It fit in a pocket, yet it could make Japanese winters considerably more bearable. Long before disposable hand warmers, this small metal case known as kairo allowed people to warm themselves without electricity. Behind this object, now housed at the Nikkei National Museum, lies an invention dating from 1923, whose mechanism is quite astonishing.
In Japan, this pocket-sized device hid inside coat pockets
With its green hue, its velvety finish, and a small red label adorned with Japanese characters, the model kept at the Nikkei National Museum looks more like a tiny candy tin than a heating device. Yet its mission was quite specific: to provide warmth to the person who slipped it into the pocket of their coat to face the cold.
The idea of carrying a heat source on one’s person is not new. In Japan, people once used heated stones, then small containers containing solid fuels to warm up. But at the beginning of the twentieth century, an invention would give the kairo a whole new dimension. This time, there would be no need to rely on embers to generate heat.
In 1923, a Japanese invention transforms the pocket warmer through platinum
In 1923, the Japanese inventor Niichi Matoba developed the Hakkin Kairo, a heater that exploits the properties of a precious metal: platinum. The principle is ingenious. A liquid fuel suited to the device vaporizes, and its vapors come into contact with a catalytic element containing platinum. Once activated by an external heat source, an oxidation reaction produces heat, without a sustained flame. A process documented by the Japanese manufacturer Hakkin.
And perhaps most remarkable is the size of this device. The Official Catalogue of Japanese Cultural Heritage records a historic specimen measuring only 10.1 cm in length, 6.7 cm in width, and 1.5 cm in thickness. Dimensions that made it easy to carry this compact heat source. It is, however, the model preserved at the Takaoka Museum.
What still sets this Japanese kairo apart from today’s warmers
Unlike disposable heaters, the Hakkin Kairo could be used multiple times. It only required refreshing its fuel and maintaining its components. The manufacturer still offers replacement catalytic elements, a design quite different from the small warming pouches commonly sold today.
Because modern disposable warmers operate differently. As explained by Nippon.com, they typically contain iron powder that oxidizes on contact with air, gradually releasing heat. There is no longer any need for liquid fuel or a platinum catalyst. As for the small green case kept at the Nikkei National Museum, it bears witness to an era when a few centimeters of metal were already enough to carry a heat source in one’s pocket…
