Tumble polishing, or tumbling, is a technique for smoothing and polishing a hard substance. Within the fields of metal work, this is known under the term of barrelling, or barrel polishing, and is subtly different, but operated under the same principles.
For tumbling of rocks, as a lapidary techique, a plastic or rubber barrel is loaded with consignment of rocks, all of similar, or the same hardness, and some abrasive grit and a lubricant. Silicon carbide grit is commonly used here, and water is the universal lubricant. The barrel is then placed on slowly rotating rails so that it turns. This causes the load of rock to slide past each other, and with the abrasive grit between the rock. 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. Initaly smoothing is done with a coarse grit (such as 80 mesh), followed by washing and use of finer grits (220 then 400 mesh) before the use of a polishing compound (such as cerium oxide). The precise time is determined by many factors, including the hardness of the rock, and the degree of smoothing desired in the coarser steps.
For barreling metals, a barrel with internal veins, typically 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 lubracant, and to prevent the rusting of the metals, whilst the material is tumbled as for rocks. Only ony 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, only water as a lubricant to remove the sharp edges so that the glass may be handled safely. As little as 8 hours tumbling may be sufficent for tumbled glass.
These techiniques, 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/hobbiest use. Additionally, very large barrels can be used, allowing a lot of work to be done at once for professional use. The main disadvantage is it's limited scope - stones will be smooth, and have semi-random shapes (like pebbles from the beach), and metals need to be relativly 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 could be produced. It is still limited to rounded shapes. Preforms may use less time with the coarsest step, or skip it alltogether.