By Adam Kelliher
The short answer: silicon carbide does the grinding in the first two tumbling stages, and aluminum oxide is the go-to pre-polish and polish because it's cheap, hard and works on most stones. Cerium oxide and tin oxide are older polishes that still have their uses, mostly on glass, obsidian and some softer or feldspar stones.
People come to the counter with a bag of grit and a question: why does it matter which powder goes in which week? Here's the plain version. Most of it comes from the abrasives chapter of my book Beyond Cabbing.[1]
Everything in the barrel wears down
This is the bit that makes the rest make sense. Rocks are hard, so as you grind them the abrasive wears down too, and so does the barrel. In a tumbler you're trying to wear the rocks down faster than you wear down everything else. That's why particle hardness and size matter so much.[1]
Silicon carbide: the grinder
Silicon carbide is what you use for the stock-removal stages: 60/90 for rough and 120/220 for medium.[1] I use blends rather than a single size. The mix of particle sizes cuts better, and if you use straight 220 for medium you need about two extra days.[1]
Two warnings from the book. First, stay away from 46 grit in a tumbler. It's too coarse, and it's too big to rinse through a kitchen strainer, so it hangs around and contaminates every stage after it. Second, buy lapidary-grade grit, not sandblasting grit. Sandblasting grit costs about half as much but is more porous and breaks down in a couple of days instead of more than a week, so in my experience you get about a quarter of the work out of it.[1]
The pre-polish
Pre-polish is roughly 800 grit. Aluminum oxide is the most popular and the most versatile.[1] One trick I like: equal parts 600 and 1000 silicon carbide. Silicon carbide breaks down quickly, so by the end of the week it's finer than 800, which sets you up nicely for the polish.[1]
Aluminum oxide: the polish
Aluminum oxide is the same chemical as corundum, the mineral behind ruby and sapphire, and it sits at 9 on the Mohs scale.[1][2] It's my go-to polish because it's economical and effective on just about anything 6 or harder. The particles also round over as they wear, much like the stones do, which is part of why it buffs so well.[1] A 3000 grit aluminum oxide is the usual final polish, and you can find grit and polish in our lapidary tools.
Cerium oxide, tin oxide and chrome oxide
- Cerium oxide. Lapidary-grade cerium is reddish-brown, cheaper and common in starter kits. Optical-grade cerium is white or pale pink and is preferred for glass, quartz, obsidian and other silica-based stones.[1] Many tumblers also like it for feldspars such as moonstone and labradorite.[3]
- Tin oxide. A white polish that was a favourite in the early days of tumbling. People still use it on obsidian, glass and softer stones, but it costs several times as much as aluminum oxide.[3]
- Chrome oxide. The classic green compound. It's mostly a wheel and buffing compound, good on nephrite jade and other stones around 5 to 7.5 on the Mohs scale.[1]
Can you reuse it?
Yes, for the last two stages. I save used pre-polish and polish and cut it with about 50% fresh powder next time. Rough and medium leave too thick a slurry to reuse, so always start those with fresh water and grit.[1]
Soft stones and a matte finish
Stones softer than about 5 on the Mohs scale won't take a real shine in a tumbler.[1] If yours come out with a frosted, sugary finish, the book suggests soaking them in mineral oil for about 15 minutes and wiping them dry. It brings the colour back, a bit like oiling wood.[1]
You'll find grit, polish and tumbler parts in lapidary tools. New to tumbling? Start with our step-by-step tumbling guide.
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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), Aluminum Oxide: TXP and Rapid Polish for Rock Tumbling (Al2O3, Mohs hardness 9).
- RockTumbler.com (Hobart King), Which Rock Tumbler Polish? Aluminum, Tin, Cerium, Chrome Oxide.