Mercerization
A caustic treatment that makes cotton stronger, smoother and more lustrous.
Mercerizing treats cotton under tension with caustic soda. The fibre swells, becomes rounder, takes dye more deeply and gains a soft sheen. It also improves strength and reduces shrinkage.
It is a finishing step, quoted separately. “Mercerized cotton” on a label should be verifiable in the price — if it is not, ask.