About Tumbling Grit

Abrasives used in the art of tumbling come in a wide range of grits, and in a lot of different materials. This can be confusing to the newcomer. Here are some of the terms you'll need to know:

*Grit refers to the size of the particles, and as with sandpaper, the smaller the number, the larger the particles are. 60/90 –- which means a range of particle sizes between 60 and 90 grit -- is the coarsest grit generally used in tumbling, though 46/70 is available. 500F or 600F is generally used for the prepolish step, although some people use 800F or even finer for prepolish. (The "F" means "and finer".)

*Mesh is a little more precise, referring to the screen through which a particular grit must pass. The term mesh is more commonly used for abrasive sizes 1000 and higher. For practical purposes, however, grit and mesh can be considered the same thing: 1000 grit = 1000 mesh.

*A micron is 1/1000 of a millimeter. About the only time you'll come across this measurement is with certain polishes, like Linde A and Linde B. In the case of microns, the larger the number, the larger the particles -- the inverse of how the terms grit and mesh work. Therefore, 120 grit equals about 100 microns, whereas 220 grit equals 70 microns and 600 grit equals 25 microns.

*Silicon carbide is the basic abrasive used in tumbling. It is very hard –- Mohs 9+ -- and each time it breaks, a new sharp edge is exposed. This makes it long-lasting and quite effective.

*Boron carbide serves much the same function as silicon carbide, except that it is slightly harder. This makes it the abrasive of choice for tumble-polishing corundum, although it is both expensive and messy to use.

*Aluminum oxide is a synthetic material most often used as a prepolish and polish, although you can get it in coarser grades. The advantage of aluminum oxide (or alumina) is that it breaks down to a more rounded edge than does silicon carbide, which is what you want in the finer stages of processing a batch. Aluminum oxide is Mohs 9.

*CPP tumble polish is an effective and economical aluminum oxide-based polish for barrel tumblers. If you just want one polish for most stones, this is the one to go with.

*Cerium oxide is a natural polishing compound particularly useful for materials that are between Mohs 5 and 7, like glass and quartz. It is more effective in a rotary tumbler than in a vibratory one, as the intense vibrations of the latter break the grains down very quickly.

*Tin oxide is 99.9% chemically pure and finely graded in the submicron size. It's recommended as a final polish in flat lapping and for metal/stone combinations.

*Chrome oxide is the medium of choice for getting a good polish on jade, lapis lazuli, rhodonite, peridot, and other colored stones.

*Pro Polish polishes a wider variety of gemstones than any other material, working like Tin Oxide but at half the price. It is composed of finely-graded aluminum oxide in the 1/2 micron range. Polishing Tip: works best in a thin slurry.

*Vibra-Dry is a proprietary prepolish and polish compound specially designed for vibratory tumblers. It requires no liquids, which makes inspection and cleanup very easy, and is excellent for soft, fragile, or difficult-to-polish materials. Vibra-Dry is also reusable. Processing cycles are somewhat longer than with polishes that use a liquid medium.

*Sapphire powder -- also sold as Linde A (0.3 micron) and Linde B (0.05 micron) –- is a synthetic polish manufactured from aluminum oxide. It has a hardness of Mohs 9, and is therefore most useful for polishing very hard and/or difficult materials (but not corundum).

*Diamond powder is the hardest abrasive available –- Mohs 10. It's also probably the most expensive, and is generally sold by the caret! However, if you need to get a good polish on corundum, diamond powder is probably the most effective way to go.


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Information from How to Tumble Polish Rocks into Gems by Edward E. Smith and various other sources.


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