Slipping a warm object under the covers is an ancient, universal way of warming up the bed. In the past, that object could be a brick or a flat stone previously warmed by the fire, a simple object capable of radiating low heat that the sheets and blankets helped keep inside the bed. In post-war Europe, bed warmers were rubber bags filled with hot water, very similar in size and shape to the yutanpo. Yutanpos developed in China and came to Japan in the 15th century. Three centuries later they had become a common household object. In those days, they were earthenware and had a dome shape. In the first decades of the 20th century, mass-produced yutanpos became oval in shape, made of zinc-plated iron with a rippled top and bottom to optimize heat radiation. In the 1950s, annual production in Japan was about a million units. Production slowly decreased as fossil fuel heating systems became widespread. But today, the demand is steadily increasing again, and the functional principle of the yutanpo has been reevaluated as an environmentally friendly, easy to use heating alternative. A yutanpo holds about 2 liters (0.5 gal.) of heated water, which will still be warm in the morning, ready to be reused for washing your face or the breakfast dishes. Because they are easy to ship, inexpensive and transportable, a novel use for yutanpos has emerged as temporary heating units in relief efforts where existing energy infrastructures have been severed.
