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What is Silicon Carbide Grit?

Most of the tumbling done in a rotary tumbler follows a four-step tumbling process. The grits used are typically coarse (60/90 grit), medium (150/220 grit), fine (500 grit), and a rock polish such as TXP aluminum oxide.

This image shows the relative size of coarse, medium and fine grit compared to the size of a grain of rock polish.

This image is a close-up photo of 60/90 ungraded silicon carbide grit. The scale at the bottom of the photo has one millimeter increments. You can clearly see that there are a range or particle sizes as expected in an ungraded grit.

One of the Hardest Known Substances

Silicon carbide, the material used as “rock tumbling grit” is a compound composed of silicon and carbon with a chemical formula of SiC. It is one of the hardest known materials with a Mohs scale hardness of 9+. It is also inexpensive, selling for just a few dollars per pound as a screened and packaged product. The combination of hardness and low price make it an excellent abrasive for rock tumbling, lapidary work, sand paper coating, sand blasting and water-jet cutting.

Silicon carbide works extremely well for rock tumbling because most tumbling roughs are materials such as agate, jasper and petrified wood. These have a hardness of about 7 on the Mohs scale, while silicon carbide at 9+ is much harder. This hardness difference makes silicon carbide an effective rock tumbling abrasive.

Synthetic Silicon Carbide

Silicon carbide occurs naturally as the mineral moissanite; however, it is extremely rare, only found in tiny quantities in a limited number of locations. All of the silicon carbide sold as an abrasive is manufactured. It is produced by placing a mixture of coke (a fuel and carbon source), sand (a silicon source) and sawdust (another carbon source) in an electric resistance furnace and heating to a very high temperature. The silicon carbide is then crushed and screened into specific sizes. These “grits” and “powders” are used as cutting, grinding and polishing media.

grit microns
20 841
30 595
40 420
50 297
60 250
80 177
90 166
100 149
120 125
150 100
180 83
200 74
220 68
250 58
270 53
325 44
400 37
500 25
600 20

“Grit Sizes”

The numbers used for grit sizes are similar to the numbers used to grade sand paper. Small numbers are used for the coarser grits (larger size particles), and large numbers are used for the finer grits (smaller size particles).

The chart at left has grit sizes in the left column and the particle size in microns (a micron is 1/1000th of a millimeter) in the right column. You can see that a particle of 60 grit would be about 250 microns across.

Some silicon carbide is sold as a “graded grit.” That means that all of the particles are very close to the same size. For example, an abrasive sold as “80 grit” would have almost all of the particles about 177 microns in size. Graded grit must be carefully processed to be sure that all of the particles are the same size.

In contrast, “ungraded grit” has a range of particle sizes. For example, an abrasive sold as “60/90 grit” would have a range of particle sizes smaller than 60 grit but larger than 90 grit. This does not require the same level of processing needed for graded grits and can be sold for a lower price.

For rock tumbling, the less expensive ungraded grits work fine and that is what most people use.

Rock Tumbler Polishing Sequence

In rock tumbling a four-step procedure is usually used. This typically begins with a coarse grit (60/90 grit), then a medium grit (150/220), then a fine grit (often a graded 500), and finally a polish such as TXP aluminum oxide.

Using the grit in sequence works best. The coarse grit does the rough shaping of the rock, the medium grit smooths the surface, the fine grit prepares for polishing, and the polish is the appropriate size to bring the rock to a bright luster. (Polishes are usually very fine powders with a particle size of three microns or less.)

Using Silicon Carbide Grit

In most rotary tumblers, about two ounces (two tablespoons) of silicon carbide grit is used for each pound of rock being processed. That is the typical charge if you are going to tumble for about seven days. That much grit distributes through the barrel and does a great job at grinding the rough rocks. The grit particles will break and wear down as they work, so at the end of a week any grit that you find in the barrel will be smaller in size. If you run it long enough the grit will have broken into very tiny particles and be difficult to find in the barrel. That is evidence that you got your money’s worth!

Happy Tumbling!