Kariyasu is a grass that grows on sunny mountain slopes in Japan, and the finest grass was said to come from Mount Ibuki. In late summer, the grass sends up upright flower spikes, and up close these show tiny dark purple parts among fine hairs that are often reddish. Yet since at least the eighth century, people have used this grass to dye cloth yellow. How can a plant with purple flowers give a yellow color?
The answer is that the color does not come from the flowers. Dyers cut the stems and leaves in the fall, dry them, and boil them in water. The yellow comes from natural chemicals called flavonoids in the leaves and stems. The color of kariyasu's flowers, in other words, says little about the color of the dye.
Boiling, however, is only the first step. Used alone, a dye like this often washes out. To make the color last, dyers add a mordant, often a mineral salt, that binds the dye to the fibers. The word comes from the Latin for "to bite," because a mordant was thought to help the color grip the cloth. The mordant can also change the color: with alum, kariyasu gives a mustard yellow, and with iron, a soft green.
The yellow can be changed in one more way. Cloth dyed with kariyasu and then dipped in blue indigo turns a bright grass green. So the purple of the flower spikes was never what dyers were after. The colors they wanted were hidden in the plain green leaves and stems.
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