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Tumbling is the process of agitating a collection of rocks (or other articles) in a barrel along with abrasives, to grind and/or polish them. There are several types of tumblers, the most common and least expensive being the rotary, or rolling barrel, type. Next up the food chain is the vibrating or sonic types; they vibrate their loads rather than roll them. They are considerably more fast and efficient, although more finicky as to the kind of loads they will accept. A load of very angular rocks, for example, may not churn properly in some vibratory machines, and for such work a rotary is preferred. There also exist magnetic tumblers and centrifugal disk finishers, but these are used mainly in the metal-finishing industries and will not be further considered here.


'4 pound' and '12 pound' tumblers.




Tumblers must be operated with a full load. In rotary tumblers, that means 2/3 of the barrel filled with rocks, along with whatever abrasives and other ingredients may be required. You cannot tumble a single rock.


Rock tumbling proceeds in typically 4 stages, beginning with the coarse abrasive and then moving up sequentially to the medium, fine, and polish abrasives. Typically, between each stage, the rocks should be run through a brief cleaning cycle in a mild soap solution (consult machine manual for acceptable types of soaps); subject, however, to the following caution: there is a risk of chipping the rocks during this process because the cleaning solution offers less drag to prevent rapid collisions than does a proper slurry. The problem is particularly significant with glass and obsidian, including Apache tears. For these types of rocks, hand washing is recommended. In the barrel, the addition of a large quantity of plastic pellets to the soap solution may be an answer, but we have not specifically tested this method. There are several different abrasives available for each of the grinding/polishing stages, but they are all used essentially the same way. One difference is that some polish agents do not require a final soap burnishing or cleaning cycle, but check the label or ask the vendor.


In a rotary tumbler, the required tumbling time is typically 1 week per stage, although this varies considerably depending on the rocks, especially in the coarse grind. Vibrating/sonic tumblers are considerably faster.


It is possible to get poor results from a tumbler when using good materials, by improper composition of the load. Probably the biggest factor in good tumbling is the consistency (viscosity) of the slurry. Too thick a slurry will gum up the process and prevent grinding, and may cause separation of the rocks from plastic pellets (if used). Too thin will allow the abrasives to wash between the rocks ineffectually, while also allowing action that is excessively violent, leading to chipping, pitting, or spalling of the surface, which will make a good polish impossible. The optimum slurry will be tacky enough to hold abrasive grains onto the rocks, but yet allow the rocks to slide past one another.


In the rotary tumbler, the desired action is a sliding down the slip zone (upper 1/3 or so of the load. Rocks (speckled with grit in the coarser stages) should slide upon one another, not roll on one another nor pound one another. This condition is achieved by not including many rocks that are too large for the size of the barrel; and by using a mix of different sizes of rocks; and by maintaining the right viscosity of slurry. Note also that the tendency to chip or pit will be more pronounced if the average rock size is considerably larger than the usual average value of about 5/8 of an inch, because of the greater weight of each rock. this problem can be minimized, When using the usual hobby-size tumbler, by including very few oversize rocks. But when dealing with bigger barrels in which the main objective might be to produce considerable quantities of larger-than-usual gemstones, special measures may be needed.


Can all mineralogical classifications of rocks be tumbled together? Largely yes, but glass, Apache tears, other obsidian, and clear crystalline quartz chip easily when impacted and must be protected more than other rocks need to be. The obsidians and glasses are also somewhat softer on the Moh's scale than most other rocks. For this reason, it is advisable to use cushioning (e.g., plastic pellets) in all stages when doing these items, which preferably should not be mixed with other types of material. Also, certain very soft materials such as calcite may abrade quite a bit faster than most other materials and are best done with similar materials, and may require additional cushioning as with the glass etc.


Specialized tumbling: Rocks that would not normally be considered appropriate for tumbling, such as 3-inch x 1/4 inch slabs, and such as 8 inch bulky rocks, have been successfully tumble-polished, but these will typically require special methods.





However, we have crushed rock suitable for tumbling, and abrasives, and books on tumbling and other lapidary arts.


We also offer a limited inventory of parts for Lortone tumblers. If you need a part, please give us a call.