DYEING is a messy business. Factories that colour natural fabrics use masses
of salts—sometimes as much by weight as the fabric itself—to
encourage dyes to stick to the material. But those salts often end up in waste
water, killing fish in rivers or ruining crop soil. Now there may be a way to
avoid the salts altogether when dyeing cotton.
Dyes and natural fibres both become negatively charged in water, so they tend
to repel each other, making it tough to get strong colours to bind to cloth
quickly. Adding salts such as sodium sulphate helps overcome this.…
