Tumble polishing, or tumbling, is a technique for smoothing and polishing a hard substance. Within the field of metalwork, this is known as "barrelling", or "barrel polishing", and is subtly different, but uses under the same principles.
For tumbling of rocks as a lapidary technique, a or rubber barrel is loaded with consignment of rocks, all of similar or the same hardness, some abrasive grit, and a lubricant. Silicon carbide grit is commonly used, and water is a universal lubricant. The barrel is then placed upon slowly rotating rails so that it rotates - if round - at about (where Din is the inside barrel diameter in inches) RPM. A hexagonal barrel will rotate at about RPM [1].
This causes the rocks within the barrel to slide past each other, with the abrasive grit between them. The result of this depends on the coarseness of the abrasive, and the duration of the tumble.
Typically, a full tumble polish from rough rock to polish takes around 4-5 weeks, and is done in 4 steps. Initially, the rocks are smoothed with a coarse grit (such as 80 mesh), followed by washing and finer grits (220 then 400 mesh), before the use of a polishing compound (such as cerium oxide or ferric oxide). The precise tumbling time is determined by many factors, including the hardness of the rock and the degree of smoothing desired in the coarser steps.
During the 1970s, small rock tumblers were a common hobby item, and jewelry decorated with tumbled semi-precious stones was very much in fashion. Likewise, dishes and decorative glass jars filled with tumbled stones (often including common rocks not suitable even for costume jewelry) were frequently used as household ornaments.
For barreling metals, a barrel with internal veins, typically made from rubber, is used. The work parts are placed in the barrel along with polished steel shot and a solution of a barreling soap. The soap acts as a lubricant, and prevents rusting of the metals, whilst the material is tumbled as for rocks. Only one stage is used, although the length of time determines the final polishing.
Stained glass shards used for mosaic glass are also tumbled. No abrasive is used, to avoid clouding the glass; there is only water as a lubricant. The object of this tumbling is to remove the sharp edges from the glass, so that it may be handled safely. As little as 8 hours of tumbling may be sufficient for tumbled glass.
These techniques, although they take a long time, involve very little operator intervention and thus are very cheap. Small tumblers (one pound capacity) are available and inexpensive for home/hobbyist use. At the other end of the scale, professionals can use very large barrels to do a lot of work at once. The main disadvantage of tumbling is its limited scope - stones will be smooth and have semi-random shapes (like pebbles from the beach), and metals need to be relatively simple shapes, with no fine work.
Sometimes, stone "preforms" are used. This refers to cutting shapes from the rough rock, before tumbling. This gives more control over the final piece, so shapes such as a tear drop can be produced. The technique is still limited to rounded shapes. Preforms may use less time with the coarsest step, or skip it altogether.
Tumbling is used to polish and smooth dice for recreational use, but it has the unfortunate effect of making their sides and faces somewhat uneven and thus making the dice less than fair. Lou Zocchi's Gamescience company is known for making dice without tumbling them; the edges of these dice are very sharp, but the dice are guaranteed to be fair.
References
- ^ Raymond Rodebaugh/Alan Silverstein. Big Tumbler, December 17, 1999, p. 10
Categories: Decorative art | Gemstones | Arts and crafts | Hobbies | Industrial processes
