TL;DR
Unsealed stone and wood mortars absorb soap, so the standard method uses no detergent at all -- scrub with warm water and a stiff brush, then grind a spoonful of uncooked rice or coarse salt to lift oils out of the pores. Glazed ceramic and glass mortars are the exception and wash normally with dish soap.
Why the material decides everything
A mortar is one of the few kitchen tools where the cleaning method flips completely depending on what it is made from.
Granite, marble, and basalt mortars are unsealed on purpose. The rough interior is what grinds. Rough means porous, and porous means it soaks up whatever liquid you put in it, including detergent.
Unglazed clay and wood are even more absorbent than stone.
Glazed ceramic, porcelain, and glass have a sealed surface. They wash like a bowl.
Soap in an unsealed mortar does not simply rinse away. It works into the pores and comes back out into the next thing you grind, which is why a curry paste can end up tasting faintly of dish soap weeks later. That absorbency problem shows up across the kitchen in every unsealed material, as covered in the guide to cleaning kitchen tools without damaging them.
How do you clean a stone mortar and pestle?
Rinse it with warm water and scrub with a stiff nylon brush, using no soap. For oily or strongly flavored residue, grind two tablespoons of uncooked rice or coarse salt into a paste against the walls, discard it, rinse, and repeat until the rice comes out white. Air dry fully before storing.
The everyday method, step by step
1. Scrape out the solids. A rubber spatula gets the paste out without pushing it further into the surface.
2. Rinse with warm water. Warm, not hot -- extreme temperature swings can crack stone, and hot water sets protein residue.
3. Scrub with a stiff nylon brush. The brush does what soap would do in a smooth bowl. Work in circles around the walls and into the bottom curve.
4. Rinse again and check the smell. Garlic, chili, and cumin all linger. If you can still smell them, move on to the rice scrub.
5. Air dry completely, upside down or open. Trapped moisture in porous stone or clay leads to musty smells and, in wood, to splitting.
The rice scrub, and why it works
Uncooked rice is mildly abrasive and very absorbent. Ground against the walls of a mortar it acts as a dry scouring agent that also soaks up oils.
Add roughly two tablespoons of plain white rice, grind it with the pestle until it turns to powder, and look at the color. Grey or brown powder means it pulled residue out. Discard it and repeat with fresh rice until the powder stays white.
Coarse salt does the same job and is slightly more aggressive, which suits granite. Some cooks add a splash of water to make a scrubbing paste. Both approaches leave no flavor behind, unlike soap.
Rice and salt scrubbing is also the traditional way to season a new granite mortar. A brand new stone mortar sheds grit for the first several uses, and grinding rice until it comes out clean and white removes that loose material before you grind anything you plan to eat.
Removing strong odors
Garlic, ginger, chili, and fish sauce are the usual culprits.
The first move is another rice or salt scrub, which handles most of it because the odor lives in oils sitting in the pores.
If a smell persists, leave the mortar open in a dry, airy spot for a day or two. Air does more than water for a porous surface. A bowl of dry baking soda set inside overnight will absorb some of what remains.
Do not try to solve it with bleach. Porous stone and clay absorb it, it will not fully rinse out, and bleach must never be combined with ammonia, with vinegar or any other acid, or with rubbing alcohol, because each of those combinations releases a toxic gas.
Ceramic storage vessels behave the same way, which is why a garlic keeper or ceramic crock is also cleaned dry rather than washed with detergent.
Wooden mortars
A wooden mortar and pestle is washed like any other wooden utensil: quickly, never soaked, never in the dishwasher.
Wipe with a damp cloth, use a brush and water for stuck material, and dry immediately with a towel rather than leaving it to air dry face up. Standing water is what splits wood.
Once every month or two, wipe the inside and the pestle with a little food grade mineral oil to keep the wood from drying out. The full routine matches the one for how to clean wooden spoons.
What ruins a mortar
Dish soap in unsealed stone, clay, or wood. It absorbs, lingers, and flavors your food.
The dishwasher. Detergent, prolonged heat, and a long soak damage every mortar type except glass and glazed ceramic, and even those risk chipping when they knock against racks.
Acidic cleaners on marble. Vinegar and lemon juice permanently etch marble, travertine, and limestone. Etching is physical damage, not a stain, and it does not clean off.
Sealing a grinding mortar. Sealing the interior defeats the purpose. The rough, unsealed surface is the tool.
Storing it damp. Musty smells in porous material are hard to reverse.
When to just wash it normally
If your mortar is glazed inside -- a smooth, glassy surface -- treat it like a bowl. Warm water, dish soap, a sponge, and a rinse. Glazed ceramic and glass do not absorb, so there is nothing to gain by avoiding detergent.
Check by dripping a few drops of water onto the inside. If the water beads and sits, it is sealed. If it darkens the surface and soaks in, it is porous, and the no soap method applies.
Grinders behave differently again, since their working parts are metal or ceramic burrs rather than a bowl -- see how to clean salt and pepper grinders.
Cleaning a mortar well reduces flavor carryover and residue in your food. It is not a food safety system on its own.
Source: USDA Food Safety and Inspection Service guidance on cleaning food contact utensils
FAQ
Can you use soap on a mortar and pestle?
Not on unsealed stone, unglazed clay, or wood. Those materials absorb detergent into their pores and release it into whatever you grind next. Use warm water, a stiff brush, and a rice or salt scrub instead. Glazed ceramic and glass mortars can be washed with dish soap normally.
How do you get garlic smell out of a mortar and pestle?
Grind two tablespoons of uncooked rice or coarse salt against the walls until it powders, discard it, and repeat with fresh rice until the powder stays white. The abrasive lifts the oils that carry the odor. Then leave the mortar open in a dry, airy place for a day.
How do you season a new granite mortar?
Grind a couple of tablespoons of uncooked white rice into powder, discard it, and repeat until the powder comes out white rather than grey. New stone sheds loose grit for the first several uses, and this removes it before you grind food you intend to eat.
Can a mortar and pestle go in the dishwasher?
No, with the narrow exception of glass or fully glazed ceramic, and even those risk chipping. Detergent, heat, and a long wet cycle damage stone, clay, and wood, and detergent absorbed into porous material will flavor your food afterward.
Why does my mortar smell musty?
Because it was put away before it had fully dried. Porous stone, clay, and wood hold moisture deep in the material. Dry it thoroughly, then leave it open in a dry, airy spot for a day or two, and store it uncovered rather than sealed in a cupboard.
Is it safe to use vinegar in a marble mortar?
No. Vinegar and other acids permanently etch marble, travertine, and limestone, leaving a dull rough patch that is physical damage rather than a stain. Use plain warm water with a brush, and a rice or salt scrub for anything stubborn.
Kitchen tools reward a little knowledge, but the room around them is where the hours go -- the range, the counters, the floor under the stove. If you would rather hand that off, get a quote or read more about house cleaning services.