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Two methods of tumbling being discussed will be the rotating tumbler, and the vibrating tumbler. For both methods the grinding medium used is silicon carbide.
The Grits: Rough grit (anything under 100) is used for the first stage. Medium grit (100 – 300) is used in the second stage. Fine grit (400-600) is used for the third stage.
Estimating Cost: As a “rule of thumb”, silicon carbide, 220 to 400 grit, weighs approximately 0.8 ounces per tablespoon. Fine grits, 400-600, and polishing compounds, weigh approximately 0.5 ounces per tablespoon.
Weigh your stones when you get your tumbler barrel 2/3 to3/4 full. A six-pound tumbler doesn’t always hold six pounds of rocks. As a rule of thumb, use one tablespoon of grit per pound of stones, or one tablespoon per two pounds of the manufacturer’s weight rating for the tumbler. Most tumbling books suggest more grit than is really necessary.
Volume shrinkage: Roughly 25% of the beginning volume tumbled in step one will turn to mud, so run two loads of step one. This will provide the filler material to replace those that were ground away or thrown away from the first step.
Size and hardness of stones: A superior polish requires a variety of sizes, with the emphasis on lots of small chips that help polish the larger stones. All stones in a load should be of the same approximate hardness. Softer stones will not polish and may be gouged by harder stones.
A Handy Hint: Have a notepad next to the tumbler to record the date, time, and condition of the stones during the various stages, steps, and grit changes.
Polishes: There are many different types of polishes for the final stage. The best one for you will be determined by experimentation. Some of them are: Rapid 61, Aluminum Oxide, Cerium Oxide, Tin Oxide, Chrome Oxide, and Tripoli. Suggestion: Rapid 61, Cerium Oxide, or Tin Oxide.
Burnishing: Use a soap rinse between the third cycle and polish, then after the final polish. Bar Ivory Soap is the only pure soap readily available. Chemical additives in other soaps and detergents seem to leave a residue on the stones. The dark color when this soap rinse is poured off shows how much grit was still on what you thought was a clean stone. Shave the bar with a knife or whatever method you wish. The water-soap combination reaches a balance when little bubbles appear on the surface of the moving material and should remain this way throughout the soap cycles. The soap bubbles provide a cushion for the stones. Excess water slows down this process.
If you are concerned about gas buildup in the
tumbler barrel, add a teaspoon of baking soda for each three pounds of
material in each grit cycle.
Spalling is a “ground glass” appearance
on the edges of the stones. If this occurs in step four using a rotating
tumbler, go back to step three and add filler material.
Filler materials may be purchased and may
include the following: Plastic pellets, ground black walnut shells, etc. A
material that floats is easier to remove from the polished stones. Vibrating
tumblers do not generally need filler materials.
Tumblers: If your vibrating tumbler has gray sludge running down the
outside during grinding, whitish sludge during polishing, or soapy stuff
during burnishing, you most likely are using too much water and grit, polish,
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VIRGIL G. RICHARDS