Do not wash a spice grinder with water: steel burrs rust, moisture can swell the seat of a ceramic burr, and wet spice powder sticks to the burr teeth like paste. Clean the grinder dry: add a handful of rice or coarse salt, grind it on the coarsest setting, and discard the resulting powder. This removes both residue and old odors.
The working part of a spice grinder consists of two interlocking conical burrs. Whole spices fall between them and are crushed as they move downward; the grind size changes when the gap between the burrs is adjusted. For this mechanism to work properly, two things are needed: the burr teeth must stay sharp, and the gap between them must not become clogged. Water damages both. That is why the first rule of grinder cleaning is simple: do not wash the grinder with water or put it in the dishwasher.
Water damage varies by material, but in both types it can be permanent. In grinders with steel burrs, moisture means rust; rusted teeth become dull and mix rust particles into the ground spices. In models with ceramic burrs, the burr itself does not rust, but the shaft and seat that hold it are usually metal, and that is where the main problem occurs. Wet spice powder also sticks like paste between the teeth; when it dries, it leaves a layer as hard as stone and locks the burrs. If you force a locked grinder to turn, the teeth can break and the device can become unusable.
Dry cleaning is both easy and effective. Empty the grinder’s hopper, add a handful of dry rice or coarse sea salt, set the grinder to its coarsest setting, and grind it all the way through. As the hard grains pass between the burrs, they carry out oily spice residue left there and absorb odors from the teeth. Do not use the resulting powder; discard it. If needed, repeat the process one more time. Then turn the grinder upside down, tap it gently, and wipe the outside with a dry cloth.
The reason for keeping separate grinders for salt and black pepper lies in this mechanism. Salt absorbs moisture and is corrosive to metal; grinding salt in a grinder with steel burrs wears them down in a relatively short time. That is why salt grinders have ceramic burrs. Oily spices such as black pepper, cumin, and coriander seeds leave a thin film of oil on the burrs; when that film combines with salt, it clumps. Alternating between the two ingredients in the same grinder compromises the result of both. For the same reason, the grinder’s adjustment nut can also start to seize over time in a salty environment.
There are a few additional limits. Grinding spices in a coffee grinder is not recommended because their volatile oils settle in the hopper and on the blade and carry over into the next coffee; even dry cleaning cannot fully remove this odor. Very oily or moist ingredients, such as fresh ginger or fresh red peppers, should never go into the grinder. If the grinder will not be used for a long time, leave it empty and dry. If spices are left in the hopper, they can absorb moisture and clump, locking the burrs the first time you turn the grinder.