A new stone or unglazed ceramic mortar has fine stone dust left over from manufacturing; the first spices you grind pick up that dust, leaving a gritty texture in the mixture. Seasoning the mortar removes this dust: grind and discard a handful of rice until the resulting flour is white, then coat the surface with coarse salt and garlic.
A mortar is less a tool than a surface. Stone mortars are carved from granite, marble, or basalt; a ceramic mortar is glazed on the outside and deliberately left unglazed on the inside. The working surface needs to be rough, because that roughness does the grinding. But the same roughness also holds fine dust created during carving and grinding. A new mortar looks clean after being wiped by hand; the dust is deep in its pores and comes out only when something is ground. On the first use, that dust goes straight into the food.
To remove the dust, run the mortar empty once. Add a handful of dry rice to the mortar and grind it with circular, pressing motions until the rice becomes flour. The flour that comes out is gray or dirty beige on the first round; that color is the stone dust being removed. Discard the flour, wipe the mortar, and repeat the process. Color is the only measure of when the job is done: when the resulting flour is completely white, the pores are clear. Hard mortars are done in 2 rounds, while porous basalt mortars may need more rounds.
The second step is to seal the surface. Mash one garlic clove and one pinch of coarse salt in the mortar into a paste, spread it over the entire inner surface, and let it sit for a while; then rinse with hot water. The salt acts as an abrasive to remove any remaining dust, while the garlic’s aroma settles into the pores. This step is not required, but it reduces how much aroma the mortar transfers during its first few uses. Some kitchens use cumin or black pepper for the same purpose, with the same result.
The main rule for cleaning a mortar concerns soap: do not wash the mortar with soap. Porous stone and unglazed ceramic absorb detergent just as they absorb water; soap left inside transfers flavor to the next mixture, and washing does not remove that flavor. The right way to clean it is with hot water and coarse salt: add one pinch of salt to the mortar, scrub it with the pestle, then rinse. After a greasy mixture, first wiping it dry with a paper towel also helps. Do not put the mortar away until it has dried completely after washing.
Two common mistakes can damage the mortar. First, striking it: if the pestle is brought down like a hammer, the stone can crack and the ceramic mortar can break. Grind by pressing and twisting. Second, trying to break very hard ingredients in the mortar; break ingredients such as cinnamon sticks and coconuts by hand first, then put them in the mortar. Keeping a separate mortar for very aromatic spices also helps, because an aroma that settles into the pores can remain for years and spoil a mixture meant for dessert. If you clean the mortar right after grinding an aromatic mixture, the smell does not have time to settle into the pores.