Mud dyeing, a natural dyeing technique handed down on Amami Oshima in Kagoshima Prefecture, is a method of dyeing fabrics from brown to black by chemically reacting the tannins of the techi tree (sharinbai) with the iron of the muddy fields. First, the fabric is dyed with a boiled solution of the techi tree, and then the mud from the muddy fields, which contains a lot of iron, is rubbed into the fabric to cause a reaction. By repeating this process alternately several times, from a dozen times to a dozen times, sometimes even several dozen times, the color gradually deepens and becomes fixed in the fabric.

The muddy fields used for dyeing are made from red soil unique to Amami, which originates from volcanic geology, that has been precipitated and decomposed over a long period of time. It contains a lot of iron, but the particles are very fine, making it easy for the dye to penetrate the fabric. If the mud does not have enough iron, cycad leaves and other materials may be added to supplement it.

This dyeing method, which combines plants rooted in the land with soil, is a unique culture nurtured by the climate of Amami. The layered, deep colors are achieved only through the combination of geology and vegetation conditions, as well as a time-consuming and labor-intensive process.