The thread is dyed in a mud field. Ōshima tsumugi involves two processes that are hard to explain and harder to watch without wanting to try: dorozome, dyeing the yarn in iron-rich mud, and shimebata, a loom on which the pattern is bound into the thread before any cloth is woven. In dorozome, yarn dyed with teechi wood is worked into the mud so that the iron reacts with it, and the black deepens with each repetition.

Before the war a quicker method was common, and it left dye that had not penetrated the thread, producing cloth that wore badly under friction. Naze has a hospital and shops; Kagoshima is about an hour away by air; and this is an island where preparing for typhoons is a fixed part of the annual routine rather than an occasional emergency. The weavers' association was founded in September 1901. Post-war output stood at a little over three thousand tan in 1946 and had fallen to six hundred and eighty-one by 1950. The association now has eighty-three members.

Mud dyeing cannot produce the same colour twice. The composition of the mud and the temperature differ from day to day, so the depth of the black varies from one bolt to the next, and there is no way to treat that as anything other than variation between individual pieces. A trademark, registered in 1955, is one answer to that problem, and a fairly elegant one. It gathers work that cannot be made identical under a single name, and what it guarantees is the method rather than the result — which is the only thing a mud-dyed cloth could honestly promise.