Originally, koshiki were made of pottery and used to steam rice for eating.However since bamboo seiro replaced koshiki hundreds years ago, the word koshiki is used mainly to refer to steamers used in the sake industry.
A lot of mico breweries use traditional koshiki for steaming rice. In order to produce good steamed rice (hard on the outside and soft on the inside), the use of koshiki is very specific.
First a big pot of water is heated by a boiler. As it boils, the vapor steams the rice and also heats the inside of the pot. Because of the high heat, the humid steam gradually becomes dry vapor.The combination of humid and dry vapor makes it possible to have the steamed rice remain hard on the outside while being soft inside. (Some pots have a heating system which is under the water but as the water boils off, the element is exposed, further drying the air.)
The period of steaming is around 60 minutes after boiling, during which period up to 2000kg of rice can be steamed.