Once you have gained some experience and knowledge with rock tumbling, it can be fun to venture out into alternative shaped stones to polish. Have you ever seen a clockface made out of polished rock? Wonder how that happens? You can actually tumble polish small slabs in a rotary rock tumbler. It takes some work, and the large slabs you see on clocks are usually done in a very large tumbler, but you can get some experience with smaller slabs in a regular tumbler and have fun while doing it.
For most normal hobbyist rock tumblers, the biggest slab you will be able to polish is around 2 to 3 inches. As with all rock tumbling projects, the slabs will need to be close in hardness in order to keep harder slabs from tearing up softer slabs.
One of the secrets to a successful tumbling of slabs, is to load the tumbler in a certain way, that way being layers. Another key is to use regular small stones to offset the slabs. So you will load a layer of slabs, then a layer of small stones. Repeating this pattern keeps the slabs separated and ensures that you will get a good abrasive polish on all surfaces. You want around 80 percent small stones and only 15 to 20 percent slabs.
This layering technique will need to be repeated after the rough grind, and before all subsequent grinds, ie. intermediate or medium grind, and fine grind and polishing stages as well.
Once each grinding phase is complete, be sure to inspect everything as with normal stone polishing. You want to make sure there are no odd chips or sharp flaking edges on either the slabs or the stones that will contaminate the other grind phases.
The polishing phase is critical when tumbling small slabs. You might want to even use a softer stone for the regular stones in order to make sure the slabs do not get scratched by the regularly shaped rocks. You can always go back and polish the stones at a different time. The focus should be on the slabs and getting them polished correctly.
One special note to remember is for polishing small slabs that have been sliced from geodes. The crystal interior must be surrounded by a complete circle of exterior rock in order for the slab to make it thought the whole process. Trying to tumble a partial circle will only result in cracking and ultimate failure, possibly of the entire batch when the geode disintegrates and contaminates the whole lot.
Another thing to consider with crystal of any kind, is that abrasive grit can easily be lodged in the exposed crystal and contaminate a successive grinding stage. Any rougher grit that is lodged in the crystal will be exposed in the next tumbling cycle and possibly foul the whole operation. Be especially careful in the inspection phases between each grind level when working with exposed crystal. A few minutes of extra time cleaning can save days and days of sabotaged work if you fail to remove the abrasives.
Larger slabs require a larger tumbler. It is possible to tumble one large slab at a time in order to maximize your slab size for your tumbler. However, there is a breaking point where you will just need a larger rock tumbler. Things like tabletops and clocks require huge tumblers that are outside the scope of a normal hobbyist's enterprise.
Obviously, tumbling slabs is not for the beginner. Or maybe it is…
Remember when you first started with rock tumbling? It was all brand new and everything felt like it could go wrong at any moment. It was also exciting and the payoff when a batch turned out right was like no other. Moving on to odd shapes like slabs can be just as exciting. Don't be scared of trying and failing to do something as intricate as slab work. It is all a part of the journey and failing a batch or two (or three) is just part of the process. Those failures will make the successes all the more sweeter when you inevitably have them.