Making Perfect Slurries
The Absolute Key to Successful Rock Tumbling
Trust me, effective slurries are the absolute key to successful rock tumbling. Without knowing how to make a perfect slurry for your tumbler, everything else you do and buy will be wasted. Rotary slurries are worlds apart from vibratory slurries. You need to know how to make each one perfect every time. Here are the secrets:
A. Slurries for Your Rotary Tumbler
Perfect rotary slurries begin with good barrel-loading techniques. Your goal is to load the tumbler barrel to the ¾ level (75% full, 25% air, with the lid in place) with clean, rinsed rocks, no matter how many rocks you have.
Build or Buy a Template
This involves making or buying a template which measures when you are properly full. To make a template, measure the height of the actual tumbling area, assuming the lid were closed. In a 3-pound barrel, this measurement is about 4-inches. Now measure the distance from the lip where the lid rests to the top of the barrel--for 3-pound barrels, this is about ½-inch. If the barrel were 75% full, there would be 3-inches high of rocks inside. Now cut a piece of plastic pipe or wood 1 ½ inches thick, and you will see that it will fit on top of the rocks in the barrel and be perfectly level with the top of the barrel. You can follow this process to make a template for any size barrel. Note: Little Red Store has prepared just such a template for 3-Lb rotary barrels called the AccuFill Ring Template. Check out the two photos to see how it works.
All subsequent barrels of rocks should be loaded exactly the same way--rocks plus template equals full-to-the-top. If you have too many rocks, take some out. If you have too few rocks, use Ceramic Shapes to make up the difference. You can even put in 1 rock--say a cabochon--and fill to the 75% mark with Ceramic Shapes!
Now that the rocks are properly loaded, add the proper amount of powder (SiC or polish), and add water to about 1/8-inch below the top level of rocks. You should be able to see the water, but it will be 1/8-inch below the level of the rocks. That’s just perfect.
Now install the lid on the barrel and immediately start tumbling the load. If you set the barrel aside instead of immediately beginning the tumbling process, there is a chance that the slurry materials will fall to the bottom and cake together. This “slurry cake” must be remixed with a “Stir Stick” before tumbling begins.
Normally, we let a properly-prepared rotary batch tumble continuously for a period of 7 days. It takes that long to break down the Silicon Carbide particles. If you are polishing, the harder polish particles will last much longer than a week--I always let the polish slurry go for a total of two weeks, running continuously.
When you clean up and dump the slurry at the end of the week (or 2 weeks for polish), it should have the consistency of thin pancake batter (80-grit slurries will usually be thicker than polish slurries, however). If the slurry pours easily, you have confirmation that it was, indeed, a perfect rotary slurry.
B. Slurries for Your Vibratory Tumbler
The perfect slurry for a vibratory tumbler looks a little different than a perfect rotary slurry, but it is no less important. Vibratory slurries are thicker--about the consistency of thick pancake batter. This consistency depends on how many rocks, how wet they are and other factors. Therefore we do not measure out the powder for a vibratory slurry--we build it through careful observation, until it, literally, sticks to the rocks themselves.
Creating the Required "Toroidal Motion"
Start by placing your clean, rinsed, wet rocks in the empty, clean vibratory bowl (no additional water is added)--the level should be anywhere from ½ to ¾ full--you can adjust with Ceramic Shapes as needed--you do not need to use a template for this process. Leaving the top of the bowl off, now start the tumbler and watch for the “toroidal” or “donut” motion which is peculiar to a vibratory tumbler. The rocks should be rotating around the bowl horizontally while they are also rotating down to the bottom on one side and back to the top on the other side. If this dual motion is not present, adjust the rock level higher or lower until you are able to observe these two motions occurring simultaneously.
Now Create a Perfect Slurry
Once you have observed that the toroidal motion is present in your batch, it is time to sprinkle in a tablespoon of grit or polish (vibratory slurries normally start with 220-grit, not 80-grit powder). Wait 10 seconds and see if the rocks become coated with the slurry. If not, sprinkle in another tablespoon of powder and observe again. Keep following this procedure until the rocks become coated with a “thick pancake batter” of slurry. If you are using cerium oxide polish, the rocks will all suddenly turn pink--with silicon carbide grit, the rocks will all turn gray. Once the rocks all change to the powder color, you have achieved the perfect vibratory slurry. There should be NO standing water on the bottom of the bowl, and the rocks & ceramic shapes should be thickly coated with the slurry. Now you can fasten the top on the bowl and continue tumbling.
This Perfect Slurry Must be Re-established Often
Because each rock is fully coated with slurry, every one of the 3600 rubs/minute will cause the desired abrasion or polishing effect, whether the rock is at the top or the bottom, or somewhere in-between. The vibratory process is very fast ONLY when the slurry is perfect! Therefore, it is important to check the slurry consistency every 3-4 hours (use of a timer is very helpful). The slurry will tend to “dry out” as it gets contaminated with rock dust. To make it perfect again, just give your rocks one or two squirts of water from a spray bottle, and the slurry will return to the consistency of thick pancake batter. Eventually your coarser slurries will become really contaminated with rock dust--that’s a sign it’s time to wash up and start over. Thus, you will want to check your 220-grit slurry more often than your polish slurry, because the 220-grit powder makes much more rock dust.
A Real Slick Tip
One little tip when washing up--add a whole cup of water to the bowl about 10 minutes before you shut down to clean up. This will dilute the slurry so it will come off the rocks very easily. Thus, wash up will be much faster. Try it, you’ll like it!
If you would like more information about key procedures for rock tumbling, we advise that you pick up a copy of our book, Modern Rock Tumbling, which will make you the neighborhood expert on how to do it and why!