May Flours
Nope, that’s not a typo. This month a short note about how the way your materials are processed can change a glaze - things like silica flour. One common aspect is the mesh size of ingredients. Generally, the finer the grind of the material the more easily and quickly it is incorporated into the glaze melt. The larger the mesh number, the finer the grind. Silica is commonly supplied in a number of different mesh sizes. The most common form of silica for glazes is 200 mesh and finer, but it’s also available in 90 mesh (or even coarser 30 mesh silica sand) and 320 mesh. 320 mesh silica is sometimes specified in glaze recipes, and may make noticeable a difference when it is or isn’t used in that particle size. Most other ceramic materials are provided in the United States in powdered form, ground to 200 mesh or finer so they can be directly added to glazes and mixed.
Occasionally, materials come a little more coarsely ground, or you may find your own natural source. In this case, you may want to ball mill the glaze before using to reduce the particle size for a smoother melt. However, too much ball milling can grind the glaze to too fine of particles, making it melt more fluidly than expected. Another side effect of too much grinding can be that the glaze shrinks more in drying on the pot, creating application problems. And grinding some materials isn’t recommended at all, as it may cause other problems such as increased leaching of soluble portions of the material in the raw glaze slip, or in the case of zirconium encapsulated inclusion stains (highfire red stains, for example) grinding may even destroy the stain.