A true ball mill is a porcelain jar partly filled with spherical or rounded cylindrical porcelain balls. Liquid glaze is poured in, a lid secured, and it is rotated on a motorized rack, sometimes for many hours. The tumbling of the balls within grinds particles smaller and smaller. The creamier glaze consistency that it produces applies better, has more stable viscosity, fires more consistently and cleaner with less specks and imperfections (eg. pinholes and blisters), and melts better.

Potters and hobbyists are generally not aware of the importance of the ball mill to industrial ceramic ware production. For a small-scale stoneware operation it is possible to survive without one if you use a narrow range of glazes. But when production is ramped up consistency, reliability of the glaze appearance and defect free ware become increasingly important. Many materials in ceramics are simply not ground fine enough for glazes (they produce speckle or defects related to expulsion of gases around larger particles); ball, native and slip clays are an example. In other materials fine particles agglomerate into larger ones (e.g. barium carbonate, tin oxide). Others are supplied as a grain-type material rather than a powder and obviously have to be milled (eg. lithium carbonate, alumina hydrate). Some materials melt significantly better or sooner if they are ground finer (e.g. silica, calcined alumina, feldspar).

Ball mill jars need to be heavy and thick to withstand use. They are expensive, a 1 gallon jar typically costs about $100 US. These jars are not easy to make. They must have a water tight lid. A jar must be a true cylinder or it won't rotate without jumping on the rack. Jars are not glazed on the inside. Lighter duty jars are best made by casting a low shrinkage porcelain, heavy duty ones must be made using some type of turning process that produces an accurately vertical and round sidewall. It would be extremely difficult or maybe impossible to dry and fire a suitable thick jar made by throwing a cylinder shape from a plastic porcelain. Plastic jars are an option for studio use. The grinding action between the jar walls and balls is lost but the balls will still crush the particles between them if you mill for much longer periods. Plastic mill jars also run much quieter. Typically small jars have a range of ball diameters from 1-4 cm. The pebbles cost about $6 US per pound and you need about 8-10 lbs for a 1 gallon jar.

A small mill rack is $700-1300 US. However you can build your own. You can find information on this at http://www.digitalfire.com/gerstleyborate/ballmill/
Or you can email Steve Harrison about his booklet "Thoroughly Modern Milling". It is intended to assist the potter in building a ball mill with a roller mechanism to handle a jar in the 3 to 5 gallon range. The text describes how to assemble the parts illustrated in the detail drawings and briefly describes making your own jar and ball from porcelain clay body. A4 size, 6 pages of text and 6 x A3 pages of detail mechanical drawings. There is one colour photograph. Au$25 plus postage.

If you are using a ball mill in your operation resist the temptation to think that using one is just a matter of throwing in some pebbles, pouring in the glaze, and turning it on for an hour or so. As a general rule you should mill for the same amount of time, fill the jar to the same level, use the same charge of pebbles and the range of sizes of the pebbles should be controlled (the pebbles wear down over time).

There are many finer points to know about using ball mills and industry uses the term "mill practice" to embody them. Variation caused by poor mill practice can create a number of significant changes in glaze firing behavior and application performance for many applications and formulations. To learn more check the book 'Ceramics Glaze Technology'. You should be able to find a copy at one of the used ceramic book vendors.