations:

These are pretty much the same as for rotary tumbling, with the exception that vibratory tumblers can be very noisy. Placing your tumbler on a piece of carpeting can help moderate this.

In choosing appropriate rocks to tumble in a vibratory tumbler, you have quite a lot more leeway than in a rotary machine. You can process rocks (and even organic materials like bone and coral) from about Mohs 2 on up, though the softer materials require a range of special techniques. Extremely hard materials, such as emeralds and corundum, can also be processed in a vibratory tumbler. For specific hints on tumbling corundum, see How to Tumble-Polish Corundum. Emeralds can be processed using ordinary silicon carbide grits, though they will need to be cushioned as they are quite prone to fracture.

You can also effectively process rocks of different hardnesses at the same time -- say, Mohs 5 through 7 -- as long as you use the appropriate abrasives and techniques for the softer material. Otherwise the guidelines for choosing rocks are the same as for use in rotary tumblers.

Again, you'll find it helpful to keep a notebook showing, for each batch, the rocks processed, the abrasives and media used, and the time for each step.

 

Step One:

Load your hopper 3/4 to 4/5 full of the rocks you've chosen. A good mix of sizes is advisable. Some of this can be ceramic media, and in the case of soft, irregular, or large pieces of rock, you'll definitely want to include some. Jagged, hammer-broken rocks, for example, won't tumble properly without small, rounded pieces to roll on. Use 60-90 grit silicon carbide on rough rocks that are from Mohs 7 to 8-1/2. If they are smoother and softer to start with, you may want to start at Step Two. Use 1-1/2 to 2 tablespoons of grit per pound of load, and about the same amount of water. Sprinkle the grit into the load while it is running, to help distribute it evenly. If the rocks are soft or fragile, you may need to cushion them. For more on this, check out How to Tumble-Polish Glass.

As the rocks are tumbled, the slurry will thicken. Check every few hours, and if the slurry is thick enough to impede tumbling action, add a small amount of water.

The goal of this step is for all surfaces of your rocks to be well-rounded. This should take 1 to 2 days. Repeat this step with fresh grit if necessary.

When the surfaces are rounded to your satisfaction, clean your rocks and the tumbler hopper thoroughly as outlined in Rotary Tumblers, above. DON'T WASH THE SLURRY DOWN YOUR DRAIN! The ceramic media is also reusable, in any step, once it is clean of all traces of grit. A colander with small holes will be useful for this.

 

Step Two:

Put the cleaned rocks (and ceramic media, if used) in the hopper, filling it to the 3/4 to 4/5 mark. For this step use 120-220 grit silicon carbide in the same amount as in the first step, 1-1/2 to 2 tablespoons of grit per pound of load sprinkled carefully into the load with the vibratory tumbler running. Also as before, use about the same amount of water as grit, and add more as necessary as the slurry thickens. Check frequently as it processes.

You'll want to produce a smooth, matte finish. This will take 2 to 4 days. When you are satisfied with the results, clean your stones, hopper, and any ceramic media thoroughly.

 

Step Three:

Fill the hopper as usual, using the same amount of water and grit, but using 500 grit silicon carbide this time. Check frequently for thickening of the slurry. Processing will take 2 to 4 days. This step is complete when rubbing a sample rock on felt or wool cloth produces a dull gloss.

Clean rocks, hopper, and ceramic media thoroughly. If your rocks are Mohs 7 or above, you can probably go on to Step Five. Otherwise do the prepolish step.

Step Four (Optional Prepolish):

For this step you will use 600 grit aluminum oxide, which wears down to a more rounded (and thus less harsh) surface than silicon carbide. Otherwise the directions are the same as for the earlier steps. Process for 1 to 2 days and clean everything thoroughly.

 

Step Five (Polish):

Fill the hopper – preferably one reserved for polishing and burnishing, or otherwise very well-scrubbed -- with your clean rocks and ceramic media as for the earlier steps. Use CPP polish at the rate of about 1-1/2 tablespoon per pound of load. If you use a different polish – and there are many available – follow the manufacturer's directions. (Cerium oxide is often suggested for emeralds and for glass.) Add the same amount of water, or enough to make a thick slurry. A small amount of dishwashing liquid can improve the tumbling action (in earlier steps as well). The rocks should be evenly coated during the tumbling process.

Run this step for 2 to 4 days, until the (cleaned) rocks look the same wet as they do dry. Clean everything thoroughly. If your rocks show a slight haze or film at this stage, go on to Burnish. If not, you're done!

Alternate Step Five (Dry Polish):

To polish your rocks without water or other additives, use about 25 to 30% of the load by volume of Vibra-Dry. Start with #1700 mesh, and run for 2 to 3 days, then switch to #14000 mesh for another 2 to 3 days. Rocks that are Mohs 5 to 6 may require a third step, using #25000 mesh for the same amount of time.

Vibra-Dry polishing compound is reusable.

 

Optional Step Six (Burnish):

Put cleaned rocks and ceramic media in the hopper. Add about 1-1/2 teaspoon water per pound of load, and start the machine at low speed. Add about 1/2 ounce soap powder per pound of load to develop a thick foam. Process for up to 4 hours, checking hourly. What you are looking for is an enhanced gloss.

Clean up everything and admire your polished stones!