Usu are large mortars used to pound cooked sticky rice in preparation for making mochi. It can also be used to pound millet.
Though you can get smaller ones for table top use (if the table is sturdy), most are about the size of a medium-sized barrel, about 1 metre (3 feet) tall, about 30 cm (1 foot) wide, and made of wood or stone.
The pestle that goes with it is called a “kine.” It’s like a wooden mallet, with a handle on it up to 1 metre (3 feet) long.
The process of pounding the rice in it is called “mochitsuki.”
First, the rice is steamed and then let cool a bit, then the pounding starts.
You have to be careful not to wallop the edge of a wooden usu, or you can cause splintered wood to come off into the rice. The flat edge of the mortar must come down flat into the mortar. If it comes down on an angle it might cause the edge of the mortar to give off splinters.
It’s best to have two people working it: one person to swing and pound, and the other person, between swings, to stir the rice with his or her hand.
When the mass is smooth and pasty, then it can be formed into mochi.
Now, there are electric machines to do this in homes, called “mochitsuki-ki.” The machines both steam the rice, and convert it into the paste.
But mochitsuki with a usu is still done in some settings such as restaurants, rich or rural households, or households that like the tradition, because they think it tastes better done the old way in a usu.
The preparation and the pounding has traditionally been part of the work-up to New Year’s celebrations, when mochi are traditionally served.