Molcajetes are used to make guacamole and grind spices. Matates are used to grind corn and spices. Both are made from volcanic or lava rock. Because of this, they must be treated before use so that loose pieces of the rock does not get ground into the food.
Place your molcajete or matate in the sink and wet thoroughly. Also the roller or pestle must be rinsed thoroughly. Take handfuls of raw rice and place either the molcajete or matate. Add some cold tap water and begin to grind the rice into the bowl. Repeat several times, making sure that you have ground the entire interior surface. Rotate the pestle as well so that its surface is cleaned as well.
This needs to be done until the water runs clear. No bits of stone should be coming off as you rinse.