Tumbling Grit:
Tumbling grit is an abrasive material that is placed in the tumbler barrel with your rocks.
Silicon carbide is the most commonly used tumbling grit. It is much harder than the rocks -
and when caught between tumbling rock particles it creates tiny scratches on them. This
continuous scratching wears down the rocks and rounds them. Sharp edges on the rocks are worn
down more rapidly than flat surfaces because the grit is rubbing against them on two or more
sides. The result of this grinding is smaller, more rounded particles.
There are different types and sizes of tumbling grit. If you bought a kit to go with your tumbler, then you will be using what’s provided. If you would like to choose your own, or if you have used up what came with your kit, here we have outlined the different abrasives.
Silicon carbide grit ranges in size from very coarse (like grains of sand) to very fine (like a powder). The sizes are graded with
numbers, and the smaller the number, the coarser the grit.
The tumbling process is usually done in five steps as shown in the table below. The first three steps use silicon carbide tumbling grit (some dealers sell prepolish grits that are not made of silicon carbide and we have used them with good results.) The fourth step uses a polishing compound such as aluminum oxide. The fifth step is a burnishing step with soap to clean up the rocks and make them sparkle.
Amounts of grit, polish and burnishing soap recommended for rotary tumblers.
Rock Polishing Compound:
A variety of polishing compounds are available for tumbling gemstones. We use aluminum oxide for hard materials such as quartz varieties. If you purchase tumbling rough from a dealer they can frequently tell you what polishing compound to use (many of them will recommend a polishing compound on their website). The polishing compound is a very fine powder designed to buff off the scratches left behind by the prepolish grit. Occasionally dealers sell aluminum oxide for use as a prepolish instead of the extra fine silicon carbide grit.
Plastic Pellets:
Plastic pellets work nicely to cushion the rocks from banging into each other during the later stages of tumbling. They should be used in the prepolish and polish steps for cushioning, and in the earlier steps if necessary to bring the volume of the barrel to 2/3 full. The pellets also help distribute the grit and polishing agents to all surfaces of the rocks. It is helpful to note that although the plastic pellets can be reused, they are reusable only for the particular step they were used in before. For example, pellets used in the fine grind will have particles of fine grit in them, and will scratch the rocks if used in the prepolish step. If you’d like to reuse the pellets, it will help to keep them separated in marked containers or bags.
