Glutinous rice is soaked in lye, the alkaline liquid drawn off from wood ash or bamboo ash. Then it is wrapped up in the sheath of a moso bamboo and boiled in more of the same liquid for several hours together.

What comes out of the pot is amber-colored, translucent, and keeps for a remarkably long time. The alkali is doing two separate things simultaneously here: it softens the fibers of the rice, and it suppresses the growth of bacteria. Neither of those effects is decorative. This is preservation chemistry, worked out and relied upon centuries before anybody had a theory of chemistry to explain it.

One account says it began when Shimazu Yoshihiro of Satsuma carried it to Sekigahara as a food that would last the length of the campaign. Given the properties, that is at least plausible.

It is now made at home and bought in shops for the boys' festival in May, and eaten with kinako, or brown sugar, or white sugar dusted over the top of it according to preference. Something carried into a battle four hundred years ago is now handed to a child on a spring afternoon in May. What survived the transition intact is the gesture: rice, ash water, bamboo leaf, several hours of boiling.