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Fundamentals of Lapidary Part 3, Tumbling
By Donald Clark CSM
Introduction
... Tumbling is a simple process where the rough material is put in a revolving barrel with abrasives. Progressively finer abrasives are used until a polish is obtained. This process closely resembles what happens to rocks in a stream or on the beach, except that the level of polish is much higher.
... This is the simplest and least expensive of the lapidary arts. It requires a minimum of equipment and the minimal skills of being able to read the instructions and measure the abrasives. It is an ideal activity for children, or for families to do together. It is especially enjoyable if you are working with material you have found yourself. In addition, you get some beautiful treasures for your efforts.
Preliminary Considerations
... Before you buying a tumbling machine, there are some important considerations to bear in mind. In order for the tumbling action to work, the stones must be a variety of sizes. They need to range from, not more than half the diameter of your barrel, to about 1/10th the size of the largest stone. You cannot simply put a few large pieces in the tumbling barrel and expect them to come out nice.
... Now, simply gathering an assortment of sizes is not too challenging. If all the sizes you need are not readily available, simply use a hammer to create them. However, if you intend to work with stones the size of melons, you had better be prepared to gather a lot of filler!
... If your intention is to just tumble a couple dozen agates you found on vacation, you have different considerations. A machine large enough to do them all at once might be more than you need. A smaller tumbler will require you to process your stones in two or more batches. However, if the machine is going sit idle most of the year, it would be a waste to purchase a larger unit.
... All the stones in your barrel need to be the same hardness. The reason is that harder stones will scratch softer ones. If you had even one stone that is harder than the rest, it will put scratches on everything else in the barrel. Of course, stones that are used just for filler can be softer that the gems you want to polish.
... Measuring the hardness of your rocks may seem daunting, but it can be accomplished with simple tools. A pocketknife is about 6 to 6.5 in hardness; a file is closer to seven. You can also assemble a set of stones of known value for your testing.
... Materials of the same hardness will not scratch each other. Therefore, if you want to tumble some agates, use a piece of agate to scratch the other stones you intend to tumble with them. It should not scratch any of them, nor should it or show any wear. If suddenly it wears away on a stone, that piece is much harder and should not be included in your tumbler.
... Not all stones will take a polish, just the harder ones. A simple field test will save you from bringing home a lot of useless rock. Try scratching them with a pocketknife. If you cannot scratch the specimen, it will take a polish. You will find others are nothing more than compressed mud. Not only will they not take a polish, they create quite a mess!
... Location, location, location. While you are familiar with that saying for locating a business, you need to consider it for your tumbler. These machines are noisy and run 24 hours for days on end. Loretoneä makes tumblers with a rubber barrel to minimize the noise. While this helps, they are still noisy. You need more than one wall between your bed and the tumbler if you want to get any sleep. If you are using a steel barrel, you will need more than that!
... Do not forget about your neighbors either. Simply putting it in the garage may help you sleep, but it may not ingratiate you with your neighbors.
... If you do not have a secluded room or basement, consider making a container for it. A cardboard box lined with Styrofoam, or covered with an old blanket, will reduce the sound considerably.
Machines
... There are two kinds of tumblers, rotary and vibratory. Rotary tumblers are the ones most people are familiar with. They can be found at any rock shop, scientific supply houses, and many stores that sell toys. They are the least expensive and come in the widest range of sizes.
... Vibratory tumblers shake the ingredients, rather than rolling them over each other. They have the single advantage of being faster. What would take weeks in a rotary tumbler can be done in a couple of days in a vibratory. They also have a significant disadvantage. The barrels do not stand up to coarse tumbling and wear out quickly. Hence, these are usually reserved for finishing and are not recommended for general-purpose use.
... When choosing a tumbler, the primary consideration is what size stones you want to tumble. About half the diameter of the barrel is as large as you can finish. Therefore, your least expensive models with a barrel four inches in diameter can only finish two-inch diameter stones. Moving up to a machine twice that size will cost twice as much, require twice the abrasives, and finding twice the amount of filler. For the serious lapidary this is reasonable, but for the part time hobbyist it may not be justifiable.
... When considering your budget, include an extra barrel for your machine. Reserve it for polishing only. This is especially important if you are using a rubber barrel. They are very difficult to get completely clean. If you have any abrasive, (even one or two little pieces,) left in the barrel when you try to polish, it will scratch your stones. They will look awful and you will have to go back, clean the barrel more thoroughly, and polish them again.
... Successful results have more to do with your choice of abrasives than the machine. Most companies that sell machines also offer abrasives in a kit. They may even be included with the machine. Polishing kits are recommended for the beginner. As you gain experience, you can try different polishes, different combinations and save money by purchasing in quantity.
Tumbling Procedures
... It is best to follow the instructions that come with your machine. They are much more specific than this article can be. However, here is a general outline to show the beginner how the process works.
... Fill the barrel 2/3 to 3/4 full with stones. Make sure that they are all the same hardness and in an even variety of sizes. For example, if your largest stones are three inches in diameter, there should be just as many two inch stones, one inch, one half and one quarter inch stones. These proportions are by volume, not by count.
... Next, add water to the top of the stones. Measure the proper amount of abrasive and put it in. Put the top on the barrel and start it in motion.
... This stage will take about a week. However, you should open the barrel and inspect your stones every day. Some stones release gases that build up inside the barrel. It is possible for the gas to blow the top off the barrel, creating a terrible mess! Simply opening it every day is enough to eliminate this problem.
... You also need to keep track of your progress. The only way to do this is to take a few stones out, rinse them off and have a look at them. Seeing the daily changes adds to the fun of the project!
... Do not rinse your stones in a sink; take them outside. The waste from your tumbler can plug up a drain. In fact, it is nicknamed the "Plumbers Best Friend."
... When your stones are all nicely rounded and have no angular surfaces left, it is time for the next stage. Take them outside, place them in a colander, and rinse them thoroughly. Wash the barrel out completely using soap, not just water. You need to get all the coarse grit out before moving on to the next step.
... Place the stones back in the barrel and add water to the top of the stones. Do not fill to the same level as before. Your stones are now smaller and will not fill the barrel as high. For the abrasives to work properly, your water level needs to be to the top of the stones in every step. If you did not have quite enough stones in the beginning, you are now short. You may still be able to finish, but it will take longer as there isn't as much friction as when it is properly filled.
... Add the proper abrasive and set the machine to work.
... This second step is usually done with fine grit silicon carbide. It also takes about a week and you should inspect your progress daily. It is complete when all the coarse scratches are removed and the stones are smooth as a baby's bottom.
... Now variations enter and you need to read the instructions that came with your machine, or the abrasive kit. Some processes will go directly to the polish stage; others will have a prepolish first.
... Regardless of which step you are on, the procedures remain the same. Clean the stones and barrel outside. Replace them, add water and abrasives, and then set the machine to work. Check the progress daily.
... If you are using a prepolish, your work will be complete when they have a smooth and satiny surface when dry and look polished when wet. In the polishing stage, you are finished when sparkle and shine from every direction. If ever you aren't sure, give the stones an extra day or two. More time will not hurt them, but too little will result in a substandard polish.
... The whole process will take approximately a month to complete, although the actual work is only a couple of hours. If you go away for a few days, you can simply turn the tumbler off and restart it when you come back.
... As with any lapidary project, it is that final day when it all becomes worthwhile. You have spent considerable efforts in all the preliminary stages. While it is fun to see the progress, your great reward is when you dry your stones and they glisten with vibrant, beautiful colors. They are now a delight to the eye, and a welcome gift for friends and family.
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