Geology And Geophysics

Beginning Rock Tumbling



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Though it requires patience, rock tumbling and polishing can be a fun hobby that the entire family can get into, and that results in stones of astonishing beauty.

The first thing to get is a quality tumbler. There are basically two kinds of tumbler; the agitator, which basically just shakes the stones, and the rotary, which moves them in a steady circular motion. My preference is the rotary. Rotary and other tumblers come in different sizes, and this has to do with the number of batches that can be going at any one time, and also the size of the stones. Buying a tumbler that will hold 1 pound of stones may make really nice polished rocks, but they are going to be very small. For the average beginner, I'd recommend a 2 tank rotary, with each tank holding 3 pounds of rocks. I will address this article to this sort of tumbler. If you purchase another, please keep in mind that you will have to alter the steps below for what you have. I want to mention that the newer rotary tumblers do not make nearly as much noise as a person might think.

When putting the rocks in the tank (or canister), make sure that they are all of about the same hardness. Putting hard rocks in with softer rocks will pulverize the softer rocks, and also ruin the grinding solution. Putting the rocks in the tank is called loading.

Make sure that you follow the instruction manual for the tumbler. If there wasn't one, make sure that the tanks are clean inside and out, put a drop of fine oil, like sewing machine oil, on the ends of each of the rollers, and make sure that the belt is as loose as possible while still being able to turn the rollers under load. This will prolong the life of the tumbler, and result in a better end product.

Use different sized stones. You won't get a very good result if the stones are the same size. With a 3 pound tank, it should be loaded about 1/2 to 3/4 full of stones, and of these, the large size should be about 1" in diameter, with about a quarter or more of the stones being smaller. Keep in mind, though, that the polishing process can remove 20% of the stone, so very small stones may be ground to such a small size that they aren't worth much.

Polishing the stones:

Keep in mind that all of these steps take a lot of time.

First, load the tank. It should be 1/2 to 3/4 full of stones. Too few stones, and the result will be poor. Too many, and it puts too much of a burden on the tumbler and may burn out the motor or break the belt. Make sure that the seal is firmly seated on the tank and put it on the tumbler.

Add water to about 1/4 of the way deep on the top layer of stones. To this, add 2 tablespoons of coarse grit. This is 60 to 90 grit size (relatively large grit).

Run the tumbler for about an hour, making sure that it is rotating properly. At the end of an hour, check for water leaks, turn off the tumbler, remove the tank, and carefully remove the top to release built up gases. Replace the lid, reseating the seal, and turn the tank to the tumbler, turning it on. Check it again in 24 hours, in the same way. At that time, check the water solution. The grit should not be clumped up, nor should it be a slurry, looking like pea soup.

Run the tumbler for a week with the coarse grit, then stop and thoroughly clean the stones with running water. (An old toothbrush is great for this step, and subsequent cleaning steps.) Also clean out the tank well. Important: When rinsing the stones in this, and later steps, do not do so down the sink. The grit can plug drains.

Replace the stones, again add water as before, and add 2 tablespoons of medium grit. (Note that polishing grit is often sold in coarse, medium, fine, and polish grades. Usually you don't have to ask for a specific grit size.) Again run the stones in this solution for a week.

Repeat this process using fine grit, and run for 7-10 days. Then move to pre-polish grit and polish grit. With this last step, it is helpful to include some of the small plastic pellets that are made specifically for this purpose, to prevent the rocks from scratching each other or chipping.

Between each step, make sure that you clean off the rocks and both the inside and outside of each barrel. After the polish step, if you remove any of the stones and find that it is cloudy rather than glossy, run the tank a final time with water and a small amount of mild powdered soap (NOT detergent) like Ivory.

This is very time consuming, but done properly, this will turn rough uninteresting stones into things of beauty, and perhaps some, into semi-precious stones that are totally gorgeous.

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