By Adam Kelliher
The short answer: rock tumbling is four grit stages in a rotary barrel: rough (60/90 silicon carbide), medium (120/220), pre-polish (about 800) and polish (aluminum oxide), each run for about a week with a thorough wash in between. Fill the barrel about three-quarters full with a mix of big and small rocks, add water to the top of the rocks, and soap-wash after every stage from the medium onward.
Most people meet tumbling as a birthday present. I get it. But in my book Beyond Cabbing I make the case that most hobbyists underestimate what a tumbler can do, and the difference usually comes down to a few habits, not a better rock. Below is the process I actually run, with the reasons behind each step. Where it disagrees with the advice you'll find online, I'll tell you why.[1]
What you need
- A rotary tumbler from a lapidary brand. Thumler and Lortone are the names you'll see most. Skip toy and general-tool brands. When a beginner does everything right and still gets a dull polish, it's often the machine, not them.[1] You'll find tumbler barrels, belts and parts in our lapidary tools.
- Four grits: 60/90 silicon carbide, 120/220 silicon carbide, an 800-ish pre-polish and an aluminum oxide polish. Grit and polish are in lapidary tools.[1]
- Plain bar soap. Unscented Ivory or Dove, grated into flakes. More on why below.[1]
- Rough rock in mixed sizes, a kitchen strainer, and a bucket for the slurry.
Loading the barrel
Fill it about three-quarters full. A lot of online guides say half to two-thirds.[2] I go to three-quarters because tumbling needs a slope inside the barrel for the rocks to roll down. Too few rocks and the slope is too shallow, so they get thrown instead of rolled. Too many and there's no room to move.[1]
Mix the sizes. Grinding only happens when a grit particle is trapped between two rocks under pressure. Small rocks also get into the dips of the bigger ones, which nothing else will reach. And never put in a rock bigger than about a quarter of the barrel. That one big stone just drops on everything else and bruises the load.[1]
Add water to the height of the rocks, then the grit. For a 3 lb barrel I use about two heaping tablespoons of grit per stage.[1] You don't need to scrub rough rock first. Knock off caked mud, sure, but you're about to grind away the outer couple of millimetres anyway.[1]
Do you need to sort by hardness?
This one goes against the usual advice. Plenty of guides tell you to load only rocks of the same hardness. I don't bother sorting a mixed load. I do pull out anything I spot that's softer than about 5 on the Mohs scale and save those for their own barrel, because soft rocks won't take a shine from a tumbler no matter what you do. But I don't lose sleep over the odd one that slips through.[1] If you want the background on hardness, read our post on Mohs and true hardness.
The four stages
- Rough (60/90), 8 to 10 days. The standard advice is 7. I like a few extra days here to really wear the rocks into smooth shapes. This is the stage that does the shaping. Use the 60/90 blend, not straight 46 grit, which is too coarse and too big to rinse out of a strainer, so it ends up contaminating every stage after it.[1]
- Medium (120/220), 7 days. The blend cuts better than straight 220. If you only have 220, add a couple of days.[1]
- Pre-polish (about 800), 5 to 7 days. Aluminum oxide is the easy choice. A less common trick from the book: mix equal parts 600 and 1000 silicon carbide. It breaks down fast and ends the week finer than 800, which leads nicely into the polish.[1]
- Polish (aluminum oxide), about 5 days. Five days is plenty, and anything past seven is overkill.[1]
When you open the barrel after stage 1 it'll be noticeably lower, because the rocks have worn down. Top it back up to three-quarters before the medium and pre-polish stages.[1]
Fillers: rocks, not plastic
A lot of guides tell you to top up a low barrel with plastic pellets.[3] I don't. Pellets wander off, they're bad for the environment, and you need a separate batch for every grit stage. Plain ceramic media (with no grit baked in) works better, but my first choice is more rock. Crushed milky quartz from a garden centre is ideal. It suits most tumbling material, it comes out polished too, and the white is easy to pick out of a finished batch.[1]
Wash and soap between stages
Clean the rocks, barrel and lid completely between every stage. Even a few grains of coarse grit will scratch a polish.[1][2] From the end of the medium stage onward, I also run a soap stage: washed rocks, water to the rock line and a teaspoon or so of grated bar soap, tumbled for 4 hours, then rinsed again.[1]
Why plain bar soap? Not because it's cheap. The chemicals in liquid dish and laundry soaps break down rubber barrels. If you ever see your water turn black, that's the barrel.[1]
A few things people get wrong
- Burping. You'll read that you should open the barrel after the first day to release gas.[3] In my experience you only need to burp a barrel when a stage runs past about 10 days.[1]
- The drain. Never pour slurry down a sink. It sets up and clogs plumbing.[1][2]
- Speed. If the edges come out frosted with little impact marks, that's bruising. The rocks are being thrown, not rolled. Check that the barrel is full enough.[1]
- Store-bought shapes. The round, even tumbles you see in shops were usually shaped by hand on a grinder before they went in. Yours will look more like the rocks you started with, and that's fine.[1]
Need something to put in the barrel? Start with the rough rock collection, and grab grit and parts from lapidary tools. To see what finished stones should look like, browse tumbled stones. Once you've got the hang of it, the next step is cutting a cabochon.
See what's new: browse our newest arrivals for the latest pieces added to the shop.
Sources
- Adam Kelliher, Beyond Cabbing (A Lapidary Handbook), Section 1: Tumbling and Cutting a Cabochon; Volume II: Abrasives & Compounds and Lapidary Safety.
- RockTumbler.com (Hobart King), Rock Tumbler Instructions: the four-step process, fill levels, cleaning between stages and not pouring grit down a drain.
- Kingsley North, Rotary Rock Tumbling Instructions: plastic pellets as filler and opening the barrel after 24 hours (the book's advice differs; see above).