A tool used to crush or grind substances in a mortar. Before many artists' materials were manufactured and made commercially available, artists and their assistants produced paints and other media from raw materials, grinding many of them to prepare colorants, binders, glazes, investments, etc.If you're missing your pestle, consider improvising with a river rock; well rounded by long exposure to the smoothing action of flowing water. Look for a thin, oblong stone. Also see trituration.