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Is the Hobby of Rock Tumbling Changing?

People ask me what has changed in the world of rock tumbling over the past 30 years. They remind me that we have had both rotary and vibratory units during that period, that the Silicon Carbide grits have remained the same, and the polishing powders are pretty much the same. So what has changed in our little world of rock tumbling since 1970?


My answer is the increasing use of non-abrasive ceramic shape mixtures (CSMs). CSMs are now being stocked by enlightened lapidary houses and used successfully by tumbling operators everywhere. They usually consist of several sizes of “angled cylinders”, some spheres and sometimes one or two other shapes--these are made out of very hard, nonabrasive,  ceramic materials with a relatively low Specific Gravity, like Aluminum Oxide and Porcelain.

How Are the Shapes Used?

You put the CSMs into the rotary or vibratory tumbler at any stage in order to (a) cushion the tumbling action, (b) to make up for lost volume, or (c) to act as a “carrier” for the slurry. The CSMs roll and vibrate right along with the rocks, and since they are too hard to embed with the sharp Silicon Carbide particles, you can wash the CSMs with the rocks and let them go right along into the next stage of tumbling without fear of contamination. These mixtures of ceramic shapes are relatively expensive to purchase--about $43.00 for 6 pounds--but they last for many, many batches, and they save you a great deal of time and trouble. Considering the amount of improvement in the shape and finished quality of the rocks, the ceramic shapes are definitely cost effective.

CSMs replace the rubber bands, sawdust, bits of plastic, poly pellets, leather strips, molasses, honey, oatmeal, and many other “favorite” media we have used in the past to alter the internal environment of the barrels, mainly to reduce the force of rock-to-rock collisions. Today, you can toss out these old remedies and use just the shapes.

Let’s cover a little more on how the CSMs are used. First, in batches of soft material like obsidian and apache tears (Mohs-4 or 5), I advise that you start out Stage 1 with a 50-50 mix of rocks and CSMs. As Stage 1 wears down the rocks and creates rock powder, rock-volume is lost. You need to make up this lost volume with more CSMs. I keep an AccuFill template on hand to measure and re-adjust the batch volume so I start out each new week with the barrel 75% full of rocks & 25% full of air, with the lid in place (see the TechTip on making a Perfect Slurry). This consistency of weekly rock volume allows me to measure out the same volume of water for each barrel, which in turn causes the resulting slurries to be very consistent in their viscosity (thickness). Consistently perfect slurries mean that the cutting or polishing action will be very efficient as you work through the Stages of a batch.

If I am tumbling agates, which are quite hard (Mohs-7), I generally won’t add the CSMs in week one, but use them primarily for volume makeup.

The Old "Balanced Load" Theory

50 years ago, the rock tumbling literature was full of formulas for achieving a “balanced load”, where the rocks had a smooth size distribution from small to large. Distribution of sizes is important to insure that the slurry is deposited in all the “nooks and crannies” for even size reduction and finishing. Today we can put just one rock in a tumbler, add a pound or two of CSMs, and the slurry will be carried evenly to every part of the rock--a "balanced load" can thus be achieved using only one rock!. This is especially important when we have 1 or more preformed shapes (crosses, cabochons, etc.) we want to process. CSMs allow us to vary the amount of rocks while keeping our quality high. This is a huge benefit for the custom jewelry maker who assembles one or two pieces at a time.

Volume replacement is less important in vibratory tumblers, because we establish the perfect thicker-than-average viscosity visually, as we make very small adjustments in water and powder, until the slurry is clinging to the rocks as they follow their toroidal path around the bowl.  When tumbling hard rocks in a vibratory tumbler, we may not use the CSM's at all.  We recommend you start all Obsidian and glass batches with 33 to 50% shapes so they won't spall and fracture as they bump into each other.  After that, there's no reason to add more.

And the Winner is...

These are just a few of my reasons for nominating non-abrasive Ceramic Shape Mixtures as the most significant rock tumbling product improvement over the past 50 years. It’s probably time for you to purchase a supply and start reaping the advantages which  they bring to the rock tumbling process.

Little Red Store has been selling Ceramic Shape Mixtures since it opened in August of 2008.