The Art of Rock Tumbling


By Bruce MacLellan

Fill your barrel 3/4 - 7/8 full of broken or beach rock (no larger than 1/5 the width of the barrel & smaller). You’ll get the best results if you make sure that there is a good mixture of sizes with more in the medium size range. Hardness is important. A load with rocks that are all harder than a knife blade or carpenter's nail will polish better than a mixed load or one of softer stones. Unless they are very pretty & warrant a lot of work, eliminate any stones with cracks or pits as these may hold grit between stages & contaminate the next stage.

Once you are happy with your load (hardness, size, etc.) put in enough water so that you can see it between the top layer of stones. Do not cover the stones. Add your grit. Generally, in Steps 1 & 2 (of the 4-step process) you should use 1 ounce of grit for every pound of rocks & a little less in the pre-polish and polish stages (Steps 3 and 4). Using the measuring cups we provide, per pound of rock, you need:

Step 1 Coarse (60/90) 1 tbsp.
Step 2 Fine (220) 1 tbsp. + 1 dram (5 drams)
Step 3 Pre-Polish just over 2 tbsp.
Step 4 Polish 2 tbsp. well tamped down

Let the tumbler run for 1 week (for each stage).
STEPS 2, 3 & 4
At the end of each week dump your rocks (gently) into a screen or colander & rinse them thoroughly. Don't do this in your sink - the grit will clog up the drain. Pick a corner of your garden or yard in which to dump the water. Make sure you get every corner of your barrel & lid clean & every stone. Use a small brush (old toothbrush) on those stones with cracks or pits. Put the stones back in the barrel with enough water as in Step 1 & the appropriate grit & repeat.

1. After each cleaning, but before putting in the next grit, use water & soap** for a good rinse cycle. Let the tumbler run for an hour or so. This will make sure that the stones & barrel are clean. Dump the soap and water, rinse & proceed with the next grit. Make sure that you do this after the polish step at the very least, as this process cleans the stones & gives them a final burnishing.

**DO NOT USE DETERGENT. Detergents will interact with the rubber of the barrel, laying black stuff all over your rocks.

2. If your stones do not take up at least half of the volume of the barrel after the second step, you can add a filler - such as plastic pellets - to bring the volume back up to 3/4 and proceed to Steps 3 & 4. The plastic pellets available from Mountain Gems float. So. Add a step to your process -- throw the slurry (grits, rocks & pellets) into a bucket of water & float the pellets off. The pellets are reusable in the same grit. Do not use the pre-polish pellets in the polish cycle.

3. A better way is to run two loads of stones for the first two stages & then combine the two loads, choosing the best & filling your barrel to the 3/4 mark. Proceed to Steps 3 & 4.

4. Rejected stones might be improved by retumbling them with another load of rough stones.

5. For more rounded stones, run your coarse Step 1 for two or more weeks. Putting some fresh grit in at the end of each week will help.