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What Size Rocks Should Be Used in a Tumbler?

You should use a mix of rock sizes - none bigger than 1/2 the barrel diameter.

sizes of rock to tumble
The illustration above shows how small diameter material can increase the efficiency of your tumbling.

In Figure 1A there are only a few large rocks in the tumbler barrel. Notice how there are only a few particle-to-particle contacts where grinding can occur.

Small particles have been added in Figure 1B and they occupy the spaces between the large particles. Now there are a much larger number of particle-to-particle contacts where grinding can occur.

Two Rules for Rock Sizes

There are two basic rules that you should follow about the sizes of rocks that you load into your rock tumbler:

1) Load the barrel with a mixture of rock sizes.

2) Tumble rocks no larger than 1/2 the barrel diameter.

The first rule is one that everyone should follow. The second is for people who want to make "monster" tumbles.

Tumble a Mixture of Rock Sizes

For small tumblers with a three-pound capacity barrel such as the Thumler's A-R1 and Thumler's A-R2, a mix of rock sizes between 1/4" and 1 1/4" is recommended. The action produced in these small-diameter barrels is adequate to shape a rock up to an inch or a little more in size.

For larger tumblers such as the Thumler's A-R6 and Thumler's A-R12, small pieces of rock are still required for efficient grinding and polishing. However, these tumblers can perform well with rocks between 1 1/2" and 2" in diameter.

Small Rock Particles are Needed

Grinding in the barrel of a rock tumbler occurs at points of contact between two pieces of rock. Tiny grit particles caught between these pieces of rock scratch the rocks and remove small bits of material. If there are only a few points of contact between the rocks, then very little grinding will occur. However, increasing the number of contact points will increase the amount of grinding.

Figure 1A shows a barrel with several large rocks. Notice how there are only a few points where these rocks are in contact with each other. If you tumble a load that contains only large rocks, very little grinding will occur.

Figure 1B shows a barrel with a range of rock sizes. Notice how the tiny rocks occupy the spaces between the large rocks. Also notice how many points of contact there are between the rocks. There are many more points of contact in this load, and when it is tumbled a lot more grinding will occur compared to the rocks in Figure 1A.

This is why tumbling a variety of rock sizes is recommended.
small ceramic tumbling media
Small ceramic media: These angle-cut ceramic cylinders are about 3/16" diameter x 3/8". They work great when you need some small-size material to balance the particle size load in your tumbler. They also work well when you have a lot of flat or angular rocks that tend to slide in the barrel instead of tumbling - they get between those rocks and serve as roller bearings!

Using Ceramic Media (Ceramic Pellets)

Ceramic media (also called ceramic pellets) is used in rock tumbling as a filler and as a substitute for small-size material. If the rough that you plan to tumble is all 1" pieces then you will need some small particles to fill the spaces between them. These small particles will produce additional points where grinding will occur. If you don't have small-size rough for this purpose you can add some small ceramic media.

The most often used ceramic media shape is an angle-cut cylinder. These cylinders turn into "roller bearings" in the tumbler barrel and do a great job of moving your rough around and delivering grit to irregularly-shaped rough rocks. They can be washed with your rough between tumbling steps and accompany the rough through every step of the tumbling process.

Even if you don't need small material, a small amount (several percent by weight) of small ceramic cylinders can improve the action in a tumbler barrel. They are especially effective in vibratory tumblers when your rough consists of a lot of flat or blocky shapes. We use them frequently.

See our article on how to use ceramic media.
Large tumbled stones
Producing large tumbled stones with a spectacular appearance requires careful rough selection and a lot of patience. Don't waste your time tumbling a big piece of "junkite." Instead, pick a nice jasper with great color or a big piece of Brazilian without any hint of fractures. Good luck!

Producing "Monster" Tumbles

Happy Tumbling!

Some people like to make very large "monster" tumbled stones. These can be made if you have a lot of patience and are willing to run these large stones through the coarse grit grinding step several times. It can take several weeks for these rocks to become nicely rounded and smooth.

If you have patience you can easily tumble rocks that are 1/2 of the diameter of your tumbler barrel. Just don't put too many of them in the barrel at the same time - perhaps one or two.

We have produced 2" diameter tumbles in the small three-pound tumbler barrels of the Lortone 3A and 33B tumblers and in the three-pound barrels of the Thumler's A-R1 and A-R2 tumblers. We have made 3" tumbles in the Lortone QT6 tumbler barrels. In each of these experiments our rocks spent over one month in coarse grit.

Select your rocks carefully if you plan to make monster tumbles. Avoid any with deep indentations because those indents will take a very long time to wear smooth. Don't waste a month of tumbler time on a piece of junkite!

The instructions supplied with the Lot-O-Tumbler say that it will tumble any rock that you can fit through the opening. We have used the Lot-O-Tumbler to produce a few 2" diameter tumbles that were brought to a round shape in a rotary tumbler.

Happy Tumbling!