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GEOLOGY ARTICLE

Tumbling procedures
by David McIlwain

If you haven't tumbled stones before, these procedures may be of interest to you:

Preparation
1. Wash the stones thoroughly. Be sure there is no debris attached to the stones. Use a brush and soapy water if necessary.

2. Sort your stones by size and hardness into groups or batches. Soft stones will grind away before hard stones are ready for the next step. Stones of nearly the same size will have more points of contact and therefore will produce a more thorough and faster grinding action. If certain shapes or sizes are desired, you may want to preform your stones by grinding them on a lap first.

Coarse grind
3. The amount of stones put in a tumbler barrel depends on the size of the barrel and the stones themselves. The best tumbling action occurs when the barrel is filled to 50% to 60% of its capacity. Fill the barrel with your stones to 1/2" above the halfway mark. Remove the stones and weight them. This weight will help you to determine how much grit is needed. Record this weight for future reference. Use the following ratio to determine the amount of silicon carbide grit needed for your batch:

One pound of grit per eight to ten pounds of rock.

Put your batch of rocks back into the barrel and add grit accordingly. If the rocks are chips or have rough crude surfaces, start with a coarse grit (60/90 grit). If the rocks are water worn from tumbling in streambeds or already tumbled by ocean waves, start with a medium grit.

4. If baking soda is available, add about a tablespoonful to the mix. The soda will help neutralize the gases that might be formed. Add water into the barrel until it covers the rock by about 1/16 of an inch. Put the cover on and secure it. Place the barrel into position on the tumbler.

5. Put the tumbler into operation and observe its action for a few minutes. Check for loose or slipping belts or pulleys. Listen to the sounds coming from within the barrel. Is there sufficient amount of action taking place? If there isn't, shut down the operation, open the barrel and inspect the consistency of the mixture. If the mixture is too dry, add a little water. Put the cover back on and continue as before. If everything appears to be functioning properly, you can leave and come back periodically to check the operation.

6. It will take an average of four to six days of coarse grinding. Sharp-edged or broken pieces of agate may require as much as 360 hours of coarse grind operation. After each 24 hours of operation, shut down the operation. Remove some stones from the barrel and examine them. You can add or decrease the grinding time according to how much more rough grinding you want performed. If you want to continue with the coarse grit, inspect the grit and the consistency of the mixture. If the grit no longer has sharp edges, you may want to add more coarse grit. If the mixture is too dry, you may want to add a little more water. Place the stones back into the container, seal the barrel, place it into position, and start up the operation. Observe and listen to see if the operation is functioning properly. You can leave and check back periodically. When the coarse grinding phase is finished to your satisfaction, you can proceed to the next step. If a few stones need more coarse grinding, you can remove them from this batch and re-tumble them later in another batch.

7. Remove all of the material and stones from the barrel and place in a pan. DO NOT use an aluminum pan. It may discolor your stones. Plastic is preferred. DO NOT wash the waste material down your drains. It may harden in the traps or pipes and require major plumbing repairs to clear the pipes. Clean the stones, the barrel, the lid, the pan and any other part that has made contact with the grit mixture. Wash everything and your hands thoroughly. You do not want any previous grit particle left to contaminate the next mixture. It may produce scratches.

Medium grind
8. Place the stones back into the barrel. If they do not fill the barrel to the halfway mark, you may need some filler material. The filler material is available from lapidary supply stores. Inexpensive marbles or crushed walnut shells make good substitutes. Add the filler material until the halfway mark is reached. Add about one tablespoonful of baking soda. Add 220 or "240 & finer" grit to the mixture in the same quantity as the coarse grit. Add water into the barrel until it covers the rock by about 1/16 of an inch. Put the cover on and secure it. Place the barrel into position on the tumbler.

9. Put the tumbler into operation and observe its action for a few minutes. Check for loose or slipping belts or pulleys. Listen to the sounds coming from within the barrel. Is there sufficient amount of action taking place? If there isn't, shut down the operation, open the barrel and inspect the consistency of the mixture. If the mixture is too dry, add a little water. Put the cover back on and continue as before. If everything appears to be functioning properly, you can leave and come back periodically to check the operation.

10. It will take an average of about four days, or 96 to 100 hours. After each 24 hours of operation, shut down the operation. Remove some stones from the barrel and examine them. You can add or decrease the grinding time according to how much more grinding you want performed. If you want to continue, inspect the grit and the consistency of the mixture. If the grit no longer has sharp edges, you may want to add more grit. If the mixture is too dry, you may want to add a little more water. Place the stones back into the container, seal the barrel, place it into position, and start up the operation. Observe and listen to see if the operation is functioning properly. You can leave and check back periodically. When this grinding phase is finished to your satisfaction, you can proceed to the next step. If a few stones need more grinding, you can remove them from this batch and re-tumble them later in another batch.

11. Remove all of the material and stones from the barrel and repeat the cleaning procedures used after the coarse grit operation. Clean the stones, the barrel, the lid, the pan and any other part that has made contact with the grit mixture. Wash everything and your hands thoroughly. You do not want any previous grit particle left to contaminate the next mixture. It may produce scratches.

Fine grind
12. Extreme care should be taken from this point on to prevent the stones from chipping or breaking. Place some water in the barrel first and then place the stones gently into the barrel. If they do not fill the barrel to the halfway mark, you may need some filler material. Don't use contaminated filler material from previous operation. Add the filler material until the halfway mark is achieved. Add about one tablespoonful of baking soda. Add 500, 600, or "600 & finer" grit to the mixture in the same quantity as the coarse grit. Add water into the barrel until it covers the rock by about 1/16 of an inch. Put the cover on and secure it. Place the barrel into position on the tumbler.

13. Put the tumbler into operation and observe its action for a few minutes. Check for loose or slipping belts or pulleys. Listen to the sounds coming from within the barrel. If there isn't sufficient action taking place, shut down the operation, open the barrel and inspect the consistency of the mixture. If the mixture is too dry, add a little water. Put the cover back on and continue as before. If everything appears to be functioning properly, you can leave and come back periodically to check the operation.

14. It will take about 72 to 150 hours for the stones to be ready for the polish phase. After each 24 hours of operation, shut down the operation. Remove some stones from the barrel and examine them. You can add or decrease the grinding time according to how much more grinding you want performed. If you want to continue, inspect the consistency of the mixture. If the mixture is too dry, you may want to add a little more water. DO NOT add more grit. Place the stones back into the container, seal the barrel, place it into position, and start up the operation. Observe and listen to see if the operation is functioning properly. You can leave and check back periodically. When this grinding phase is finished to your satisfaction, you can proceed to the next step.

15. Continue to take extreme care not to damage the stones. Remove all of the material and stones from the barrel and repeat the cleaning procedures used after the coarse grit operation. Clean the stones, the barrel, the lid, the pan, the sink faucets, etc. Wash everything and your hands thoroughly. You do not want any previous grit particle left to contaminate the next mixture. It may produce scratches.

Polish
16. Allow the stones to dry and then examine them very carefully. Remove any stone that is chipped, broken, or has very sharp edges. The damaged stones will scratch the rest of the batch if left to remain. Place the stones gently into the barrel. If they do not fill the barrel to the halfway mark, you may need some filler material. Filler materials such as sawdust, wood shavings, cornmeal, walnut shells, and rubber strips (cut-up rubber bands) are ideal. They will help prevent damage to the stones by absorbing some of the tumbling (banging) action. Add the filler material until the halfway mark is achieved. Add about one teaspoon of detergent soap. Add cerium or tin oxide to the mixture in half the quantity as the coarse grit. Add water into the barrel until it covers the rock by about 1/16 of an inch. Put the cover on and secure it. Place the barrel into position on the tumbler.

17. If it's possible with your unit, reduce the speed of rotation by about twenty percent. Put the tumbler into operation and observe its action for a few minutes. Check for loose or slipping belts or pulleys. Listen to the sounds coming from within the barrel. If there isn't sufficient action taking place, shut down the operation, open the barrel and inspect the consistency of the mixture. If the mixture is too dry, add a little water. Put the cover back on and continue as before. If everything appears to be functioning properly, you can leave and come back periodically to check the operation.
18. It will take about 48 to 150 hours for the stones to be completely polished. After each 24 hours of operation, shut down the operation. Remove some stones from the barrel, wash and examine them. If they become duller as they dry, then they are not completely polished. Carefully put them back into the barrel and polish them for another 24 hours. Inspect the consistency of the mixture. If the mixture is too dry, you may want to add a little more water. Seal the barrel, place it into position, and start up the operation. Observe and listen to see if the operation is functioning properly. You can leave and check back periodically.

Final clean-up
19. When you are satisfied with the polish results, Remove all of the material and the stones from the barrel and repeat the cleaning procedures used after the coarse grit operation. Clean the stones, the barrel, the lid, the pan, the sink faucets, etc. Wash everything and your hands thoroughly. Carefully place the stones back into the barrel. Add enough detergent soap powder and water to make a thick soapy solution. Tumble the stones for 6 to 12 hours. Remove the stones from the barrel carefully and place them in a plastic colander. Wash them thoroughly. Spread them on a cloth or towel to dry. Those that are properly polished will have the same appearance as when they were wet. If some stones are not satisfactory, you can re-run them later with another batch as fillers.

Tid-bits
Always take notes. Record your procedures, any changes made, the time you start and stop an operation, and how long each operation takes for the type of material you are tumbling. Don't be afraid to experiment.

If your procedure is different than ours or you have some tid-bits that will help others, please let us know.

This article was written by MAGS member, David McIlwain, ©2002