Theory and Terminology
Jonathan Kaplan via Clayart:
Yes its true that for he most part, the materials we purchase to make glaze are finely ground. Once they become part of a glaze mixture, a suspension of materials in an aqueous solution, one would expect that these materials and their respective particles are uniformly distributed. For the most part, yes. But it might be worthwhile to think about how these materials melt in the kiln, how they sit on the surface of the ware as they melt, and what influences an optimum glaze surface. Theres no need for lengthy chemistry and physics here. Suffice it to say that it is the grind of these materials, the nature of glass structure relative to the grind that plays a very important part to optimum surface development. For instance, lets say you go to Feldspar mine in Spruce Pine NC and get some crude feldspar before it is selectively processed. And lets further say you hand grind it in your morter and pestle or other some such device. Add some water, make a crude solution and apply it and fire it. Chances are that the surface will be rough, perhaps the glass will be undeveloped. Run the same test with some processed spar, and the results will be dramatically different(same temperature). Voila, illustrating that the grind of the material is directly responsible for the optimum surface development, the melt. Sure if you fire any material to a hot enough temperature it will flow, but thats not the point here. It is the action of milling that effects the particle size and hence the melt, the fluidity of the melt, etc etc.
So ball milling renders a solution more homogeneous by the milling action of the grinding media, the amount of charge in the mill, the speed of rotation in the mill, and the time the solution is in the mill being processed. There is an optimum balance between all these factors relative to the glaze. Too much milling will result in a glaze solution that has absolutely no surface tension whatsoever and will run off the pot.
Do we need to ball mill all our glazes? Probably not, and as most on the list who make glazes have seen that a simple jiffy mixer followed by some selective seiving will produce a glaze that for the most part, will work well. But, we also know that all glazes are not the same and perhaps some may require ball milling to produce the proper effect. Ball milling is also one of the things that may be out of reach for studio potters in that a one gallon ball mill jar is faily heavy and large, the mill needed to turn it is substantial, and if you have a 30 gallon batch, you gonna mill 30 times? Plus that one gallon jar doesnt hold one gallon. And on it goes.
So we also need to get some terminology right.
What we often refer to as ball mills is kind of generic. Refers to a rotating jar with a charge of grinding material and stuff to grind up. According to US Stoneware (one of the largest producers of mills) ball mills of their manufacture range from 12-210 gallon capacity and ar quite substantial pieces of equipment, both in the size (physical footprint) and price for most potters. These are some hefty pieces of equipment.
What is more akin to what we use are called jar mills and can be as simple as one jar, multiple jars on one tier, or on multiple tiers, short rollers or long rollers, etc.etc. These can be belt driven, chain driven, one speed or multiple/variable speeds. More affordable.
Why so expensive? The simple answer is just because. These are industrial manufactured devices and I am sure carry the manufacturers hefty overhead, especially for items like liability insurance and a myriad of others. In concept, yes, these are simple devices and are well within the scope of shop built equipment if you need one. I would posit that most studio potters dont need a jar mill for their glazes unless the specific glaze is extremely problematic and can be beneficiated by selective milling. Val suggests that terra sigs should be milled, This is a specific application in which a mill could be and probably is very useful.
Grinding Media
Dan Taylor via Clayart:
You need grinding media in a ball-mill that has some tooth, like pebbles of porcelain. Glass or steel is too smooth. I use high fire, very dense porcelain balls of various sizes. Industry says you should fill your mill jar half full of balls, half of which (by weight) are the largest, and two other sizes 50/50. Im not sure what ratio I use and I dont think you have to be that specific but it is most efficient if all the balls are not the same size. You can make your own. They will wear down in time but the amount that is left in your glaze is so miniscule as not to matter.
Steve Grimmer via Clayart:
You can extrude balls for your mill from a nice, tight porcelain body. Use a round die about 3/8 in diameter. Cut the long coils into cylinders about 3/8 long, fire to maturity in a pile on a shelf or in a bowl, and then run them in your mill by themselves for 12 hours or so to knock off any rough points. Now you are in business.
Speed as related to internal jar diameter
From Ray Aldridge via Clayart:
The right speed is a function of the jars internal diameter. According to Cardew, the right speed is 64 to 87% of critical speed, which can be found by using this formula:
Critical speed in rpm=54.19 divided by the square root of R (when R is the internal radius expressed in feet)
Mill Speed - Critical Speed
http://www.pauloabbe.com/size-reduction/resources/mill-speed-critical-speed
Books
Chinese stoneware glazes by Joseph Grebanier
Thoroughly Modern Milling by Steve Harrison
Pioneer Pottery by M. Cardew
