Mercerisation is a treatment for cellulosic material, typically cotton threads, that strengthens them and gives them a lustrous appearance. The process is less frequently used for linen and hemp threads.
Mercerisation alters the chemical structure of the cotton fibre. The structure of the fibre inter-converts from an alpha- and beta-cellulose mixture to a thermodynamically favourable cellulose II polymorph.
Mercerising results in the swelling of the cell wall of the cotton fibre. This causes an increase in the surface area and reflectance, and gives the fibre a softer feel. An optional last step in the process is passing the thread over an open flame, or singeing. This incinerates stray fibers, improving the fabric's appearance. This is known as "gassing the thread" because of the gas burner that is typically used.