Why does indigo have to change its chemistry before it can dye blue jeans?
Indigo is intensely blue but practically insoluble in water. Sodium dithionite provides the reducing power needed to convert it into a water-compatible reduced form that can reach cotton fibers.
The yarn is dipped into this reducing dye bath and then exposed to air.
Oxygen reverses the reaction, turning the dye back into insoluble blue indigo—now associated with the fiber.
The cycle can be repeated several times to deepen the color.
So traditional indigo dyeing works through an elegant chemical trick: reduction makes the dye mobile; oxidation brings the blue color back and makes the pigment insoluble again.
Global Chemical Database