Questions & Answers

(Questions from our Customers which we have answered--feel free to send us your own questions)

Question #1: I have been tumbling rocks on a rotary tumbler for about 6 years, and I notice that when I clean up Stage 1 (80-grit SiC) batches, sometimes the spent slurry is thick and other times it is very thin. I carefully measure everything, but the results are not consistent. What is happening to cause these variations? (From John in Tennessee)

Answer #1:    Hi John. The variations you have noticed in spent (used up) slurries are very common and are caused by two things. First, there are often differences in the height of the rocks stacked within the tumbler barrel before you begin the process of adding the powder and water. If you add water up to the bottom of the top layer of rocks, and the height of the top layer changes from barrel to barrel, you will add different amounts of water to each barrel. The solution is to fill the barrel with rocks up to the same level each time--sometimes you may need to add a few ceramic shapes to get the level properly adjusted. I like to use the AccuFill Ring Gauge to insure the level is the same each time. Then I add the grit or polish and finally add the water up to 1/8-inch below the top level of the rocks.

Second, softer rocks usually produce more rock dust than harder rocks in a given amount of time. The more rock dust, the thicker the spent slurry. Stage 1 is where most of the rock dust is generated--that is why it is called the “shaping” stage. I normally let agates run a whole week before washing up. With softer materials, cleaning up after 4 or 5 days will help keep the slurry inside from becoming too thick to flow properly.

Question #2: I just got done with my rotary-tumbled batch and the rocks aren’t polished--they’re pretty dull. What went wrong and what can I do to get them polished? (From Mary in Washington)

Details: The rocks are Mexican Crazy-Lace Agates, 7 in hardness. I tumbled them in Stage 1 (80-grit Silicon Carbide) for 5 weeks, Stage 2 for 2 weeks (220-grit Silicon Carbide), Stage 3 for 1 week (600-grit Silicon Carbide), and Stage 4 (polish) for 3 weeks. I used no plastic pellets to cushion the stones. My polish is a white powder called Levigated Alumina, and I followed all the directions for loading the barrel, quantities of abrasives & polish, and water.

Answer #2:    Good job Mary--five weeks in Stage 1 is excellent! Those rocks got pretty well rounded if you cleaned up at the end of each week and re-loaded new abrasive each time. The times in Stages 2 and 3 look good also, providing you thoroughly cleaned up between stages and didn’t let any coarser grit contaminate the finer grit. 

The first thing you need to do is gather some information. Examine your stones to see if they are chipped or just dull. Use your hand loupe or magnifier to determine if the scratch sizes are from the 220-grit or the 600-grit abrasive. This information will help you decide if you need to go back to a previous stage or if you can continue with the polish stage. My guess is that you used the wrong polish for this hard stone. Levigated Alumina is actually “Calcined Alumina”, which is not for rock tumbling at all, but rather, for polishing rock slabs and glass microscope lenses.  The particles are flat platelets (like pancakes) and are very compatible with flat surfaces--but they don’t tumble rocks very well. This product is often confused with its cousin, Fused Aluminum Oxide, which is more effective in polishing rocks, especially in a vibratory tumbler.

My own choice would be to run another 2 weeks in my favorite polish for agate, Cerium Oxide. With hard, tough rocks such as these, you shouldn’t need to use plastic pellets in any of the stages.

One last thought is to review how you re-fill your barrel to the desired level after cleanup. In Stages 1 and 2, you lose rock volume as the abrasive makes each rock smaller. This loss in volume is significant and must be “made up” with other material to get back to the correct barrel fill-level. The older method is to make up the diminishing volume with more rocks. If these rocks are less rounded than those in your batch, they will actually score and gouge your smoothed rocks and increase the time it takes to make them smooth.

The best way to make up lost volume in the barrel is to add smooth, hard, non-abrasive ceramic shapes (CSMs) to each barrel before you attach the lid and begin the next week. These will tumble with the rocks and actually improve the grinding/polishing action by carrying abrasive to all the “nooks and crannies” in your stones. There is no need to segregate your CSMs by Stage number--they are so hard that they do not embed with Silicon Carbide--simply wash them with your rocks and put them all together into the next Stage.

Question #3: I just got back from Lassen Creek in California with several pails of high-quality Obsidian. I have heard it is difficult to polish. How do you recommend that I polish Obsidian in a rotary tumbler? (From Terry in California)

Details:   Obsidian was once part of the molten magma inside a volcano. The magma cooled so rapidly that crystals did not have time to form. It became soft (Mohs 5.5) volcanic glass, and because it has no regular structure, it breaks in a conchoidal manner to form incredibly sharp edges--it was once used for arrows & spears and today, obsidian is still used in cardiac surgery because obsidian blades have a cutting edge up to five times sharper than high-quality steel scalpelsObsidian is 70% silicon dioxide (SiO2), and is colored black by finely dispersed magnetite (Fe3O4). It’s a member of the quartz family. Obsidian, an igneous rock, is relatively soft--it can be scratched easily, and it exhibits poor toughness (it breaks and chips easily). It is thus more difficult to polish than a harder, tougher stone such as an agate. 

Answer #3:     Terry--Properly polished Obsidian is quite spectacular, and you will be amazed at how high a polish those stones will take. Here’s my basic procedure for polishing obsidian:

First, I polish Obsidian in 5 stages rather than the normal 4 stages. Being softer, the Silicon Carbide abrasive will make much deeper scratches than with harder stones, and it is necessary to add another stage to get the surfaces properly smoothed prior to polish. Because Obsidian chips so easily, I also “cushion” each barrelful to keep the rock-collisions less violent. Begin with a properly filled barrel of 50/50 sharp obsidian mixed with CSMs, add your 80-grit Silicon Carbide and water, install the lid and start Stage 1. After a week, break down the barrel and wash up. Inspect the obsidian for roundedness, and, using CSMs to make up lost volume, keep going another week until you are satisfied with the rounded shape of the rock.

To remove the scratches from the previous stage, Stage 2 (220-grit Silicon Carbide) should take 1-2 weeks, Stage 3 (600-grit Silicon Carbide) about a week, and Stage 4 (1000-grit Silicon Carbide) about a week. Remember to use your hand-loupe or magnifier and let the stones tell you when they’re ready. Be meticulous about cleaning everything between stages.

I use Cerium Oxide for polishing Obsidian in Stage 5. After letting the tumbler run for 2 weeks continuously, I usually see a wonderful polish on those little pieces of volcanic glass.

If they’re not quite right, I would add a 2-week continuous stage of polishing with Tin Oxide.  Always look at your stones under magnification and try to figure out just what they’re missing. Once you see them up close and personal, you will be amazed that they will actually tell you what they need to look beautiful. If you are persistent and careful, you can (and will) get the polish of your life!

Question #4: How does rock hardness and abrasive grit size relate to tumbling? Are they important concepts to understand? (From Janice in Tennessee)

Details: Most rock-tumbling advertisements indicate that all stones can be tumble-polished together and that you can get highly polished multi-colored rocks from the same batch within 4 weeks. This is simply not true. It is imperative that you have a thorough understanding of rock hardness and grit size, because these are key elements in the production of truly noteworthy tumbled stones.

Answer #4:    God made rocks with a wide variety of hardness. Sometimes the same type of rock from two different sites have differing hardness. Rocks from the surface may be weathered and softer than rocks which are dug. Rocks of different hardness cause a problem when they are tumbled together. Either the softer rocks won’t polish because the harder ones keep scratching them, or the harder rocks take longer to polish because the softer ones don’t apply as much abrasive pressure. Generally, by the time you get the hard rocks polished, the soft ones still haven’t polished and now they are too small to use. Some people enjoy the challenges presented by multiple-hardness batches, but I don’t. I usually break up only one kind of rock for any given batch. Then I am certain that the hardness of one stone will closely approximate the hardness of all the others.

Another successful method is to actually measure the hardness of your various rocks and store them by individual hardness. Then, when you get ready to load a barrel, you load it from the storage box labeled “Hardness-7” or “Hardness-5.5”. To check the hardness of a specific rock, you will need a hardness testing kit. The cheaper ones supply rocks of different hardness, using the theory that when two rocks are rubbed together the harder one will scratch the softer one. The better kits (and I think more practical) have a variety of hardness picks, each of which has different hardness, and numbered from 2 to 9. They operate on the same theory.

It is also true that harder rocks are generally easier to polish than softer rocks. Don’t worry, though, because most igneous rocks above a hardness of 5 polish very well.

The rock hardness scale we normally use was developed by Friedrich Mohs in 1812. It is accurate but not linear. That is, an 8 hardness rock is not twice as hard as a 7 hardness rock. We generally use the Knoop hardness system to see the linear relationship between rocks of differing hardness--for example, a diamond (Mohs = 10) is actually 10 times harder than a piece of quartz or agate (Mohs = 7)! See page 50 of my book, Modern Rock Tumbling for a comparison of the two scales. 

Question #5

My 10 year old Son, Michael, and I have taken up rock tumbling together now that his Dad is no longer in the household. We recently broke down our first Stage 1--Week 1 rotary barrel of Texas Plume Agates and it was almost dusty inside--the barrel had not leaked. What did we do wrong? (From Laurie in Texas)

Answer #5:    Laurie…congratulations for taking on the role of both Mom and Dad for Michael--your efforts to raise him as normally as possible will help him in many ways--I hope you will also consider joining your local Lapidary Club to help him bond with other mentors and enjoy this great hobby of rock tumbling.

It sounds like you did everything right in loading the barrel except for adding enough water. Most tumbler instructions say to add water to the “bottom of the top layer of rocks”. This means to add water to about 1/8-inch below the top of the rocks.

Sometimes, when we look into the barrel to check the water level, we see a few droplets of water on a rock glistening in the light, rather than the actual water level, which may be several inches below. Learn to get into the habit of tilting the barrel slightly until you see water--this little procedure will make sure that what you see is the actual water level, and not merely a wet rock. Then you will add the proper amount of water to every barrel and enjoy the benefits of consistently perfect slurries.

Question #6: While inspecting the barrel contents in mid-week, the phone rang and I spent about 15 minutes talking to my Daughter in college. Then I returned to the open barrel, carefully replaced the lid and restarted the tumbler.

At the end of the week, I tried to dump out the contents to clean up, but it was all stuck to the bottom of the barrel in a gooey mess. What went wrong? (From Tom in Oregon)

Answer #6:    Tom, you were snake bitten by a process called “particle settling”. When slurries are allowed to sit quietly, the heavy SiC grit falls out of suspension to the bottom of the barrel, where it is joined by heavy, fine, wet, rock dust. In just a very few minutes, this mixture becomes hard and stiff, like cured cement. If you set the barrel to tumbling again, it will likely never mix back into suspension, even when tumbled for days.

Top avoid this problem in the future, there are three things you can do. First, don’t set down barrels with slurry in them for more than a minute or so. Then, make sure you shake the barrel to make sure the slurry is intact (you will hear the rocks rattle) before you set it back on the tumbler. And finally, if it appears that the slurry has settled and hardened (you can’t hear the rocks rattling when you shake the barrel), open up the barrel and manually stir the contents back into suspension before proceeding. The Little Red Store has developed the Stir Stick to help with this chore, since most stirrers (and finger nails) are not stiff enough to get the particles back into suspension again.

Question #7: I have been tumbling with a rotary tumbler for several years now, and have just purchased a 10-pound vibratory tumbler. I have had my rough Tigereye vibrating in the bowl for a full week, and its still sharp and jagged--it looks like the 220-grit SiC is not even working. I am ready to throw this tumbler out the window. Do you have any idea what is going on here? (From Nancy in Colorado)

Answer #7:    Nancy, you have fallen victim to a very common error which tumbler operators make with their first vibratory tumbler. You have added way too much water and caused your slurry to only work when the rocks are submerged at the bottom of the barrel (the abrasive powder is remaining in the liquid on the bottom of the bowl). This is extremely inefficient and wastes most of the power of a vibratory tumbler.

You need to learn how to make a “Perfect Slurry” for your vibratory tumbler. We have a great article on this process at the top of this page, under TechTips.  Check it out and call me if the problem persists.

(Nancy never called me back)