A metate is part of a grinding tool known as a metate y mano. The metate is the lower, fixed half of this grinding tool, used to create a hard surface for crushing grains and a variety of other foods. Metates are Mesoamerican in origin, and they continue to be used in Mexico and parts of Latin America today; they are sometimes available at markets which specialize in Mexican cooking utensils, and they can be costly.
A classic metate is a flat slab of stone, sometimes with carved legs. The top of the slab has a shallow depression which is created through years of wear. To use the metate, someone scatters grain into this depression and then rolls a mano, a rounded grinding stone, back and forth across the metate. This back and forth rolling motion pulverizes the grain in a smooth and even way.
Metates are sometimes compared to mortar and pestle grinding arrangements. The grinding action of a mortar and pestle is slightly different, however. Mortars and pestles are also used in Latin American cooking in the preparation of specific dishes, but for things like grinding the corn for tortillas and mixing Mexican chocolate, a metate is the preferred grinding tool.
Learning to use a metate can take some time, as it requires the development of strong muscles in the upper body. Metates are traditionally used by women, and after years of use, they develop very strong upper arms and backs. Typically women sit or squat at a metate, creating as much leverage as possible for the grinding so that the food is ground efficiently.
Some examples of historic metates can be seen in museums which specialize in Mesoamerican artifacts. Some of these metates are quite old, showing clear signs of decades of use; because a metate y mano is made from stone, it will potentially endure for centuries, much to the delight of archaeologists. It is also possible to see ceremonial and decorative metates, which are used in some Latin American cultures for special occasions.
If you purchase a new metate to produce Mesoamerican food at home, you will need to season it before you use it. Seasoning typically starts with an overnight soaking and then a day of drying. After this, grind white rich in the metate until it turns into a fine grayish powder. Discard the power and grind a fresh batch of rice; continue to do this until the powder you produce is white, indicating that fragments of loose stone have been removed from the metate. Some people like to finish their seasoning by grinding in a mixture of spices such as salt, cumin, pepper, and garlic.