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Tumbling

From The Rock Licker 9/00 via Golden Spike News 10/00 by Bob Hicks (Rearranged and shortened by the editor with the author’s permission)

General Informaitohn

Two methods of tumbling being discussed will be with the rotating tumbler, and the vibrating tumbler. For both methods the grinding medium is silicon carbide.

The grits: Rough grit (anything under 100) is used for the first stage. Medium grit (120 to 300) is used in the 2nd stage. Fine grit (400-600( is used for the third stage.

Estimating cost: As a “rule of thumb”. Silicon carbide, 220-400 grit weighs approximately 0.8 ounces per tablespoon. Fine grits 400-600 and polishing powders weigh approximately 0.5 ounces per tablespoon.

Weigh your stones when you get your tumbler 2/3 to 3/4 full. In other words. a 6 pound tumbler does not always hold six pounds of rocks. As a rule of thumb, use 1 tablespoon of grit per pound of stones, or 1 tablespoon per two pounds of the manufacturer’s weight rating of the tumbler. Most tumbling books suggest more grit and polish than is really necessary.

Volume shrinkage: roughly 25 percent 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.

Sizes 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 note pad next to the tumbler to record the date, time, and condition of the stones during the various steps and grit changes.

Polishes: There are many different polishes for the final stage. The best one for you will be determined by experimenting. Some of them are: Rapid 61, Aluminum Oxide, Cerium Oxide, Tin Oxide, Chrome Oxide, and Tripoli. Suggestion: rapid 61, Cerium, or Tin Oxide.

Burnishing: Use a soap rinse . Bar Ivory soap is the only pure soap we know of. Chemical additives in other soap 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 were clean stones. 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 the process.

Cautions:

1. Do not let the slurry mud dry on the material in any cycle. [If you goof] and you have hardened mud. fill the tumbler with water and let it run until the mud has washed off or is soft enough to wash off with a hose or faucet.

2. Do NOT pour slurry down the drain: unless you can afford lots of plumbing bills.

3. Some materials like jade or obsidian require different procedures.

If you are concerned about gas, include a teaspoon of baking soda for each 3 pounds of material in each grit cycle.

Spalling is a “ground glass” appearance on the edges of stones. If this happens, use a filler material in step 3.

A filler material can be purchased, plastic, walnut shells, old inner tube pieces, felt matting, cut up soft plastic from milk bottle tops, etc. They are easier to remove if they float. Vibrating tumblers generally do not need a filler.

If your vibrating tumbler has gray sludge running down the outsides of the barrel during the grinding, or whitish sludge during the polish, or soapy stuff running down during the burnishing - you are using too much water, and grit or polish, or soap.