It is a heated stone wrapped in the floss silk or the cloth that the people put in their clothes to warm the chest or belly. It was used since the beginning of the12th century through the 19th century and may be considered as a prototype of "Kairo."
In the Chinese medication, "Kairo" is applied as so‐called "On‐an‐po," which generally consists of 3 methods according to the medical text written during the 11th century Japan.
- "Sui‐on‐ho" method
- A medical pin‐shaped stone "Benseki" is heated up and dipped into the water or the liquid medicine and is applied to the affected part.
- "Kawai‐ho" method
- The pin‐shaped "Benseki" is placed in the warm ashes and is applied to the affected part.
- "Zoshin‐ho" method
- A heated stone is worn in the clothes. One of the routes of "Onjaku."
"Zoshin‐ho" is regarded as a route of "Onjaku" but its principal is also seen in today’s bedrock bathing.
As the monks of Zen Buddhism used the "Onjaku" when practicing in order to manage the severe coldness or hunger, the "Kaiseki," dishes (meaning "putting a stone in the clothes"), a light meal just to temporarily ease hunger or to welcome guests served at the Tea ceremony is said to be named after this.
As described in the above, warming has a long cultural history in the worldwide medical cares and is tightly related to the Japanese culture of life.
"Kairo‐bai": In the Meiji‐era (1868~1912), the packed hemp husks or the peat are burn in the casks.
"Benzine Kairo": From the Taisho era (1912~1926), the catalytic action of the evaporated Benzine gas and the Platinum was applied and commonly used.
The present type of "Kairo" dates back to 1950~1953. The American soldiers fighting the war in the Korean Peninsula put the iron powder and the salt into the canteens which warmed their bodies. It is the application of the oxidation of iron that generates heat. The Japanese made it with their ingenuity into the present "Kairo."
In 1978, the disposable type which can be heated just by taking out from the package and shake was introduced to the market and the Benzine Kairo were quickly driven out.
Ferric established the technology of present "Kairo" and ever since has been innovating the products mainly for the medical use. Today this medical treatment of warming is attracting attention especially from the point of preventive medicine in view of the growing costs of medical treatments.