A mortar and pestle grinds a solid down through repeated, controlled impact and friction rather than through any chemical process. The mortar, a solid bowl with a rounded or slightly rough interior, holds the material to be ground and keeps it contained as it is worked, while the pestle, a rounded, hand-held tool with a broad striking end, is pressed and turned against the material inside, first crushing larger pieces with direct downward pressure and then grinding the resulting fragments smaller with a rotating or circular motion against the mortar's interior surface.
Repeating that crush-and-grind motion steadily reduces even a hard mineral or a dry, brittle plant material to a fine, even powder, and the finer the powder, the more surface area of the material is exposed, which mattered enormously for whatever operation came next: a substance ground fine dissolves faster in a liquid, calcines more evenly in a furnace, and sublimes more completely from an aludel than the same substance left in coarse lumps. Because grinding by hand takes real, sustained effort, the choice of mortar material mattered too, a harder stone or metal mortar wearing far more slowly than a softer wooden one, and letting an operator work a tough mineral down without the tool itself grinding away alongside it.