Theory of Tumbling

If you own a Tumbler here are some instructions on how to use it:

Our Readers On How to Tumble Gems

From Gems and Minerals, January, 1955
When the tumbling project was first undertaken, it was hoped that a definite formula for successful tumbling could be devised. There are so many variables, however, that it soon became apparent that only general directions and principles could be compiled. A review of the Process and the comments of some of our contributors will easily illustrate this fact. We could not possibly print, in their entirety, all the material we received. Below you will find selected and edited excerpts from some of the accounts of hobbyists who are using the tumbling process successfully and whose remarks should prove a guide to those who wish to attempt the process or to an prove their methods.

     The first consideration is a place to operate.   People in warmer areas set up outdoors. The noise...the mess from numerous checkings and washings and the production of...gas...must be considered if moved indoors. Sheet steel is the best barrel material. The irregular, hexagonal shape, with the two longer sides being one-third longer gives a change in the angle of fall and raises the load to the greatest height with minimum wall slip. Inside lining or coating...quite often carries the material beyond the point of slide and makes cleaning out of grit more difficult. A well gasketed, solidly fastened door with no internal edges to hinder emptying and cleaning, on the long side, is desirable.
     The barrel should be mounted on the frame so as to permit the placing of containers directly below when the door is in the bottom position for emptying and washing. There is a time saving and economical advantage to multiple compartment barrels.
     A strainer, large enough to hold the entire batch, placed over the container (into which the unloading is done) is a great labor saving device and by choosing mesh size it also grades the material.
     The batch to be tumbled should consist of materials of equal hardness, broken or rough shaped to desirable sizes, with excess matrix removed. Mixing smaller pieces, down to about half the largest size, speeds abrasion.
     The best average results have been obtained by limiting (the size of the batch) to two-thirds of the barrel capacity for grinding and two-fifths for polishing.
     Initial abrasive charges of 40 to 100 grit, depending on roughness of the material, may vary from one-tenth to one-fourth of the stone volume, by weight.   For reasons of economy, use the minimum amount of abrasive found to be effective. Adding detergent (Tide) as a wetting agent, soda to neutralize the acid, and clay flour or adobe soil to hold the abrasive in suspension have proven worth while. Add sufficient water to make a cream-thick solution this will not quite cover the rocks. The polishing mixture should be somewhat thinner.
     Time will be determined by the hardness of the material and how well you have adjusted the "variables." The minimum...60 hours plus for softer materials and 300 plus for harder ones. It may take several hundred more if your variables are scrambled.
     A continuously variable speed adjustment is very desirable. The best method to judge speed is by ear...vary speed until no clicking of falling stones is heard, only a steady, swishing, grind...speeds up to 40 r.p.m. for grinding the thinner the mixture, the faster the tumbler may be operated. For polishing, speeds down to one-third of the grinding speed seem best.
     The addition of a couple of pounds of 220 grit abrasive for the last day or so is worth the additional cost. Do as much grinding as possible with the coarser grits because they cost less. When materials are sufficiently rough ground, as indicated by inspection, remove, thoroughly wash and sort for pieces needing more rough grinding. Flat pieces will take longer than rounded ones.
     Remember, the barrel must also be washed absolutely clean of grit.
     The second grinding (with 320 or 400 grit) will take the same length of time as the first (up to 60 or so hours).
     The value of adding a carrying agent such as steel balls, lead shot, leather or rubber scrap, sawdust, ground cork, etc., is very controversial. Their job is to carry the grits and polish to hollow spots and grooves. They usually slow down the overall abrading time and cause excessive sludge...and give a cloudy rather than bright finish. If used, a moderately hard carrier, in small pieces...would apparently do the best job if renewed for each operation.
     When the second grinding is satisfactory...clean the material and barrel, as before, and repeat with 600 or 1200 grit. If you think this unnecessary, just recharge with your pet polishing compound, detergent, carrying agent, thickener, and soda. Levigated alumina is a favorite polish but combinations of tin, cerium and chrome oxides work better under some conditions. It is believed Linde A would do a quicker and better job for those who can afford it.
     The polishing process usually takes two times the amount of time required for one grinding. Some batches that are stubborn will respond nicely to a couple of pounds of Tide and a cup of soda, after having been washed free of polishing agent.
     I have received information from down El Paso way that they are experimenting with tumblers for five to ten pounds of material. They make their barrels longer than wide (dia.) with off-center shafts, thus adding a sliding, rolling, flattened spiral action which theoretically shortens tumbling time (See Fig. 5).
     Another suggestion is that tiny particles left by abrasion during polishing will continue to leave a satiny finish, not matter how long the process is continued. The cure is to wash the stones and recharge with Tide only and repeat several times if necessary.
     Also, why polish for weeks trying to get a finish on a slab when a few seconds on the buffing wheel (after tumbling) will do a quicker job.

Charles N. Schwab
2745 Fort St.
Omaha, Nebr.

A Specific "Recipe"

Here is a specific set of instructions from C. L. King, King Lapidary, 667 Indian Trail, Palm Springs, Calif. Mr. King uses a hexagonal barrel, mounted through the center that holds 12 quarts of water. Speeds range from 25 r.p.m. for rough grind to 10 r.p.m. for polishing. Here are the details.

1st Step-Coarse Grind
     25 lbs. rocks
     6 lbs. 60 or 80 grit abrasive
     1 quart water
     No carrying agent
     Speed:    25 r.p.m.
     Time:     7 days or more if necessary to get shapes you want.

2nd Step-Fine Grind
     20 lbs. rock (same rock, 5 pound loss in 1st grind)
     4 lbs. 600 grit abrasive
     Water to fill
     No carrying agent
     Speed:    25 r.p.m.
     Time:     3 days

3rd Step-1st Polish
     20 lbs. rock
     1½ quarts granulated cork
     2 lbs. levigated alumina
     Water to below top layer of rock
     Speed:    15 r.p.m.
     Time:     3 days

4th Step-2nd Polish
     20 lbs. rock
     1½ quarts granulated cork
     1 lb. Tide
     Water:    2 quarts
     Speed:    8 to 12 r.p.m.
     Time:     2 days

5th Step-3rd Polish
Repeat No. 4 with fresh Tide for 3 or 4 days.

Explanation:
     With coarse grit, I get best results by just keeping the grit fluid. A little water may have to be added occasionally. My experience...carrying agent retards cutting down rough edges and shaping which is the purpose of the course grind.
     Wash rocks and barrel very carefully (between steps). Be sure no soft socks carry over into polish as some of them act as pieces of grindstone and make polishing impossib1e. (They become charged with grit.)
     The 600 grit is so fine it is always in suspension.   Therefore, the barrel may be filled almost full of water. Rocks are beginning to get slick now. Water helps retard the banging of rocks against the side of the barrel. Carrying agent is optional but I do not use it.
     I use granulated cork with excellent results. However, take care not to use too much water as the cork will then float and do no good. Put cork in first, then rocks, then polishing agent. Rocks are getting pretty slick so it is necessary to slow to 15 r.p.m.
     At the end of three days (in 1st polish) the rocks should begin to show a good polish. Then put them in 'Tide for a couple of days. This washes the rocks and removes all grit that might be carried over. Dump that Tide mixture and add new Tide and run this for three or four days. At the end of the first run with Tide you should have a good polish, at the end of the second run the polish should be mirror like.
     I used this on a run including quartz crystals, aquamarine, citrine, aventurine, smoky and rose quartz, apatite crystals and milky quartz. I got an excellent polish on everything but the apatite which, I guess, should not have been in at all.

Selected Words of Wisdom

     0. L. Jackson, Box 515, McCamey, Texas, uses both hexagonal and round barrels which he built himself. He finds the hexagonal more efficient. The barrel is 16" long by 18" diameter and runs at 40 r.p.m. for the first and second grinding and 20 r.p.m. for polishing.   Mr. Jackson says- 50 pounds of rock, 10 pounds of grit and water to cover plus two inches. I use 80 grit first, 600 grit second, and levigated alumina for polishing. On additives: I use clean silica sand (blasting sand) in equal amounts with the grit in the 1st and 2nd operations. Tide or other detergent with the 2nd. Have not been able to prove to myself that sawdust or cornmeal is any benefit with my polishing compound. Suggests baking soda if out of detergent. Never uses mechanical additives such as balls, etc.
     On Time. It takes four days for inspection to locate and orient plume, etc. Then, depending on roughness and hardness, it will take maybe 50 days. On Water. I ruined a batch with too little water. The grinding reduced the water to a paste and rocks were carried to the top of the tumbler and dropped, breaking corners and edges of many of them. Level the load and then add water at least two inches deeper than the previously added dry ingredients.
     Most of the time soft materials should be run slower than agate. Definitely, materials of different hardnesses should not be mixed-had a batch of agate "eat up" some opals and other stones of similar hardness.
     The first step is very important. If it is not carried on long enough, the others will require much more time than they should.

A Water Tank Rig

R. A. Hullett, Florissant, Colorado, says he made his own tumbler by cutting out a 16" section of a water tank. He turns the barrel at 33 r.p.m. First grind is 20 pounds of hammer trimmed rock, 6 pounds of steel ball-bearings, 5 pounds of 80 grit. This is run for 96 hours or until rocks are smooth. Second grinding is with 500 grit, same amount, plus hard leather, for about 48 hours. Polishing is done with Tide and split cow-hide leather. Water enough to cover the rocks plus a half inch is used.

Additional Technical Data

     The following information was supplied by Pernel Barnett, 751 W. Chapmen Ave., Orange, California.
     The articles I have read on the mechanics of rock tumbling usually leave out some essential points...particularly, one should know the diameter if the speed is given or, more exactly, the peripheral speed. Peripheral speed is found by multiplying the r.p.m. by the circumference of the drum, the result being in feet per minute (f.p.m.) or inches per minute (i.p.m.)
     I use a back-geared motor of   h.p. and a three speed transmission together with a chain reduction of 2 to 1. This gives final speeds, with a drum of 16" diameter by seven inches width, of 10, 12, 22½, and 32 r.p.m. or peripheral speeds of 41,50, 94, and 135 f.p.m. The motor is ample when the drum is fully charged. A reverse electric switch is also used.
     The drum is lined with ¼" thick rubber. This means the stones tumble no more than 15½ inches. The lining keeps the stones from being nicked by the drum, cuts down noise, and makes the drum last longer...much gas is generated with a resulting pressure of perhaps 50 pounds per square inch (p.s.i.). Serious injury can result if too much pressure is built up. Therefore, an alkaline, non-foaming detergent should be used to neutralize the acid, such as "All" or "Oakite". Use about an ounce or two, at most, for 30 to 40 pounds of agate. It is safer to use an indicator (of acidity) so the proper amount is used. A ph of 8 is sufficient.
     I first used 30, then 220 grit, then a polish, but found the jump too much. It is better to use a course grit (30 to 100), then 220, then 440 and then polish. Ten pounds of grit are used for 30 to 40 pounds of agate. This seems to be the maximum and minimum load for this size drum.
     Grit used with clay can be reclaimed. Dilute the mixture with several volumes of water, mix thoroughly and allow to settle a minute or two. The grit, being heavier than the clay, will settle out first. Then pour off the top water. Repeat until all clay is removed. Discard the clay as it is contaminated with rock dust.
     From 16 to 80 ounces of a swelling, bentonite clay were used. Y-O-Jell does good work. A lesser amount of clay was used as the grit size decreased. Enough water is added so that when the clay quits swelling every rock is covered. The clay keeps each rock covered with a uniform coating of cutting material. As grits cut the rock, the mixture becomes thicker...and more water must be added. If the clay is too thick, the smaller rocks will float and there will not be sufficient movement for them to be cut. It is best to soak the Y-O-Jell overnight before using.
     H. L. Zollars, in The Voice suggests that grit in the amount of 25 percent of the weight of the rocks be used but only 15 percent added at first, the balance being put in at one of the later impections.

Plus the above suggestions, nearly everyone suggested one basic ingredient that has not been mentioned specifically. They all agree that nearly every process or ingredient is subject to modification except one. So be sure to add a little- PATIENCE.

Author Unknown. From "Gems & Minerals," January 1955, pp 28, 30, 32, 57-61.

Last Updated 12/03/2014

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