|Monthly Tech-Tip |
A method of grinding particles in ceramic powders and slurries. A porcelain vessel filled with porcelain pebbles tumbles and particles are ground between colliding pebbles.
A device used to reduce the particle size of materials, bodies or glazes. A ball mill is simply a container that is filled with pebbles (either of porcelain or stones e.g. Flint) into which a charge (powder or slurry) is put and that is then mechanically rotated to cause the tumbling pebbles to crush particles that happen between them. Ball mills can be continuous or periodic, they can be small or gigantic, low speed or high speed, rotated or vibrated or both. For maximum efficiency a ball mill should be made of, or lined with, a porcelain or other very hard surface (so grinding also occurs between the wall and the balls), the balls should be of a range of sizes (to maximize points of contact), the mill should have the correct quantity of balls, the slurry should be the right viscosity and the charge should be an optimal amount (over charging reduces efficiency). Various compromises are often made (for example rubber lined mills to reduce wear and noise).
Large manufacturers hire ball mix supervisors, operators and mechanics. Technicians occupy themselves with getting a consistent and predictable product (surface area and particle size distribution), they employ mathematical formulas to determine the amount of balls needed, distribution of ball sizes and other operating parameters like duration and speed. They are wary of grinding products as mixes, it is often better to mill hard and soft powders separately and combine them later. Engineers typically use surface area measurement instrumentation to evaluate mill efficiency.
Ball mills can reduce particles to the nano sizes, the process is very important in creating powders used in hi-tech industries (e.g. alumina). Ball mills are slow compared to other methods of grinding, it could take hours, for example, to grind all the particles in a clay to minus 200 mesh. Industrial mills seeking nano-sizes might run 24 hours or more! Ball milling is normally done in consort with wet screening and/or roller-milling/air floating, for example, so that large particles have already been removed by the time the material reaches the ball mill. Air floating can also be done in consort with dust ball milling. The milling process can also reduce particle sizes by too much for an application, so a means of measuring the distribution of ultimate particles is important to be able to set the parameters for the process.
A clay body that has been ball milled will be more plastic, potentially much more plastic. Ball milling of the body or selected body materials will reduce or eliminate many types of fired glaze imperfections (especially specking, blistering and pin-holing). That being said and as already noted, iron particulates are best removed before milling).
Milling a glaze will produce a cleaner fired result with less imperfections. Materials deliver their chemistry to the glaze melt only if their particles dissolve in the melt. But some glaze materials are refractory and resistant to dissolving (e.g. silica, alumina). When silica does not completely dissolve in a transparent glaze it will fire cloudy and its actual thermal expansion will be higher than it would otherwise be. By ball milling silica to very small particle sizes all the particles dissolve, producing a much better fired product.
Milling of slurries presents less technical challenges than dust milling. We have found that thicker creamy slurries mill better than watery ones. A simple ball mill can be constructed by almost anyone, but obtaining the hard pebbles with the correct range of sizes for inside the mill can be challenge (they are expensive).
You can measure to see. How? Wash a measured amount through a 200 mesh screen and note the amount of residue. These two show the oversize on a 200 mesh screen of 100 grams of glaze slurry. On the left: Unmilled. On the right: Milled 1 hour. Clearly it needs more than 1 hour in this mill. A factor here is the high percentage of silica in this recipe. And the fact that US Silica #95 rather than #45 was used.
Courtesy of Lawrence Weathers
Yes, the granular and powdered grades are the same material. But grinding it is very difficult. Commercial ceramic grade powder is minus 325 mesh, the companies doing this obviously have very good grinding equipment. They also have patience because even in this efficient porcelain ball mill, 90 minutes was only enough to get 50% to minus 325 mesh! The color of the powder is a good indication of its quality, the finer the grind the lighter will be the tan coloration.
These DFAC testers compare the drying performance of Plainsman A2 ball clay at 10 mesh (left) and ball milled (right). This test dries a flat disk that has the center section covered to delay its progress in comparison to the outer section (thus setting up stresses). Finer particle sizes greatly increase shrinkage and this increases the number of cracks and the cracking pattern of this specimen. Notice it has also increased the amount of soluble salts that have concentrated between the two zones, more is dissolving because of the increased particle surface area.
This 1000 ml 24 hour sedimentation test compares Plainsman A2 ball clay ground to 10 mesh (left) with that same material ball milled for an hour (right). The 10 mesh designation is a little misleading, those are agglomerates. When it is put into water many of those particles break down releasing the ultimates and it does suspend fairly well. But after 24 hours, not only has it settled completely from the upper section but there is a heavy sediment on the bottom. But with the milled material it has only settled slightly and there is no sediment on the bottom. Clearly, using an industrial attrition ball mill this material could be made completely colloidal.
Types of Ball Mills
Make Your Own Ball Mill Stand
Pictures of a ball mill rack that you can make yourself
Ball Milling Glazes, Bodies, Engobes
Industries ball mill their glazes, engobes and even bodies as standard practice. Yet few potters even have a ball mill or know what one is.