Extraordinary effects of mortar-and-pestle grinding on microstructure of sintered alumina gel

Abstract

The use of the mortar and pestle in the laboratory dates back to the earliest attempts to understand and use the materials around us. Even now, it is difficult to imagine an alternative to crushing and grinding for the preparation of many samples. Traditionally, mortar-and-pestle mixing or grinding has been held to be a relatively mild and controlled process, although it was recognized that the grinding of materials harder than the material of the mortar and pestle would result in some contamination. Thus, good practice forbade grinding of, for example, mullite and alumina in agate mortars. That this technique is not so innocuous was demonstrated by Dachille and Roy¹ with respect to the stresses generated. They showed that phases usually obtained only at 10–20 kbar can be obtained metastably by simple grinding in a laboratory mortar. There are numerous examples of solid–state phase transformations being produced by prolonged or intense comminution², two of the best–documented cases of stress/shear–induced transformation being the conversion of calcite to aragonite¹, and of quartz to amorphous silica^3,4, both by prolonged grinding in a laboratory mortar and pestle. Here we show that the very small amounts of material generated by the wear of mortar and pestle surfaces by even mild grinding can also have substantial effects on the micro-structure and transformation kinetics of certain ceramic systems so treated.